The first Block IIF satellite destined for orbit arrived at the Navstar Processing Facility at Cape Canaveral, Florida, aboard an Air Force C-17 cargo aircraft on February 12. It is now undergoing preparations for its launch this spring on a Delta IV rocket. Block IIF will enhance GPS performance by reportedly providing twice the navigational accuracy of heritage satellites, more robust signals for commercial aviation and search-and-rescue, and greater resistance to jamming in hostile environments.
New L5 Signal. The new IIFs will broadcast the operational civil L5 signal, whose spectrum allocation was secured by broadcast of the signal by a IIR(M) satellite last year. L5, at 1176.45 MHz, lies in the Aeronautical Radionavigation Services band and can be used for safety-of-life aviation. It will be compatible with Galileo, GLONASS, and QZSS, with the goal to be interoperable as well. L5 will transmit at a higher power than current civil GPS signals, with wider bandwidth, and lower frequency that may enhance indoor reception.
More L2C Beacons. The IIF generation will also add to the number of satellites on orbit that broadcast the L2C signal at 1227.6 MHz, bringing it closer to full operational capability. L2C enables the development of lower-cost, dual-frequency civil GPS receivers for correction of ionospheric time-delay errors. Once the control segment modernization is complete, enhancements such as dataless and pilot channels for improved performance and an improved navigation message with more precise clock and ephemeris information will be available. L2C will also be interoperable with the Quasi-Zenith Satellite System (QZSS) under development by Japan.
Long Life. Built by Boeing, the IIF has a longer design life of 12 years, faster processors, and more memory. It will be followed by 11 other IIFs before modernization shifts into a higher gear with the GPS III generation.
It takes four hefty guys to wheel the new satellite along the tarmac, but it will only take one Delta IV rocket to lift it 20,171 kilometers into space on May 13.
Some Receivers Run Afoul of GPS Ground Control Software Update
On January 11, 2010, when the GPS Wing and the 2nd Space Operations Squadron (2SOPS) loaded the updated AEP 5.5C software to the ground control segment, a problem surfaced with a specific subset of GPS selective availability anti-spoofing module (SAASM) receivers.
The GPS Wing did not revert to the previous AEP 5.4 because of the upcoming IIF-SV1 launch. The scheduled sequential AEP 5.5C and AEP 5.5D updates are required before the ground control segment can adequately manage the more advanced capabilities of the IIF satellites.
One purpose of the 5.5C AEP update is to enable SAASM functionality in coded receivers. The software for this functionality has been resident in various certified SAASM receivers for some time, but was never implemented in the ground control segment. The update alleviates that problem for the majority of SAASM receivers, but for one manufacturer it has caused problems. The updated software sends a specific code to SAASM receivers that enables them to authenticate the message and ensure that the code is correct, and is being sent from the GPS and not some other source. For most receivers this worked without a hitch, but for one manufacturer, a software (SW) bug or glitch occurred that must be corrected before the receiver can authenticate. This fix is in progress and will most likely be implemented as a software or firmware update to the receivers.
Timing. Another problem with a different set of receivers manifested itself exactly two weeks after the AEP 5.5C update occurred. Those that have researched this problem in some depth feel that the problem is totally unrelated to the AEP update and would have occurred regardless.
This is also considered to be a receiver software bug for the manufacturer, and that process is ongoing.
ICD. Prior to activating the software update, the GPS Wing issued an updateable ICD or Interface Control Document that all receiver manufacturers use as a voluntary guide to determine compliance. Strict compliance by the manufacturer with the receiver interface control document (ICD) may have prevented the first issue, but the second may be a serendipitous event of the type that occurs from time to time no matter what precautions are taken.
The GPS Wing has issued two Notice Advisory to NAVSTAR Users (NANUs) for civilian and commercial GPS users and for military users, asking for user comments.
Letter to the Editor. Meanwhile, a reader wrote in: “I have issues with misleading e-mails containing inaccurate titles of articles posted on the site. There have been multiple cases recently claiming AEP software (SW) upgrades caused problems with receivers. In fact, and as proven by the vendors involved and others analyzing the problems, the AEP SW did not cause any of the observed conditions. ICD noncompliance of SAASM user equipment (UE) caused the problems, and the AEP SW upgrade allowed DoD, FAA, and vendors to finally discover the noncompliance issues and begin the process to resolve them. The community should view the 5.5 SW upgrade for what it is: a valuable new capability implemented correctly, which helped us all understand some unexpected shortcomings in UE.”
The editor concurs, and apologizes for misleading article titles. However, hard information was scant — in fact, completely unavailable — at the time.
GLONASS Gets Regional; Beidou Moves; Galileo Inks
The three new GLONASS-M satellites launched on December 14 have been set operational: GLONASS 730 in orbital slot 1 was set healthy on January 30, joining 734 and 733, which were set healthy earlier in the month. This brings to 18 the number of satellites currently in service, although GLONASS 722 continues to provide a healthy signal only on its L1 frequency. At present, the constellation only suffices to provide a 24-hour regional signal over Russian territory, although satellites can and frequently are pulled in by global high-precision users to complete an RTK solution, along with GPS.
Two satellites are in maintenance mode and set unhealthy, and two others, launched in 2003 and 2005, respectively, are in the process of being decommissioned.
The next GLONASS launch, of the GLONASS Block 40 satellites originally set to rocket up last September but returned to the Reshetnev factory with problems in the signal generator, is scheduled for March 2. Three more will rise in August, and a November 10 booster will put two GLONASS-M satellites and the first GLONASS-K satellite into orbit.
Beidou. According to tracking data from the United States Strategic Command, Beidou’s G1 satellite has drifted from its original location of 160°E and is currently at about 147°E longitude, that is, no longer in geostationary lock. Perhaps it is moving to another assigned Beidou slot, to back up or replace one of the other satellites in the constellation, but this can be no more than speculation. Hard data on the Beidou/Compass system is extremely difficult to come by. The new Chinese government Beidou/Compass website does not provide up-to-date information on the status of the constellation — something we take for granted with GPS, GLONASS, and Galileo.
Galileo. The European Space Agency signed contracts for Galileo’s full operational capability phase on January 26: with OHB for the manufacture of 14 satellites, delivery of the first in July 2012, followed by two satellites every three months; for launch services with Arianespace; and for system support with Thales Alenia Space.
24+3 FAQ
Survey editor Eric Gakstatter posed these questions to the GPS Wing; their answers follow.
Will the satellites (SVN24, SVN26) remain healthy during their repositioning journey?
Yes. The satellites will be set unhealthy for the initial Delta-V, but will return to healthy status approximately 24 hours after initiation of the Delta-V. Initial Delta-V for SVN24 was accomplished on 13 Jan 10 and returned healthy on 14 Jan 10. SVN24 will take up to a year to reach its final destination. Initial Delta-V for SVN49 was accomplished on 21 Jan 10 and will arrive at its expanded position in Jun 10. Initial Delta-V for SVN26 will begin early Feb 10.
Why the two-year timeframe to realize the benefits when all repositioning will be complete in 12 months?
The two-year timeframe is a conservative estimate which takes into account potential operational necessities which could extend the time required for completion. We must take a disciplined approach to cover possible failures and ensure continuity of coverage during the transition. We will be adding GPS IIF vehicles to the constellation and older vehicles may fail during the transition timeframe. As vehicles are added and removed, the current plan is subject to change in order to provide the best service to all civil and military users. Some of these decisions could require additional time to complete the expanded constellation. However, benefits will likely be realized well in advance of 24 months.
What is the reasoning behind using SVN49 as a key component of the 24+3 configuration since it won’t benefit a significant portion of the civilian user community, namely aviation and marine navigation as well as other SBAS (WAAS) and DGPS users? In my understanding, the FAA’s and the Coast Guard’s user bases are primarily single-frequency pseudo-range, users who won’t be able to use SVN49.
SVN49 was selected because it is a brand new satellite with four good clocks. Although issues with SVN49’s navigation signals may make it unusable for all civil use, it could still put out a valid set of signals for military use. The Air Force team is continuing to work “open book” with civil and industry GPS experts to determine the possible outcome of SVN49. Although SVN49 is not currently healthy, GPSW and 50th SW are actively working a mitigation that may allow setting the vehicle healthy in the future. As a mitigation in case we are unable to set SVN49 healthy, SVN30 will be rephased to the same slot following a successful launch and on-orbit checkout of IIF-1. We expect to have either SVN30 or SVN49 healthy and broadcasting from the expanded slot within a 24-month timeframe. At this time, no decisions have been made and no options have been ruled out regarding SVN49.
At the start of a new decade, let’s examine the state of the GNSS consumer market and technology. In the December 2009 issue of GPS World, I described the developments that put GPS in cell phones over the last decade. That technology revolution has brought GPS a very long way. Having come this far, we can ask that most famous of all navigation questions:
Are we there yet?
In this column, I focus on the question for the consumer segment of GNSS. Has the consumer market reached the point we expected it to be by now? Has the technology reached levels we anticipated?
The cell-phone GPS revolution began with the catalyst of U.S. E911 legislation, which mandated that when an emergency (911) call is made from a cell phone, the location of the cell phone must be provided. Among several competing location technologies, GPS proved to be the big winner, thanks to seven technology enablers: assisted GPS, massive parallel correlation, high sensitivity, coarse-time navigation, low TOW, host-based GPS, and RF-CMOS.
All of these together enable very low-cost implementation of GPS in cell phones, even phones on networks such as GSM and W-CDMA that do not have fine-time synchronization (that is, they are not precisely synchronized with the GPS system). GPS is now found in roughly 500 million phones in use today.
Four Milestones. From a consumer market perspective, we have exceeded forecasts. From a technology perspective, we have kept track with Moore’s law. Chips and receivers are cheaper than expected — because, as well as Moore’s law, we have seen greatly increased volumes and competition. Low-cost chips have not come at the expense of performance; in fact, the opposite — as chips have evolved, they have become less costly and better performing.
Small, cheap antennas have affected performance, but given the same antenna, I will demonstrate that a receiver with a single-die GPS chip costing less than $4 can outperform a $19,000 receiver.
This sounds paradoxical, even impossible — indeed many of you may be penning letters to the editor right now! But the time-to-first-fix, sensitivity, and urban-accuracy data will prove my point.
As a consequence of chip evolution, we are reaching plateaus of development for GPS-only systems. However, there remain many problems to solve, especially in urban canyons and indoors. These problems may never be solved with GPS alone, or with any single system alone. This decade will be characterized by GPS-plus; the days of GPS-only will soon recede into the past.
Don’t interpret this as a failing of GPS — quite the opposite. Because GPS-only systems have worked so well, they have found their way into half a billion cell phones, and we are boldly taking GPS to places no navigation has gone before. As we do, we start to encounter the limitations of GPS-only performance.
We will see the proliferation of GPS-plus: GPS+MEMS, GPS+Wi-Fi, GPS+NMR, and GPS+GLONASS, Compass, QZSS, and Galileo. The winners will be those with the greatest levels of integration. To paraphrase Winston Churchill, this is not the end of GPS, it is not even the beginning of the end. But it is, perhaps, the end of the beginning.
GNSS Consumer Market
For market forecasts made a few years ago, we can look at summaries provided in GNSS Markets and Applications, by Len Jacobson: a 2006 Frost & Sullivan report estimated the market for PNDs and handheld devices (not including cell phones) in 2010 would be $2.7 billion, with 8.3 million units, at an average selling price (ASP) of $325. In fact, this market today is approximately $6 billion, with 40 million units, at an ASP of $150.
Twice the Size. The consumer market, not including cell phones, is twice as big (in dollars) as forecast just a few years ago, even though prices are less than half forecast. Unit sales are more than four times forecast.
For the cell-phone market segment, in 1999 when the E911 rules were enacted in the United States, it was anticipated that A-GPS would be adopted only in fine-time (synchronized) networks, such as Verizon and Sprint CDMA. In coarse-time (non-synchronized) networks such as GSM, the expectation was that terrestrial wireless location techniques, such as time-difference-of-arrival (TDOA) and enhanced-offset-time-difference (E-OTD), would dominate. Today, only a few niches use TDOA, E-OTD is extinct, and GPS rules in coarse-time networks worldwide, including GSM in Europe and North America, and W-CDMA in Japan.
The consumer market, in particular the cell-phone market, has grown so rapidly that more receivers have been built in cell phones in the last three years than all other GPS built, ever. Today, L1 C/A-code GPS accounts for more than 99 percent of all GNSS receivers manufactured each year.
From a consumer market perspective, have we reached the point we expected to be by now?
Yes!
Not only have we arrived, we have far surpassed expectations.
GPS and Moore’s Law
Moore’s law says that for a given number of transistors, the chip size will halve every two years. Table 1 shows what this looks like in practice. For a particular class of GPS chip, the A-GPS receiver with massive parallel correlation, it shows release dates of different generations of these chips, and the technology process, which is the linear dimension of a single gate on the silicon die. As this dimension reduces to 70 percent of the previous value, the 2-dimensonal chip size reduces by 2 times. You can see Moore’s law in action here: approximately every two years, the technology process moves to the next level, and the chip size reduces by 2X. People are now talking about GPS chips in 45 nanometers, the next step.
For a comparison, consider the Broadcom BCM 4751 chip, designed for cell phones. This chip is 2.9 X 3.1 millimeters, the size of the letter B on this page. This is a single-die host-based GPS/SBAS receiver, including RF front end, low-noise amplifier, baseband, and power management unit. Ten iterations of Moore’s law have passed in the last 20 years. The same chip, had it been built 20 years ago, would have been 210 times (a thousand times) bigger.
There were never chips that big. GPS chips aren’t just getting smaller with Moore’s law, they are getting vastly more complex and more capable.
Performance
At an elemental level, a GPS receiver does just three things: it starts, it tracks weak signals, and it computes position, velocity, and time. Strip away the obfuscating details, and performance may be summed up by: how fast, how sensitive, how accurate.
Since the 1990s, time to first fix ( TTFF) and sensitivity have improved dramatically, thanks to the seven technology enablers discussed earlier. TTFF for assisted cold starts, or unassisted warm starts, is now as good as one second, even without fine-time. This is a 45X improvement on typical GPS performance of the 1990s. Sensitivity increased roughly 30X (to -150 dBm) in 1998, then another 10X, (to -160 dBm) in 2006, and perhaps another three times to date, for a total of almost 1,000X extra sensitivity.
What about accuracy?
Some perceive low-cost chips as synonymous with low accuracy. This is not true. It is true that small, cheap antennas reduce accuracy; but given the same antennas, the lowest cost receivers on the market today will outperform the most expensive in typical environments where cell phones are used. The following figures show data to prove this point.
First we connect one of the smallest, lowest cost GPS receivers t
o one of the best antennas, a choke ring, on a rooftop with a clear view of the sky. Figure 1 shows the scatter of positions. The blue circle shows the median distribution, which is 0.9 meters for this dataset of 2000 fixes.
FIGURE 1a. Low-cost GPS with large, rooftop antenna.FIGURE 1b. Survey-grade GPS with large, rooftop antenna.
The adjacent plot shows the positions obtained from a $19,000 survey-grade GPS receiver, connected to the same antenna. The survey-grade GPS, with a median distribution of 0.3 meters, shows a 60-centimeter advantage over the cell-phone GPS, or maybe a 3X advantage depending on how you look at it. But don’t get too hung up on this result, because this is neither the typical consumer scenario (on a rooftop with choke-ring antenna), nor the main challenge facing us today.
Next we look at the accuracy achieved with a more typical consumer antenna, in a more typical environment. Figure 2 shows the positions obtained in downtown San Jose with an active patch antenna, such as found in PNDs. San Jose is a fairly typical U.S. city, not the hardest place to use GPS, but not the easiest either. Lightstone Alley, adjacent to tall buildings, is only five meters wide.
FIGURE 2. Performance of cell-phone GPS (white) versus truth-reference system (blue). Median accuracy 4.4 meters, 67 percent 5.6 meters, 95 percent 11.2 meters.
To evaluate accuracy we used a truth-reference system combining GPS and a tactical-grade IMU with ring laser gyro to produce the blue dots on the figure. The white dots are the low-cost GPS positions. Most of the time, the white dots appear to be on top of the blue, but occasionally you see some separation, and there the red lines show the horizontal error. The median horizontal error is 4.4 meters.
Figure 3 shows the comparison of low- and high-cost receivers, with the survey-grade receiver connected to the same patch antenna as the cell-phone GPS. There are many position gaps from the survey-grade receiver, and the position walks around when the vehicle is stationary (at the intersections, bottom left and top of the figure). This is because of the weak signals available in the urban environment. But don’t get too hung up on this result either, since we are still not at the real challenge of consumer GPS: location in severe urban canyons, such as San Francisco, New York, Chicago, Shanghai, Taipei, Shinjuku, and similar. In these, typically, only one or two GPS satellites can be seen directly. Other satellites may be tracked, but only by observing purely reflected signals. This is not classic GPS multipath, the combination of a direct and reflected signal; instead this is the combination of nothing but reflected signals. The direct signals are usually completely blocked by many buildings, and are not observable at all. So the whole premise of GPS — observing range from time of flight — breaks down, and it is very difficult to get good accuracy.
FIGURE 3. Comparison of cell-phone (left) and survey (right) receivers, both with patch antenna
Figure 4 compares the cell-phone GPS with the survey-grade GPS, connected to the same small antenna, under such circumstances in San Francisco’s Financial District. There are no fixes at all from the survey-grade receiver. Why?
FIGURE 4. Cell-phone (left) and survey (right) receivers, in severe urban canyon
In Montgomery Street, there was only one directly visible satellite, with a signal strength of -132 dBm. All the other satellites were at -140 dBm or weaker, and traditional GPS receivers cannot acquire signals at this level. Hence the only receivers that work in this environment are modern high-sensitivity receivers most commonly found in cell phones.
You can see that the move to lower-cost receivers has not come at the expense of performance. In fact, the opposite: TTFF and sensitivity have improved dramatically, while accuracy has not been compromised, and is in fact much better in urban environments than legacy receivers, and even modern survey-grade receivers.
But are we there yet?
Although the consumer GPS market has irrefutably arrived, from a technical perspective the answer is more nuanced. Consumer GPS technology has made tremendous leaps forward. But precisely because of these improvements, we are taking GPS where it was never expected to go. It is no longer enough for GPS to work indoors (which it can). The demand is now for it to work as well as if it were outdoors (which, presently, it cannot).
Performance improvements seen with GPS-only will almost certainly not continue at the recent rate. We do not anticipate yet another 45X improvement in TTFF, or another 30 dB of sensitivity, for GPS alone. However, we do expect order-of-magnitude performance increases with the addition of other technologies. Figure 5 shows data from a TomTom 950, a GPS+MEMS containing the same GPS chip used in the earlier tests, MEMS accelerometers, and MEMS rate gyros. When tightly integrated and tested in the same deep urban canyons of San Francisco, the effect on position is good: median accuracy improved by 30 percent, worst-case errors are more than halved. But the result on heading accuracy is especially dramatic.
FIGURE 5. PND position accuracy (left), and heading accuracy (right), San Francisco
The bar graph shows the worst-case heading accuracy in each street. With GPS-only (red), the worst-case error is around 45 degrees, a familiar result to anyone who has used any GPS-only device in a similar environment: sooner or later the map will veer erroneously. However, with the integration of the MEMS rate gyros (blue), the worst-case heading errors drop to around 3 degrees, a 15X improvement in a key metric, similar to the improvements of the last decade, but now thanks to the effect of GPS-plus.
We will soon see GPS-plus many other technologies: Wi-Fi, NMR/MRL (power measurements from GSM and 3G phones), and of course GPS+GLONASS, Compass, QZSS, and Galileo. Because many mobile devices now include GPS, Wi-Fi, and 3G, there is a natural path for the evolution of GPS technology to include Wi-Fi and MRL measurements.
There is a also natural trend to source different radios from the same chip supplier. After all, why would you wish to undertake a do-it-yourself effort at removing co-existence issues in different radios, when a chip supplier has already done it for you?
Looking forward, it is very likely that this new decade will be characterized by GPS-plus other technologies, and the winners will be those with the greatest levels of integration.
Frank van Diggelen is senior technical director of GPS systems and chief navigation officer for Broadcom Corporation. He holds more than 45 U.S. patents, has a Ph.D. in electrical engineering from Cambridge University, and is the author of A-GPS: Assisted GPS, GNSS & SBAS.
A Prototype System for Navigation in GPS-Challenged Environments
By Chris Rizos, Dorota A. Grejner-Brzezinska, Charles K. Toth, Andrew G. Dempster, Yong Li, Nonie Politi, Joel Barnes, Hongxing Sun, and Leilei Li
A team of Australian and U.S. researchers have integrated a ground-based system with GPS and INS to create a hybrid system that provides precise and accurate position information continuously in a variety of environments where GPS alone comes up short.
INNOVATION INSIGHTS by Richard Langley
GPS HAS ITS LIMITATIONS. Although it is a 24/7 global system, it doesn’t work everywhere. The microwave radio signals transmitted by the satellites are rather weak, and although they can provide excellent positioning performance when a receiver’s antenna has a direct line-of-sight view of a sufficient number of satellites well spread out in the sky, positioning accuracy degrades or becomes impossible when the signals are effectively blocked by obstacles such as trees, rock faces, and buildings outdoors and by roofs, ceilings, and walls indoors.
In many obstructed environments, the signals aren’t completely blocked but rather their power is severely attenuated so that they are no longer strong enough to be acquired and tracked by a conventional GPS receiver. Remarkable progress has been made in the development of super-sensitive receivers that, in conjunction with an appropriate antenna and assistance information provided over a mobile phone network, can provide position fixes in such environments. However, the precisions and accuracies of these pseudorange-based positions are often very poor — perhaps as low as 100 meters or more.
So, is it possible to obtain precise and accurate positions in obstructed environments? Well, we could add measurements from GLONASS (or other satellites) to GPS measurements, but GLONASS suffers the same problem as GPS, and while the additional satellites could be an advantage in some partially obscured areas there are many places where we won’t be any better off. We could use an inertial navigation system (INS), but such devices have their own weaknesses such as the requirement of initial calibration and the accumulation of position error with time. Are there any other technologies available?
We know GPS works very well when there is a direct line-of-sight view between the satellite transmitters and the receivers and carrier-phase measurements can provide decimeter- and even centimeter-accuracies. So why not develop a ground-based system that works in a similar way to GPS, which would allow you to place the transmitters wherever you like? Well, such a system has indeed been developed and in this month’s column, a team of Australian and U.S. researchers describes how they integrated the ground-based system together with GPS and INS to create a hybrid system that provides precise and accurate position information continuously in a variety of environments where GPS alone comes up short.
“Innovation” features discussions about advances in GPS technology, its applications, and the fundamentals of GPS positioning. The column is coordinated by Richard Langley, Department of Geodesy and Geomatics Engineering, University of New Brunswick.
The determination of the position and orientation (or “pointing direction”) of a device (or platform to which it is attached), to high accuracy, in all outdoor environments, using reliable and cost-effective technologies is something of a “holy grail” quest for navigation researchers and engineers.
However, ongoing research has identified two classes of applications that place stringent demands on the positioning/orientation device: (a) man-portable mapping and imaging systems that operate in a range of difficult urban and rural environments, often used for the detection of underground utility assets (such as pipelines, cables, conduits), unexploded ordnances and buried objects, and (b) the guidance/control of construction or mining equipment in environments where good “sky view” is not guaranteed.
The solution to this positioning/orientation problem is increasingly seen as being based on an integration of several technologies: satellite (GNSS including GPS) and terrestrial ranging systems, inertial navigation systems (INSs), laser guidance/scanning systems, and even electro-optical devices such as surveyors’ total stations or laser scanners. Each has its shortcomings, but within an integrated system, advantage can be taken of the complementary characteristics of several of these sensor technologies.
Centimeter-level accuracy positioning systems for outdoor use typically have at their core the GPS technology. GPS is, in fact, the most effective general-purpose navigation tool ever developed because of its ability to address a wide variety of applications: air, sea, land, and space navigation; precise timing; geodesy; surveying and mapping; machine guidance/control; military and emergency services operations; hiking and other leisure activities; personal location; and location-based services. The varied applications use different and appropriate receiver instrumentation, operational procedures, and data processing techniques. But all require signal availability from a minimum of four GPS satellites for three-dimensional fixes.
However, one of the usual limiting factors in using GPS is the need for direct line-of-sight between the satellites and the ground receiver. In particular, the robustness of positioning is compromised when GPS receivers are near or under trees, in urban/suburban areas, or in deep open-pit mines and construction sites, where there is partial sky view obstruction by buildings or walls. The traditional means of overcoming the gaps in navigation coverage due to satellite signal blockages is to use an INS. An INS (with its inertial measurement unit or IMU) is also the most convenient means of determining the orientation of the device or platform. The integration of GPS and INS can, in principle, overcome the defects of standalone INS (sensor errors that grow unbounded with time) and GPS (signal availability requirement). But navigation accuracy degrades rapidly if there are no GPS measurements to calibrate the INS sensor errors.
A new terrestrial RF-based distance measurement technology offers promise of continuous signal coverage, even in difficult urban/rural environments. This technology is known as “Locata.”
The Locata approach is to deploy a network of ground-based transceivers that cover an area with strong time-synchronized ranging signals. When a Locata receiver uses four or more ranging signals it can compute a high-accuracy position entirely independent of GPS or INS. However, a standalone Locata receiver has its own shortcomings: (a) in some situations it may be difficult to achieve good vertical dilution of precision due to logistical constraints of placing transmitters (to give a variation in elevation angle between the terrestrial transmitters and the receiver whose positions are to be determined), and (b) as with GPS, multiple receivers/antennas are required to derive orientation information.
What is therefore required is several carefully selected navigation sensor technologies, integrated within a single hardware package, the measurements from which are simultaneously processed to provide continuous, reliable, and accurate navigation solutions (that is, both position and orientation information).
In cooperation with Locata Corporation, the SNAP Laboratory within the School of Surveying and Spatial Information Systems at the University of New South Wales (UNSW) and the SPIN Laboratory at The Ohio State University have assembled a working prototype of a hybrid system based on GPS, inertial navigation, and Locata receiver technology to provide seamless and reliable navigation aimed at supporting vehicle guidance and control, open-pit mining, mobile and GIS mapping, and industrial applications.
Locata Technology
The SNAP Lab has been conducting pseudolite research for many years, and has experimented with pseudolites in nonsynchronous and synchronized modes for a variety of applications, using both the GPS L1 frequency as well as the 2.4 GHz ISM band frequencies. Locata Corporation has developed state-of-the-art RF terrestrial positioning technology (“Locata”), which consists of a network (“LocataNet”) of time-synchronized pseudolite-like transceivers (“LocataLites”). UNSW has assisted in the development of the technology through experimental testing and benchmarking. In a relatively open outdoor environment, the LocataNet can provide real-time stand-alone kinematic positioning (without a base station) at centimeter-level accuracy. Even in an indoor environment where LocataLite signals arrive at a Locata receiver via non-line-of-sight paths (penetrating the walls of buildings), the static positioning quality can be at the sub-centimeter level, and also at the sub-meter level for kinematic positioning.
Locata has several advanced features that have been developed over a period of about 10 years through several technology generations, including a time-synchronized positioning network, network propagation to many LocataLites, improved signal penetration, change of transmitting frequency and signal structure, and spatial and frequency diversity.
In TABLE 1, the key characteristics of the two generations of Locata technology are listed. Using 2.4 GHz not only means the frequency is license-free, but also permits transceiver output power of up to 1 watt, which means greater operating distances (up to 10 kilometers). Using dual-frequency signals changes the initial phase-bias resolution from known-point initialization to on-the-fly (OTF), where the initial phase bias is resolved while the receiver is moving. The higher chipping rate (10 MHz) results in less pseudorange multipath error, because the delay in a reflected signal will rarely be more than two chips. The 10-Hz measurement rate allows relatively high velocities of the receiver.
Table 1. Specification summary of Locata’s first- and second- generation systems.
In terrestrial-based RF-based positioning, multipath error is more severe than with GPS, because the terrestrially transmitted signal arrives at the receiver at a very low (typically less than 10 degrees) or even a negative elevation angle, which can result in severe multipath signal fading. In the second-generation Locata system, spatial and frequency diversity techniques are employed. Spatial and frequency diversity are two of the three types of diversity principles (the other being polarization) that are common practices in terrestrial RF communications to mitigate against signal fading. The LocataLite transceiver uses two spatially separated (usually in the vertical) antennas, which transmit two signals at different frequencies. This gives a cluster of four diverse signals transmitted from one LocataLite. With this diversity technology, Locata kinematic positioning in moderately obstructed environments can provide centimeter-level quality with 100-percent coverage, as well as consistent geometry and high reliability. The Locata’s multipath mitigation technology is very important and relevant to this project, because the operational environments are often vegetated or wooded.
Triple Integration
As discussed in the preceding sections, there are both advantages and disadvantages to every navigation sensor. GPS and Locata have high positioning accuracy in open or moderately obstructed environments, but they are sensitive to signal blockage such as the case in dense forests, urban canyons, deep mine pits, and indoors. In contrast, INS is totally autonomous — that is, independent of external signal sources — and has high output rate for position, velocity, and attitude, but its unaided navigation error grows rapidly with time.
The most common data-processing tool to integrate GPS and INS is the Kalman filter, which forms the basis for multi-sensor integration in this research. The basic Kalman filter applies to linear system models. Therefore, several variations were developed to cope with the non-linear navigation model, such as the extended Kalman filter and the unscented Kalman filter.
The following discussion of the integration of the GPS/INS/Locata sensors is focused on two aspects: 1) the system state selection, and 2) the measurement model or integration model that decides which information to pass to the filter.
The error state vector consists of a nine-dimensional navigation error state sub-vector (three for the position, three for the velocity, and three for the orientation), an accelerometer error state sub-vector, a gyroscope error state sub-vector, and a three-dimensional gravity disturbance state sub-vector. Of course, other sensor error models can be considered for the gyroscope and accelerometer sensors, such as a combination of random constants, first-order Gauss-Markov variables, scale factors, and so on. In this case, the state space could have a dimension of more than 30. The objective is to adjust the sensor error model later based on experimental results (if needed). However, because of the limitations of observability, it is not yet known whether an augmented error state vector would give better results.
When integrating INS hardware with other sensors, the sensors cannot share the same physical location, which would be ideal from a theoretical point of view. Knowing the spatial relationship among the sensors is important to ensure the highest possible navigation performance. The displacement vectors or mounting biases are offsets, also referred to as lever arms, from the center of the IMU to the centers of the other sensors. These lever-arm parameters may be included in the Kalman filter and thus can be estimated. However, if the lever arms are precisely measured during the assembly of the system, they do not need to be included in the filter as estimable parameters.
For multiple sensor integration in a Kalman filter, there are essentially two types of general models: loosely coupled and tightly coupled. The loosely-coupled model uses a decentralized filter that has several sub-filters to process the sub-systems independently. In other words, the Kalman filter solutions from the sub-systems are combined in an overall Kalman filter that provides the integrated navigation solution. In contrast, the tightly-coupled model uses a single main filter to process the output of all sensors. In GPS/INS integration, tightly-coupled systems have obvious advantages in environments where GPS signals are frequently lost, because they can rely on the other sensor(s) when GPS positioning becomes impossible.
In the tightly-coupled model, the raw observations of all sensors will be input to the main filter. For GPS and Locata, the primary observations will be the carrier phase measurements, as code (pseudorange) observations cannot provide the required accuracy. High-accuracy GPS positioning needs to address the issue of carrier-phase ambiguity. The ambiguity can be treated as an unknown in the Kalman filter, but it may take several minutes to resolve the ambiguity using GPS alone. Using certain ambiguity resolution techniques, however, the ambiguity can be resolved outside the main filter in the GPS/INS high-precision (carrier-phase) integration filter. Note that if the ambiguity were to be resolved within the filter, this would increase the number of states of the filter. For the GPS component, ionospheric delay should be included for applications that cover a large area. Ionospheric delay can be resolved using network-based differential techniques,
but it will affect the ambiguity resolution for single baseline differential positioning if it is not included in the local solution. The filter is designed either to use, or not to use, ionospheric delay, which can ensure flexibility to accommodate network-based and single-baseline differential positioning.
As mentioned above, the measurement model in the tightly-coupled model is based on the raw observations. For GPS and Locata, the observations will be the carrier-phase observations. The approximate values for the linearization of the GPS and Locata measurement equations are provided by the INS navigation solution.
The GPS carrier-phase ambiguity is solved independently outside the Kalman filter with OTF techniques. The GPS differential positioning coefficient matrix remains the same regardless of whether or not a network-based differential technique is used. For velocity determination, the double-differenced Doppler observation is used to eliminate the clock error rate as an unknown (because it is difficult to model this in the filter). The initial carrier-phase bias of the Locata is also not included in the filter, because it can be resolved instantaneously with dual-frequency data in the Locata second-generation system.
The implementation of the filter will be flexible, so adjustments can be made to account for actual environmental conditions. The filter is designed with an open interface and is modular in structure, so that components can be added (or removed) from the model. In open-sky areas, GPS is sufficient for system positioning, so only its observations need to be processed. In moderately obstructed environments, GPS and Locata observations will be processed. In this case the number of GPS observation equations is limited and sometimes will be less than four. FIGURE 1 illustrates the flowchart of the triple-integration of GPS, INS, and Locata.
Figure 1. Workflow of the integrated GPS/ INS/Locata system.
Field Tests
For experimental purposes, we used a dual INS, based on a navigation grade unit and a tactical grade unit. In addition, a Locata receiver and a dual-frequency GPS receiver were placed on a vehicle at Locata’s Numeralla Test Facility (NTF) near Canberra, Australia. This test site features both open-sky and obscured environments, allowing for testing the system’s performance under truly challenging scenarios. The test was repeated by mounting the devices on an autonomous electrical car, driven on the UNSW campus. In both cases, the separation between the rover and the terrestrial transmitters was between a few tens of meters to several kilometers. The GPS and Locata data were processed separately (for testing the internal consistency) as well in a hybrid solution, resulting in few-centimeter-level accuracy per coordinate, depending primarily on GPS availability and the geometry between the rover and Locata devices, as well as the level of multipath fading.
Test 1: NTF. The first integration test was conducted at the NTF on March 17, 2008. The NTF covers an area of approximately three hundred acres (2.5 kilometers × 0.6 kilometers) and is ideally suited to real-world system testing over a wide area. At the NTF, a number of LocataNet configurations are possible through the installation of permanent antenna towers. The network configuration used for this experiment is illustrated in FIGURE 2.
Figure 2. NTF: LocataLite network.
Before the test, a special mounting platform was designed and built. The platform, shown in FIGURE 3, consists of a two-level metal frame. The bottom level can accommodate two inertial measurement units, while the top level can hold up to four antennas. The platform can be easily attached to either the roof of the NTF test vehicle or to the body of UNSW’s small electric car (described later).
Figure 3. Devices setup for the NTF test.
The devices used in the test include two dual-frequency GPS receivers (one used as the rover receiver and the other as the base station), one navigation grade INS, and one Locata rover unit. The GPS antenna and the Locata antenna were mounted with the INS together on the top of a truck. The GPS data rates were set to 1 Hz. The average length of the GPS differential baselines was about 1.2 kilometers. The GPS observation conditions were good during the testing period. The Locata data rate was set to 10 Hz, while INS data rate was 256 Hz, and both were synchronized with the GPS time using SNAP-Lab-developed time synchronization devices based on field-programmable gate array (FPGA) technology.
The GPS/INS data were first processed in tightly-coupled mode. The trajectory is depicted in FIGURE 4. The standard deviation of position, velocity, and attitude are shown in FIGURES 5-7 respectively.
Figure 4. The trajectory of the vehicle in the NTF testFigure 5. The standard deviation of position in the test.Figure 6. The standard deviation of velocity in the test.Figure 7. The standard deviation of attitude in the test.
In Figures 5-7, it can be seen that the standard deviations of position and velocity are less than 0.02 meters and 0.01 meters per second respectively. The standard deviations of pitch and roll angles are less than 0.001 degrees as well as that of yaw, which is less than 0.01 degrees after the vehicle starts to move, at about the 1500th second.
The Locata data was post-processed using Locata’s Integrated Navigation Engine (LINE). It provides an unsmoothed single point position using carrier-phase measurements. The initial ambiguity bias was resolved using the data from the GPS carrier-phase position. Following this initialization, the Locata solution was computed independently of GPS. A 15-meter tower LocataLite location in the vicinity of the start and end of the test (indicated by the “figure eight” pattern in FIGURE 8) allowed sufficient geometry for 3D positioning using Locata. For the rest of the data where there was insufficient vertical geometry, GPS height aiding was used. Figures 8 and 9 show the independent Locata and GPS solutions (without lever arm correction) for the section of the trajectory in the vicinity and the end of the test, respectively. The Locata solution compared to the GPS solution to within a few centimeters for the entire trajectory.
Figure 8. Section of trajectory showing independent Locata solution (black) vs. GPS (blue) with no lever-arm correction.Figure 9. End of trajectory showing independent Locata solution (black) vs. GPS (blue) with no lever-arm correction.
To test the GPS/INS/Locata integration, some GPS observation epochs were deleted to simulate two GPS blockages from seconds of week 94100 to 94250 and from 94500 to 94600. The INS standalone navigation errors with this deleted GPS data were about 8 meters and 2.6 meters, respectively.
In the final GPS/INS/Locata integration test, Locata compensated for the missing GPS data. The integration result was almost identical to the GPS/INS integration result obtained with the original GPS observed data clearly showing that the Locata system could seamlessly replace GPS in this scenario.
Test 2: Electric Car. Early in 2007, UNSW researchers established a permanent LocataNet on the university campus to provide a research and test facility at UNSW devoted to the Locata technology. The LocataNet setup at UNSW is illustrated in FIGURE 10. It consists of four dual-frequency LocataLites situated on tops of four buildings surrounding a lawn test area. The master LocataLite is on the Civil Engineering building and the other three LocataLites are synchronized to it.
Figure 10. LocataLites on the UNSW campus.
Currently, to be able to obtain a carrier-phase position solution with Locata, the initial ambiguities need to be resolved by initializing the rover receiver on a known position. For this purpose, a point in the middle of the test area was surveyed, and the coordinates were used to initialize the Locata receiver.
SNAP Lab has developed a small electric car that can be driven using an attached handheld controller (see FIGURE 11). The controller enables the car to move in both forward and reverse and to steer the front wheels.
Figure 11. The electronic car used in the test.
For these tests, the same mounting platform as the one used in the previous experiment allowed all the sensors and ancillary equipment to be attached to the car. For this experiment, we used the following equipment: a Locata receiver, two GPS receivers, a tactical grade INS, a 360-degree prism (tracked by a robotic total station), and two time-sync FPGA data-logging devices.
The starting position was the known point in the middle of the Locata network. The car was then driven in a circular path three times before finishing back at the starting position.
During the test the raw data stream from the Locata receiver, the GPS receivers, and the INS were recorded using the time-sync data-logging devices. In addition, a robotic total station (RTS), which was set up at the edge of the test area, automatically tracked the prism position (the data was recorded internally).
The Locata data was post-processed using LINE to give a single point unsmoothed carrier-phase solution. The initial ambiguity bias was resolved using the data from the GPS carrier-phase position. Following this initialization, the Locata solution was computed independently of GPS. Where there was insufficient vertical geometry (at the very west end of the trajectory shown in FIGURE 12), GPS height aiding was used. The Locata-only solution and the RTS result are shown in Figure 12. The two solutions compare to within a few centimeters of each other.
Figure 12. The trajectory from the Locata-only and robotic total station solutions.
We then carried out the integrated GPS/INS processing. To test the GPS/INS/Locata integration, two GPS outages were simulated by simply removing the data from the GPS file, between seconds of week 103703 and 103713 and 103834 and 103844, respectively.
We then carried out the integrated GPS/INS processing. To test the GPS/INS/Locata integration, two GPS outages were simulated by simply removing the data from the GPS file, between seconds of week 103703 and 103713 and 103834 and 103844, respectively.
In comparison to the original GPS/INS integration, the standalone INS solution has errors of about 35 meters and 12 meters during the first and second outages, respectively.
The Locata/INS integration significantly reduced the navigation error during the GPS outages, as summarized in TABLE 2.
Table 2. The difference between the Locata/INS solution and the original GPS/ INS solution
From Table 2 it can be seen that 3D position differences between the Locata/INS and the original GPS/INS integration result have been reduced to 1.143 meters and 0.053 meters during the two GPS outages, respectively. However, the improvement in the accuracy of the attitude angles is not obvious because a 10-second GPS outage is not long enough to cause a significant INS drift.
Concluding Remarks
The test experiments described here are a demonstration of the proof-of-concept of a triple-integration GPS/INS/Locata system. The navigation results indicate that this sensor combination may support navigation in GPS-denied environments, as long as some partial view of the LocataLites within the network is available. Further development of this triple integration system is being undertaken.
Acknowledgments
The research is funded by the Australian Research Council. This article is based on the paper “A Hybrid System for Navigation in GPS-challenged Environments: A Case Study,” presented at ION GNSS 2008, the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation, Savannah, Georgia, September 16-19, 2008.
Manufacturers
The Numerella test equipment included Locata devices, a Honeywell H-764G navigation-grade INS, a Boeing (now Systron Donner) C-MIGITS II tactical grade INS, and a Leica System 1200 dual-frequency GPS receiver. The UNSW campus test equipment included Locata devices, an Omnistar GPS receiver, a Leica MC500 GPS receiver, a Boeing C-MIGITS II INS, a Leica GRZ4 360-degree prism, and a Leica robotic total station TCRP 1203+.
CHRIS RIZOS is a graduate of the University of New South Wales (UNSW), Sydney, Australia, where he obtained a Ph.D. in satellite geodesy. He is head of the School of Surveying and Spatial Information Systems at UNSW.
DOROTA BRZEZINSKA is a professor and leader of the Satellite Positioning and Inertial Navigation (SPIN) Laboratory at The Ohio State University (OSU) in Columbus, Ohio. She received her M.S. and Ph.D. in geodetic science from OSU.
CHARLES TOTH is a senior research scientist at OSU’s Center for Mapping. He received a Ph.D. in electrical engineering and geo-information sciences from the Technical University of Budapest, Hungary.
ANDREW G. DEMPSTER is the director of research in the School of Surveying and Spatial Information Systems at UNSW.
YONG LI is a senior research fellow at the SNAP Lab. He obtained a Ph.D. in aerospace engineering.
NONIE POLITI is a graduate of the School of Electrical Engineering and Telecommunications at UNSW. He obtained a Bachelor’s degree in Telecommunication Engineering and an M.Eng.Sc. in electronics.
JOEL BARNES is director of navigation R&D for Locata Corporation and is also a senior visiting research fellow at the SNAP Lab.
HONGXING SUN is a post-doctoral researcher in the SPIN Lab. He received a bachelor’s degree in geodesy and M.S. and Ph.D. degrees in photogrammetry from Wuhan University, China.
LEILEI LI is a Ph.D. candidate at Chongqing University, China. He is also a visiting Ph.D. student in the SPIN Lab. He received an M.S. degree in instrument science and technology from Chongqing University.
FURTHER READING
• Locata
“Locata: A New Technology for High Precision Positioning” by N. Politi, Y. Li, F. Khan, M. Choudhury, J. Bertsch, J.W. Cheong, A. Dempster, and C. Rizos in Proceedings of ENC-GNSS 2009, the European Navigation Conference, Naples, Italy, May 3-6, 2009.
“Deploying a Locata Network to Enable Precise Positioning in Urban Canyons” by J.-P. Montillet, G.W. Roberts, C. Hancock, X. Meng, O. Ogundipe, and J. Barnes in Journal of Geodesy, Vol. 83, 2009, pp. 91–103 (doi: 10.1007/s00190-008-0236-7).
“High Accuracy Positioning Using Locata’s Next Generation Technology” by J. Barnes, C. Rizos, M. Kanli, A. Pahwa, D. Small, G. Voigt, N. Gambale, and J. Lamance in Proceedings of ION GNSS 2005, the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation, Long Beach, California, September 13–16, 2005, pp. 2049–2056.
“A Positioning Technology for Classically Difficult GNSS Environments from Locata” by J. Barnes, C. Rizos, M. Kanli, and A. Pahwa in Proceedings of IEEE/ION PLANS 2006, the Position, Location, and Navigation Symposium, San Diego, California, April 25–27, 2006, pp. 715–721.
• Integrated Positioning
“Seamless Navigation Through GPS Outages – A Low-cost GPS/INS Solution” by Y. Li, P. Mumford, and C. Rizos in Inside GNSS, Vol. 3, No. 5, July/August 2008, pp. 39–45.
“Ubiquitous Positioning: Anyone, Anything: Anytime, Anywhere” by X. Meng, A. Dodson, T. Moore, and G. Roberts in GPS World, Vol. 18, No. 6, June 2007, pp. 60–65.
“Photogrammetry for Mobile Mapping: Bridging Degraded GPS/INS Performance in Urban Centers” by T. Hassan, C. Ellum, S. Nassar, W. Cheng, and N. El-Sheimy in GPS World, Vol. 18, No. 3, March 2007, pp. 44–48.
“Development of a GPS/INS Integrated System on the Field Programmable Gate Array Platform” by Y. Li, P. Mumford, J. Wang, and C. Rizos in Proceedings of ION GNSS 2006, the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation, Fort Worth, Texas, September 26–30, 2006, pp. 2222–2231.
“An Integrated Positioning System: GPS + INS + Pseudolites” by Y. Yi, D. Grejner-Brzezinska, C. Toth, J. Wang, and C. Rizos in GPS World, Vol. 14, No. 7, July 2003, pp. 42–49.
• Kalman Filtering for Integrated Systems
“Tightly-coupled GPS/INS Integration Using Unscented Kalman Filter and Particle Filter” by Y. Yi and D.A. Grejner-Brzezinska in Proceedings of ION GNSS 2006, the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation, Fort Worth, Texas, September 26–30, 2006, pp. 2182–2191.
“Low-cost Tightly Coupled GPS/INS Integration Based on a Nonlinear Kalman Filtering Design” by Y. Li, J. Wang, C. Rizos, P. Mumford, and W. Ding in Proceedings of NTM 2006, the National Technical Meeting of The Institute of Navigation, Monterey, California, January 18–20, 2006, pp. 958–966.
• Data Time Synchronization
“A Time-synchronisation Device for Tightly Coupled GPS/INS Integration” by P. Mumford, Y. Li, J. Wang, C. Rizos, and W. Ding in Proceedings of IGNSS Symposium 2006, International Global Navigation Satellite Systems Society, Gold Coast, Australia, July 17–21, 2006.
Not “fly by,” but “apply.” As in W. Somerset Maugham’s advice to aspiring young writers: Apply the seat of the pants to the seat of the chair.
Get grounded. Confront the blank page, the typewriter, or the less preferable modern equipments (because instant electronics short-circuit orderly brain function) for a period of silence and contemplation. Above all, think. Then, and only then, communicate.
Some serious word-eating now ensues. In last month’s editorial, I faulted the Architecture Evolution Plan (AEP) GPS ground control software update 5.5C for wreaking havoc with fielded receivers, both military and civil. Website news stories that I subsequently posted bore headlines driving this misconception home.
I was wrong. Subsequent analysis by more knowledgeable and expert people has established that, yes, havoc was wreaked, but not by the software update. The deficiency resided in the factory-installed software of the respective receivers. See The System story on page 12 for details.
In my only defense, I state that I was groping in the dark. Very incomplete word, secondhand no less, arrived just as we were preparing the issue for press. No official announcement came from the Air Force during the days that followed. We understand that internal brouhaha was brewed, stern words were uttered and written, but ultimately it was determined that the fault lay in the user segment, not with ground control or vendors thereto.
We have it on good authority that the GPS Wing maintains, and will presumably continue to maintain, that the AEP update was absolutely perfect with no software glitches. But the Wing also realizes that none of the receiver manufacturers had any idea how the update was going to be implemented, as in what pages would be affected, and if there would be new data in places where it had not been loaded before.
A grudging admission emerges that there may be some ambiguity in the receiver interface control documents (ICDs), and that those may need some tightening up sometime soon.
We return to the theme of communications, in this case clear ones between the GPS Wing and the various user communities: civil, commercial, and military. For GPS to maintain its place as the world’s gold standard, there must be clear — and timely — communication between the Wing and its customers. All customers. Dual-use.
So far, there’s no No. 2 system trying harder in this regard. But why leave the door open?
A controversial suggestion: beta versions of future AEP updates could be released to a predefined subset of receiver maufacturers, who would test and report back concerning any glitches that occurred when their receivers saw the new software and simulated nav message for the first time.
After reviewing current performance of WAAS, EGNOS, and MSAS, the authors present expected future performance, including the benefits of GPS L5. They evaluate the impact of the Indian GAGAN and Russian SDCM systems on global coverage and examine southward expansions for the original three SBASs. Finally, a look at the impact of a second constellation of navigation satellites and the performance for a user taking advantage of two core constellations.
By Todd Walter, Juan Blanch, and Per Enge, Stanford University
The Wide Area Augmentation System (WAAS) monitors GPS and provides both differential corrections to improve accuracy and associated confidence bounds to assure integrity. The first satellite-based augmentation system (SBAS), it was commissioned for service in 2003. Japan’s MTSAT-based Satellite Augmentation System (MSAS) was commissioned in 2007, and the European Geostationary Navigation Overlay Service (EGNOS) was declared operational in 2009, with safety-of-life service commissioning expected in mid-2010. Two other SBASs are in the developmental stage: India’s GPS Aided Geo Augmented Navigation (GAGAN) and Russia’s System for Differential Corrections and Monitoring (SDCM) have fielded equipment and plan to become operational in the next few years.
Coming improvements will expand SBAS coverage areas and strengthen their performance. In the near term, these include more monitoring stations and algorithmic enhancements, with incorporation of a second civil signal in a protected aeronautical band and new GNSS constellations in the long term.
An SBAS utilizes a network of precisely surveyed reference receivers, located throughout its coverage region. The information gathered from these reference stations monitors the GNSS satellites and their propagation environment in real time. Availability of SBAS service is a function of two quantities: the arrangement of the pseudorange measurements used to determine the user’s position, referred to as geometry; and the quality of each individual measurement, referred to as the confidence bound. Although very small confidence bounds can make up for poor geometries, and strong geometries can overcome large confidence bounds, both values are generally required to be good to obtain high availability.
Geometry is determined purely by the locations of the ranging satellites relative to the user. Currently the basic geometry is provided by the GPS constellation. Historically it has exceeded commitments, and there are currently 29 healthy satellites in orbit when only 21 are nominally guaranteed. However, as satellites are taken off-line in critical orbital slots, the quality of the geometry can degrade significantly. There could be short duration losses of service daily at some locations. Since the goal is to provide service more than 99.9 percent of the time, these outages can have a dramatic impact. WAAS currently mitigates this concern by adding geostationary satellites with a ranging function virtually identical to the GPS satellites. These satellites are always in view and improve the overall geometry, although they do not eliminate the problem completely.
The confidence bounds relate to the expected error sources on the range measurements. Currently three error sources are corrected via broadcast to the user: satellite clock error, satellite ephemeris error, and delay error due to propagation through the ionosphere. These error sources are described by two confidence bound terms: the user differential range error (UDRE) for the satellite errors, and the grid ionospheric vertical error (GIVE) for the ionospheric errors.
For single-frequency SBAS, this last error source is the most significant. Users may sample the ionosphere anywhere in the service volume, but the SBAS only has measurements from its reference station locations. Thus, there is always the possibility of undetected ionospheric disturbances. This leads to larger confidence bounding terms and lower availability.
The combination of geometry and confidence bounds yields the protection levels (PL). PLs are the real-time confidence bound on the user’s position error. To match aviation requirements these are broken into a vertical protection level (VPL) and a horizontal protection level (HPL). Each SBAS guarantees that the user’s actual position error will be smaller than these values 99.99999 percent of the time. The PLs are calculated in real-time using stored and broadcast information. They must be compared to the maximum allowed value for a desired operation. The upper bounds are called alert limits (AL) and they are fixed numbers whose values depend on the operation.
In this article we are interested in the localizer performance with vertical guidance (LPV)-200 approach with a VAL of 35 meters and HAL of 40 meters. Currently, LPV aviation approaches can only be accomplished with a WAAS GPS receiver. Performance of an LPV approach allows minimums as low as 200 feet above ground level before a missed approach must be executed. As of January 2010, there were 1,930 published WAAS LPVs, with plans to add 300 per year in the United States.
Because GPS and SBAS generally perform better at horizontal positioning than vertical, the requirement that the VPL be below the VAL is nearly always the limiting constraint for these operations.
Methodology
To determine the global availability and the effect of potential improvements, we used our Matlab Algorithm Availability Simulation Tool (MAAST). This tool uses almanac data to calculate the position of the satellites for each specified epoch. The almanac chosen for this study corresponds to the GPS almanac broadcast on April 8, 2009, when there were 30 healthy satellites. However, PRNs 25 and 32 were removed to simulate a condition with 28 healthy satellites. MAAST also implements the WAAS integrity algorithms to calculate the corresponding UDRE and GIVE values. Finally, it uses these values to implement the airborne algorithms specified in the minimum operational performance standards (MOPS) for SBAS. The MOPS specifies user algorithms for determining the protection levels. For these simulations, the VPL and HPL are calculated about every 5 minutes and every two and a half degrees across the globe.
MAAST does a good job of predicting WAAS behavior. It is less accurate when predicting other systems’ performance. EGNOS has developed its own monitoring receivers and integrity algorithms and has different criteria for assigning a satellite a particular UDRE value and assigning each ionospheric grid point’s (IGP’s) GIVE value. Nevertheless, both systems are designed to meet ICAO requirements for integrity, and their performance should be somewhat similar. In observing EGNOS coverage plots and comparing them to MAAST predictions, we do see differences. However, the size of the coverage region and approximate boundaries are reasonably close and provide an idea of performance if not an exact map.
The MSAS algorithms are based upon the same algorithms used in earlier versions of WAAS. Therefore, MAAST should be slightly more accurate in modeling its performance. GAGAN uses the same prime contractor as WAAS and therefore similar algorithms may be expected. Less is known about the intended SDCM algorithms and therefore the modeling of this system faces the largest uncertainty. Again, the MAAST predictions should be viewed as indicative rather than precise. Individual availability maps will not be completely correct, but relative performance improvements should be properly indicated.
Current Systems Status
Currently WAAS is in its full LPV-200 performance (FLP) phase. It consists of 20 WAAS reference stations (WRS) in the conterminous United States (CONUS), in addition to seven in Alaska, one in Hawaii, one in Puerto Rico, four in Canada, and five in Mexico for a total of 38. The station locations are shown as blue circles in Figure 1. There are three WAAS master stations (WMS) and two geostationary satellites (GEOs). The GEOs are the Intelsat Galaxy XV satellite
at 1338 W and the Telesat ANIK F1R satellite at 1078 W.
FIGURE 1. Existing SBAS reference networks, consisting of 38 reference stations for WAAS, 34 for EGNOS, and 8 for MSAS.FIGURE 2. Simulation results from MAAST for availability of LPV-200 provided by current systems.
As can be seen in Figure 2, availability of LPV-200 service is very high for most of North America. In general, this performance meets the goals for the system. However, in some regions performance is lower than the 99 percent minimum target. The West Coast, Alaska, and Southern Mexico all suffer from reduced availability.
MSAS is in its initial operating phase. It consists of six ground monitoring stations (GMSs) on the Japanese Islands, one in Australia, and one in Hawaii (magenta triangles in Figure 1). There are two master control stations (MCSs) and two Multifunction Transport Satellite (MTSAT) geostationary satellites at 1408 E and 1458 E.
Because of the limited network size, the GEO UDREs for MSAS are set to 50 meters and therefore do not benefit vertical guidance. Further, the limited ionospheric observations offer little availability of LPV-200 service as can be seen in Figure 2. As a result, vertically guided operations have not yet been authorized based upon MSAS. The Japanese Civil Aviation Bureau (JCAB) has studied performance improvements that could allow it to provide LPV-200 operations. Until then, MSAS provides only lateral navigation.
EGNOS is also in its initial operations phase. It consists of 28 ranging and integrity monitoring stations (RIMS) in Europe, one in Turkey, three in Africa, one in North America, and one in South America (green squares in Figure 1). There are four master control centers (MCCs) and two GEOs, the INMARSAT Atlantic Ocean Region-East (AOR-E) satellite at 15.58 W and the ARTEMIS satellite at 21.58 E.
For a variety of reasons, EGNOS has chosen to implement its GEO satellites without a ranging capability. Thus, for our simulations we have set them as data-links only and do not model a ranging capability. EGNOS also currently implements Message Type 27 (MT-27) rather than Message Type 28 (MT-28) as do WAAS and MSAS. MT-27 restricts the use of low UDRE values to a box centered on the European region. Its borders can be discerned in Figure 2. Currently it has little impact on LPV-200 service, but if EGNOS is to expand its coverage, it may become a limiting factor. Availability of LPV-200 service is very high for most of Europe. However, there is a desire to expand coverage to more reliably cover Iceland, Scandinavia, Eastern Europe, and the Mediterranean and South Atlantic regions.
Near-Term Improvements
EGNOS is fielding additional reference stations in the Canary Islands, Northern Africa, and the Middle East. In the longer term, MT-28 is being considered as a replacement for MT-27. In our modeling we added seven new RIMS, shown in Figure 3, and implemented MT-28. We also improved the ionospheric mask by including additional IGPs. We did not update GEO locations nor did we model ranging capability that could further enhance performance. By comparing
FIGURE 3. Improved SBAS networks. The newly added reference stations are marked by yellow filled squares for EGNOS and yellow filled triangles for MSAS.
Figure 4 to Figure 2 improvements can be seen, in particular expanded LPV-200 operation to the south.
FIGURE 4. Improved single frequency SBAS coverage for the original three SBAS.
The future of MSAS improvements is less certain, with no firm commitments for major service enhancements. We have chosen to model fairly aggressive enhancements based upon studies made by the Electronic Navigation Research Institute in Japan. We have added 10 new reference stations in Japan and made the ionospheric threat model less conservative, in line with current WAAS algorithms. Together, these improvements offer good vertical guidance coverage over Japan.
These improvements extend coverage in the vicinity of the reference station networks, but are unable to push availability much beyond. This is primarily due to the limitations of the ionospheric corrections. Because strong gradients can exist outside of the viewing area of the network, tight confidences cannot be provided to those regions.
SBASs model the ionosphere as a thin 2-dimensional shell 350 kilometers above Earth. This works well for quiet mid-latitude and polar ionosphere. However, equatorial ionosphere often has significant vertical structure that is not well replicated by the SBAS message. The resulting confidence bounds are then too large to reliably provide LPV-200 capability. No certified algorithm capable of bounding the equatorial ionosphere is known to the authors. Instead, it is recommended that SBASs in equatorial areas wait for the forthcoming L5 signal to provide vertical guidance in their regions.
GPS L5
The next GPS satellite to be launched will contain a new civil signal, L5, centered at 1176.45 MHz and in a protected aviation band. As such, it will be approved for use on aircraft. When the L5 signal is used in combination with L1, the ionospheric delay for each line-of-sight can be directly estimated. This will dramatically lower the uncertainty of the pseudorange measurement. Thus, if the SBAS is upgraded to provide corrections appropriate for an L1/L5 user and the user similarly upgrades his or her avionics, SBAS service can be dramatically improved.
Another important advantage of the second civil frequency is its relative immunity to ionospheric storms. Because the users are now directly eliminating the amount of delay they actually experience, they are no longer affected by shortcomings in the MOPS ionospheric model. The weaker effect of scintillation may have some impact; however, we do not expect to lose vertical guidance altogether. Furthermore, the availability of two civil frequencies offers protection against unintentional interference. If either L1 or L5 is jammed, the user still has access to guidance on the available frequency.
At the moment there is no MOPS for an L1/L5 user, so any ground or user algorithms will have to be speculative. We propose basing future L1/L5 algorithms on the existing L1-only algorithms. Instead of using L1-only pseudorange measurements, the user forms the ionosphere-free combination. For the confidence term representing the total pseudorange error on a line-of-sight, the ionospheric correction terms and airborne multipath terms are replaced with a single value representing airborne noise and multipath for the ionosphere-free combination.
For a single frequency user, each line-of-sight has four confidence terms that are summed together to obtain the total confidence. These terms correspond to: the satellite clock and ephemeris corrections (σflt), the ionospheric correction (σUIRE), the airborne code noise and multipath (σair), and the troposphere (σtrop). The total one-sigma confidence bound for a particular line-of-sight is the root sum square (RSS) of these four terms:
(1)
When a user has access to two civil frequencies, they can remove the ionospheric effects by forming the iono-free combination of the two pseudoranges:
(2)
where f1 and f5 are the L1 and L5 frequencies (1575.42 MHz and 1176.45 MHz) respectively. If σ1 and σ5 are comparable then the iono-free combination has roughly three times as much noise as either single frequency term, but is substantially smaller than σUIRE . Furthermore, satellites do not need a grid correction to be used, thus satellites farther from the network and IGP mask can be incorporated into the position solution. The dual-frequency confidence bound for a single satellite is then given by
(3)
where σair is used in place of σ1 and σ5 in (2).
For the VPL we propose adding nominal bias terms to handle observed signal biases and non-Gaussian behavior of the underlying error terms. By including these terms it is possible to reduce the net impact of these biases on the user. Further, we propose tailoring the VPL equation to the most significant remaining threat to the user: single satellite fault modes. The L1-only VPL equation is appropriate for threats that affect many signals simultaneously as may happen with the ionosphere or troposphere. However, with the user directly eliminating ionospheric effects, the most significant threats come from satellite fault modes. As these faults are rare, they are unlikely to affect more than one ranging measurement at a time. Therefore, a VPL can be constructed to explicitly account for such a threat. We recommend that the dual frequency VPL take the following form:
(4)
where KHMI and σ5 is the Gaussian tail factor corresponding to the probability of Hazardously Misleading Information, s3,i is the projection of the pseudorange error onto the vertical position estimate, sff is the fault free overbounding sigma, biasnomis the nominal bias bound, Kfault is the Gaussian tail factor accounting for the probability of fault, and biasfault is a bound on the magnitude of all satellite faults. The H0 condition corresponds to the most likely condition of no faults present. The H1 condition corresponds to the unlikely event of a fault on the dominant satellite. The final VPL is the maximum across both conditions.
Because the faulted bias term covers the satellite faults the fault-free sigma term, σff, can be much smaller than the current total value (1), or the dual frequency version (3). Further, since the probability of fault is small, Kfault can be much smaller than KHMI . The net result is that the proposed VPL is smaller than the existing VPL for the same conditions. To model L1/L5 availability we chose the following parameters:
KHMI = 5.33
Kfault = 2.33
σ 2ff = (σflt / 3 ) 2 + σ 2iono_free + σ 2trop
biasnom = 0.5 m
biasfault = 5.333 x σflt
Other values follow the single frequency MOPS specifications as normally implemented by MAAST.
Given these parameters, the H1 hypothesis nearly always dominates the VPL calculation. We have used a nominal weighting scheme to optimize for accuracy. It is possible to deweight the dominant satellite to improve availability. We will be looking at practical methods for determining more optimal weighting for the VPL given in (4). However, there is a concern that such optimizations could harm accuracy. The potential benefits vs. risks will be studied.
The improvement in performance for a dual-frequency user can be seen in Figure 5. The coverage is significantly expanded. Now each region is robustly covered with large margins surrounding their intended service regions. However, coverage is still limited to the areas around these first three SBASs.
FIGURE 5. Potential dual frequency coverage of the first three SBASs including network improvements.
GAGAN and SDCM
Two additional SBASs are currently under development that will extend coverage to more regions. India is developing GAGAN. Currently it has eight Indian reference stations (INRES) all in India (blue diamonds in Figure 6). There is one Indian master control center (INMCC), and plans to use the GSAT-4 as its initial GEO. The GSAT-4 is planned for launch in 2010 and will be located near 82° E. The geomagnetic equator passes through India and it therefore faces the full impact of equatorial ionosphere. The advent of L5 will allow GAGAN to obtain high LPV-200 availability that is unlikely to be achievable for single-frequency users.
FIGURE 6. The networks of five SBAS systems are shown. In addition to the reference stations from Figure 3, the 8 Indian stations are shown as blue diamonds and the 19 Russian stations are shown as red stars.
Russia is developing SDCM. It now has nine operational measuring points (MPs) and has plans for at least 10 more locations, all in Russia (red stars in Figure 6). There are also plans to use three GEOs: Luch-5a planned for launch in 2010 and to be located near 16° W, Luch-5b planned for launch in 2011 and to be located near 95° E, and Luch-4 planned for launch in 2013 and to be located near 167° E.
Figure 7 shows the combined dual-frequency coverage of all five systems, WAAS, EGNOS, MSAS, GAGAN, and SDCM.
FIGURE 7. The combined dual frequency availability of the five SBASs is shown.
The vast majority of land masses in the northern hemisphere are now well covered by at least one of the SBASs. Figures 6 and 7 clearly highlight that the majority of development has occurred in the northern hemisphere. In fact, only two reference stations have been placed below the Equator.
Southern Hemisphere
If SBAS is to provide a global solution, its coverage must extend into the southern hemisphere. There have been many discussions with representatives of countries in the southern hemisphere. Further, the United States has had testbed receivers in South America for nearly 15 years. Europe has fielded receivers in Africa. Australia investigated its own variant of SBAS called the Ground-based Regional Augmentation System (GRAS). However, we are not aware of concrete plans for development in this hemisphere.
We anticipate that discussions will eventually evolve into firm plans and that either independent SBASs will be developed in these regions or existing SBASs will expand their networks southward. We have chosen to assume that WAAS, EGNOS, and MSAS will expand their networks to extend LPV-200 coverage to the southern portion of their GEO footprints. This is but one of many possible scenarios. The pr
oposed expansion shown in Figure 8 is not based on any plans, but is based on the notion that civil aviation authorities will want to obtain global coverage. The assumed new southern reference stations are shown as yellow-filled circles for WAAS in South America, yellow-filled squares for EGNOS in southern Africa, and yellow-filled triangles for MSAS in and around Australia. Advantages of dual frequency allow us to have much less dense networks for the expansions, in addition to allowing LPV-200 capability to be obtained in equatorial areas.
FIGURE 8. The networks of the five SBAS systems including hypothetical expansions into the southern hemisphere
Figure 9 shows the combined dual-frequency coverage for these SBASs with the expanded network. Now nearly all land masses have good LPV-200 coverage. Note that we have not attempted to optimize these networks to assure coverage to all land masses, not have we tried to find the minimum number of stations that offer this capability.
FIGURE 9. The combined dual frequency availability of the SBASs with the southern hemisphere stations is shown.
Added Core Constellations
Galileo is envisioned as compatible with GPS in that each satellite provides ranging using signals covering the L1 and L5 frequencies with similar modulations. Although the final specifications are not yet set, it is envisioned that Galileo satellites will provide a service that is fully interoperable with the GPS civil signals. Thus, we can approximately model Galileo satellites as being equivalent to GPS satellites in different orbits. In parallel, China is developing the COMPASS system whose signals are also planned to be compatible with GPS.
The Russian GLONASS system has been operational for many years. However, its current signal structure makes it less suited for incorporation into avionics. There are modernization plans to broadcast L1 signals that are more in alignment with the other constellations. Thus it, too, may one day be incorporated into SBAS. We believe that SBASs will someday broadcast satellite clock and ephemeris corrections for GPS and one or more other core constellations. These corrections will remove any difference in the reference times or coordinate frames between the two systems, allowing the corrected signals to be considered fully interchangeable.
Adding 24 or more extra ranging sources will have tremendous benefit for all civil GNSS users. The user’s geometry would be very robust to the loss of one or two satellites. Adding one or more core constellations has the potential to significantly improve SBAS coverage. We chose to model the addition of one constellation, by combining the almanac we used for GPS with one that had been proposed for Galileo. For these scenarios, MAAST is modeling 55 medium earth orbiting navigation satellites in addition to the GEOS used by each SBAS. Because the orbital repeat period is approximately 10 sidereal days for Galileo, the simulated time step and total run time were each increased by a factor of ten.
Figure 10 shows the improved coverage when the reference stations shown in Figure 6 are used. The additional satellites fill in many potential coverage gaps and now, compared to Figure 7, the SBASs all have even more reliable coverage well beyond their reference networks. Indeed, the Northern Hemisphere is now essentially fully covered. Figure 11 shows the results when the expanded networks of Figure 8 are incorporated. Compared to Figure 9, the southern hemisphere is much more reliably covered. The remaining gaps could easily be filled in with just a few more reference stations if full global coverage were desired.
FIGURE 10. The combined dual-frequency, LPV-200 coverage of the five SBAS systems with both GPS and Galileo.FIGURE 11. Combined dual-frequency LPV-200 coverage, SBASs with GPS and Galileo and the southern hemisphere stations.
Conclusions
For single-frequency SBAS the coverage is limited to areas very close to the monitoring station network. However, each region can obtain very good LPV-200 coverage within their desired service area. The addition of GPS L5 makes vertical guidance largely immune to ionospheric disturbances, and permits SBAS coverage to extend into equatorial areas. Independence from the ionospheric grid also allows service to extend farther away from the core network regions. When new Indian and Russian systems are commissioned, a very large fraction of the northern hemisphere will have LPV-200 coverage.
With dual frequency, LPV-200 coverage can be established with comparatively sparse networks in South America, Africa, and around Australia. Additional dual-frequency core constellations such as Galileo, Compass, or GLONASS could greatly expand coverage to well outside the original reference network regions. As GNSS capability is improved and expanded, we anticipate that SBAS coverage may one day provide nearly global LPV-200 or better service capability.
Acknowledgments
The authors acknowledge support of the FAA Satellite Product Office. However, the opinions and potential future scenarios reflect those of the authors and are not necessarily representative of the FAA.
Todd Walter is a senior research engineer at Stanford University. He has been active in the development of the Wide Area Augmentation System and related systems around the globe. His focus is on the provision of certified integrity for aviation applications.
Juan Blanch is a research associate at Stanford University, where he works on integrity algorithms for GNSS. He holds a Ph.D. in aeronautics and astronautics from Stanford.
Per Enge is professor of aeronautics and astronautics at Stanford, where he directs the Stanford GPS Research Laboratory. He has a Ph.D. from the University of Illinois.
Workshop participants from Cote d’Ivoire and Kenya assemble a Mindstorm robot to trial autonomous navigation.
By Patricia Doherty
Last year I helped coordinate a three-week workshop for 50 scientists from 15 African countries, introducing the basics of GPS for applications with socioeconomic benefits and scientific exploration. Held in Trieste, Italy, the workshop was quite successful, producing new initiatives on the African continent. We repeat the workshop next month, April 6–24, again in Trieste.
Since the 2009 training, regional GNSS workshops have taken place in Nigeria, Egypt, Kenya, and Ethiopia. We have initiated scientific collaborations with universities in Nigeria, Kenya, Zambia, Egypt, and Uganda, deploying GPS receivers at each institution, with the understanding that the data will ultimately be shared within Africa and the world.
This effort is a way to share with Africa and Africans the wealth that GNSS has brought to the developed world.
Africa’s 2006 Science and Technology Plan of Action states Africa’s commitment to develop and use science and technology for socio-economic transformation and full integration into the world economy. The leading problems that continue to cripple much of Africa include hunger, extreme poverty, erosion of natural resources, and natural disasters. GNSS can help address these problems and ultimately meet the plan’s goals. Specifically, GNSS applications can increase food security, manage natural resources, provide efficient emergency location services, improve surveying and mapping, and provide greater precision and safety in land, water, and air navigation systems. GNSS also has applications in scientific study including space weather, geophysics, geography, geology, ecology, and biology.
Workshop participants included professors and graduate students from Cote d’Ivoire, Egypt, Ethiopia, Ghana, Kenya, Morocco, Nigeria, Uganda, and Zambia. The more than 25 lecturers came from the United States, Europe, and Africa.
The workshop integrated formal lectures with hands-on practice in GNSS architecture, signal structure, hardware design, state-of-the-art applications, and scientific exploration. An on-site computer laboratory enabled participants to perform positioning calculations; use mapping and surveying software; plan a precision farming procedure; and analyze atmospheric and ionospheric data — all from GPS measurements. In addition, participants built Lego Mindstorm robots to demonstrate autonomous navigation.
One of the benefits of this program was that scientists and engineers from the United States had opportunities to discuss common interests with African scientists and engineers. Many research programs utilize GPS ground- and space-based measurements. Unfortunately, studies over the African region have not been possible due to the lack of dependable long-term measurements. This workshop opened the door to establishing a base of measurements for joint studies with our African colleagues.
Many lecturers remarked that this was the most enriching teaching experience of their careers. The African participants said that they learned a great deal and were very appreciative of the opportunity to participate in this program.
Workshop sponsors include Boston College’s Institute for Scientific Research (where I work), the Abdus Salam International Center for Theoretical Physics in Trieste (where my colleague and workshop co-director Sandro Radicella is head of the Radiopropagation Laboratory), Institute of Navigation, Federal Aviation Administration, Air Force Research Laboratory, National Aeronautics and Space Administration, United Nations Office for Outer Space Affairs, National Science Foundation, Trimble, and NovAtel.
To learn more about the workshop, participate, or contribute, please contact Patricia.Doherty @ bc.edu
Adding GLONASS to GPS gives a total of about 50 satellites, for a significant improvement in navigation availability, reliability, robustness, and convergence time through a new multi-GNSS precise point positioning (PPP) service. System performance and field results demonstrate that there is no need to await future constellations — better performance is available now.
By Tor Melgard, Erik Vigen, Ole Ørpen, Fugro Seastar AS, and Jon Helge Ulstein, Bourbon Offshore Norway AS
Precise point positioning (PPP) stands out as an optimal approach for providing global augmentation services using current and coming GNSS constellations. PPP requires fewer reference stations globally than classic differential approaches, one set of precise orbit and clock data is valid for all users everywhere, and the solution is largely unaffected by individual reference-station failures. There are always many reference stations observing the same satellite because the precise orbits and clocks are calculated from a global network of reference stations. As a result, PPP gives a highly redundant and robust position solution.
The results presented here represent a significant step forward in PPP GNSS research and development. Using GLONASS improves the availability and reliability of the solution. The G2 system’s horizontal positioning accuracy is at the decimeter level. These results derive from increasing the number of satellites in the constellation by 60 percent, from about 30 to 50 satellites. The outcome of the development of the G2 real-time combined GPS and GLONASS PPP service represents a next-generation GNSS augmentation. Further, the later GLONASS-M satellites have improved performance and lifetime over previous GLONASS satellites, so that results will continue to improve as that constellation is replenished.
G2 development has benefited from the close cooperation between Fugro and the European Space Operation Centre (ESOC), an establishment of the European Space Agency (ESA). ESOC has contributed its long experience and expertise on precise orbit and clock processing techniques, while the strength of Fugro is real-time positioning and navigation services.
Based on this work, Fugro has introduced the first real-time GPS and GLONASS precise orbit and clock service. The service utilizes Fugro’s own network of dual-system GNSS reference stations to calculate precise orbits and clocks on a satellite-by-satellite basis for all 50 satellites of the two global navigation satellite systems. The system comprises about 40 dual-frequency GPS and GLONASS reference stations distributed around the world as shown in Figure 1.
Raw GNSS measurement data for all satellites are transmitted to processing centers for calculation of the precise orbit and clock of each GPS and GLONASS satellite (Figure 3). The precise data generated is then broadcast to users via geostationary communications satellites with nearly global coverage, as shown in Figure 2.
FIGURE 1. The G2 reference station network consists currently of 40 GNSS receivers owned and operated by Fugro.FIGURE 2. The G2 precise orbits and clocks are broadcast over redundant geostationary satellite beams together with the other Fugro services.FIGURE 3. Dataflow from the reference stations to the redundant calculation servers producing precise orbits and clocks, then to the satellite uplink stations for broadcast over geostationary satellites to combined G2/GNSS user equipment.
Inside the end-user equipment a dual-frequency carrier-phase-based PPP solution gives horizontal positioning accuracy at the decimeter level. The PPP calculation module is provided by Fugro and is embedded in multiple GNSS receiver manufacturers’ products as well as Fugro’s own product line.
Like any GNSS technique, PPP is affected by satellite line-of-sight obstructions. Even the most precise orbit and clock data is useless if the user cannot track particular satellites. When satellite visibility is partially obstructed, a best possible service can be ensured by using the full range of satellites from both the GPS and GLONASS systems. This can occur during a survey of a dense urban environment, and for urban positioning in general. It can occur under heavy tree cover, when a cruise ship is in a high-sided fjord, when an offshore vessel is close to an oil rig or platform, or during ionospheric disturbances.
The trend clearly lies towards increasing availability of GNSS satellites on orbit; many studies predict the future benefits of combining the constellations of GPS and Galileo. There is no need, however, to wait for future constellations to reap the immediate benefits of access to additional GNSS satellites. The current GLONASS constellation may not have all the features of future GNSS systems, but it is available here and now. Recently, the Russian government has proven its commitment to enhancing the GLONASS constellation. Many receiver manufacturers have also acknowledged this fact and now provide combined GPS and GLONASS receivers.
G2 Accuracy and Statistics
In Figure 4, time-series plots show the 3D accuracy of GPS and GLONASS G2 real-time orbits on August 14, 2009. In the comparison, final orbit data from the International GNSS Service (IGS) is used as reference. PPP positioning is mainly affected by the radial orbit error, which is significantly less than the total 3D error shown here. The 95 percent 3D accuracy for GLONASS (22 centimeters) is more than double that for GPS (10 centimeters). The graph demonstrates how this difference in this case is mainly caused by a few GLONASS satellites being less accurate. Actually, several GLONASS satellites have orbit accuracy very close to the level of GPS for real-time G2 data.
FIGURE 4. GPS and GLONASS orbits compared to IGS final orbits.
Figure 5 shows the clock accuracy of the G2 real-time clocks compared to final IGS clocks. A constant bias has been removed to account for the differences in system reference time. Smaller individual clock biases for each satellite can still be observed. Small biases do not affect the final accuracy of the PPP solution, and achievable position accuracy with these clocks are significantly better than the 21-centimeter 95 percent number for GPS may indicate.
FIGURE 5. GPS clocks compared to IGS final clocks. GLONASS clocks compared to a combined solution based on IGS plus Fugro network to calculate a best possible reference solution.
The lower time series in Figure 5 shows the estimated GLONASS clock accuracy. Currently there is no comparable IGS product with precise GLONASS clocks. A post-processing of all available IGS plus Fugro GNSS stations has been made to establish a reference for the comparison. As shown, the GLONASS clocks are more variable, but still they are stable enough to allow for precise navigation.
Real-Time Positioning Results
Real-time position performance is continuously observed at the G2 operation and monitoring center in Oslo, Norway. The graph in Figure 6 shows typical G2 positioning results with the calculation engine running in dynamic mode at a fixed location for a 24-hour period. The blue lines in the north and east time series are at 20 centimeters and the scale is 61 meter. In the height graph the blue lines indicate the 30-centimeter level. The antenna is in a location with clear view of the sky, and in
dependently calculated reference coordinates are used as reference. 1-sigma accuracy statistics on August 14 are 3, 4, and 8 centimeters in easting, northing and height respectively.
FIGURE 6. G2 GPS-plus-GLONASS position monitoring results in Oslo on August 14, 2009.
Figure 7 shows GLONASS-only real-time positioning with clear view of the sky for the same day as in Figure 6 and the same antenna location. The blue line indicates the 50-centimeter level and the scale is 62 meters. For long periods, the GLONASS-only solution works quite nicely. There are, however, shorter periods with fewer than four satellites being tracked, causing the position output to stop, followed by a period of re-convergence.
FIGURE 7. GLONASS-only real-time PPP solution on August 14, 2009 for a 24-hour period.
Figure 8 displays results from May 11, 2009, when there were slightly more satellites available and just enough to have the GLONASS-only solution running for 24 hours without resets. 1-sigma accuracy statistics for this day are 11, 9, and 16 centimeters in easting, northing, and height respectively. Considering the average number of satellites of 6.14 and periods with high DOP values, this is very promising. In early 2010, 20 GLONASS satellites should be available, and by 2011, 24 are expected. In 2010, a performance similar to or better than that of May 11 should generally be expected with the new satellites. By 2011, even better performance is believed to become the norm of GLONASS-only real-time PPP navigation.
FIGURE 8. GLONASS-only real-time PPP solution on May 11 for a 24-hour period.
Even in some clear-view-of-sky situations, the addition of GLONASS may improve the navigation compared to GPS-only solutions. Figure 9 presents an example of such situations. Here the GPS-only solution suffers some multipath-like effects showing up, especially in the east component. Figure 10 shows the combined GPS+GLONASS solution for the same dataset. The distortion in position is practically eliminated. This is an example where adding GLONASS also improves redundancy and accuracy for navigation with clear view of the sky.
FIGURE 9. GPS-only results for a 3-hour period where some multipath-like effects distort the postition, especially the east component.FIGURE 10. Adding GLONASS improves redundancy and accuracy for the same time period as presented in Figure 9.
The next test further analyzes the same dataset as in Figures 9 and 10 by simulating a virtual wall to the south, blocking all satellites below 40 degrees elevation. Figure 11 illustrates this virtual wall blocking both GPS and GLONASS satellites.
FIGURE 11. GPS and GLONASS satellites blocked between the azimuths 90 and 270 degrees and elevation lower than 40 degrees, effectively establishing virtual wall to the south.
With such data blockage, the GPS-only solution fails for more than 20 minutes, as seen in Figure 12, simply because the number of satellites goes below four. Then a period with slow convergence follows because of few satellites and high DOP.
FIGURE 12. GPS-only solution fails when simulating blockage to the south.
Again, adding GLONASS greatly improves the performance, as shown in Figure 13. Now a sufficient number of satellites are tracked all the time, and there is a continuous solution with the combined GPS+GLONASS throughout the time window when the GPS-only solution failed.
FIGURE 13. GPS+GLONASS solution continues working with simulated blockage to the south.
Even with more than 30 satellites in the GPS constellation, there are situations when the satellite geometry gets poor. This occurred in northwest Europe on February 2, 2010. One of the GPS satellites (PRN17) was not available due to maintenance, and even with five to six usable GPS satellites left, the horizontal dilution of precision (HDOP) was in the range of 7–11 for about 12 minutes (10-degree elevation mask), as shown in figure 14. Such high HDOP values lie above what most user installations are configured to accept, and Fugro received feedback from clients at sea losing positioning. The G2 solution was not affected by the poor GPS geometry and kept the HDOP below 2 during this period, as shown in Figure 15.
FIGURE 14. GPS-only performance during a period with poor GPS satellite geometry in Oslo, February 2, 2010.FIGURE 15. GPS+GLONASS performance during the same period as in Figure 14 in Oslo, February 2, 2010.
Convergence-Time Analysis
As will be shown in the following analysis, adding GLONASS not only improves availability and robustness of the solution, it greatly improves convergence time. Real-time high-accuracy PPP solutions use carrier-phase measurements to achieve high-accuracy positioning. To do so, the carrier-phase ambiguities must be determined. This process takes a certain time depending on the observed satellite geometry and is commonly referred to as cold-start convergence time.
Figure 16 presents a theoretical study of the expected convergence time for a GPS-only compared to a combined GPS+GLONASS solution. The lower graph shows how the expected convergence time varies significantly for a GPS-only solution throughout the day, with a peak of 75 minutes. The combined solution shows much more consistent performance, with expected 50–60 percent average improvement over GPS-only.
FIGURE 16. Theoretical study of expected convergence time with actual GPS-and-GLONASS constellation in view of Oslo on June 26, 2009. Adding GLONASS gives a 50–60 percent theoretical convergence time improvement over GPS-only.
We compare this theoretical study to results using G2 data produced in real time in Figure 17. A cold start is performed every 5 minutes throughout the day, for six consecutive days, giving a total of 1,728 convergence tests. The convergence criterion is the time when the 3D position arrives within 40 centimeters of the reference position and remains there for a minimum of 10 minutes. The average convergence time improvement achieved in Figure 17 is 39 percent, with some variations from day to day. On the better days, the average improvement is almost 50 percent, and close to the expected performance based on the theoretical study. On other days, there is room for further improvement. Mainly two factors are expected to contribute: more and newer GLONASS satellites, and further improvements of the G2 precise GPS and GLONASS orbit and clock product.
FIGURE 17. Convergence results for six consecutive days starting June 24, 2009. Average convergence time of GPS-only is 27 minutes, and GPS+GLONASS is 16.5 minutes, a 39 percent improvement.
Dynamic Environment Results
Since late 2008, the G2 system has been installed on the vessel Bourbon Topaz, making frequent trips into the North Sea and back into port in Norway (see BOX).
All positioning data from both the G2 system and the GPS-only reference systems are logged in real time on the vessel. Figure 18 gives an example plot of the relative height estimated by the G2 GPS-GLONASS solution. In the beginning of the plot, the vessel is out at sea, clearly seen as a noise in the graph that actually is the vessel’s movement in the waves. Then the vessel comes into port and the slower tidal variations are observed for the next 12 hours until the vessel again goes back out to sea.
FIGURE 18. Relative G2 height measurements for a 24 hour period. The vessel is in harbor from 04:00 – 16:00 UTC.
On June 22, 2009, an incident was recorded where the combined GPS-GLONASS G2 solution improves performance. As seen in Figure 19, there is a period starting at 10:00 UTC where the GPS-only reference systems suffer from poorer DOP values, and this is reflected both in horizontal and vertical components of the calculated position. This particular plot shows how the height drifts off by roughly 1 meter while the G2 combined solution remains unaffected for the entire period. Generally, the G2 solution also shows a smoother height than the reference system even when such problems as shown here are not present.
FIGURE 19. Height graph from the Bourbon Topaz while in harbor on June 22, 2009. The GPS-only reference system has a period with poor DOP values while the GPS-plus-GLONASS solution is not affected.
The Bourbon Topaz carries the G2 system on operations in the North Sea, and continuously compares it with the GPS-only reference systems onboard.
Test of G2 onboard Bourbon Topaz
The Bourbon Topaz is a modern supply vessel equipped with the latest dynamic positioning (DP) systems, operating in the North Sea. The North Sea can be a harsh environment in which to operate, and we rely on good tools for maneuvering our vessels.
Early on, we recognized the need for stable, reliable reference systems, and our fleet is equipped with Kongsberg Seatex DPS700 system as standard. When we were asked to test the G2 onboard the Bourbon Topaz, we saw this as an opportunity to follow the development in the industry of such services. The DPS232 receiver was set up in connection with the vessel’s DPS700 system, and all information was logged and sent to Fugro Seastar.
We often experience that the vessel has to operate close to offshore installations, which could block good reception of signals. In these cases, the G2 offers a much better and more reliable signal reception. Our experience of the quality of the G2 system is overall positive.
User Equipment
G2 and the other Fugro services can be received from a variety of different user equipment; both Fugro-branded or manufactured equipment and third-party equipment. In most cases the L-band receiver decoding the data from the geostationary satellites, including Fugro subscription software and position calculation module, is integrated into the same box as the GNSS receiver. Both the GNSS and geostationary satellite signals can be tracked with a single antenna.
FIGURE 20. Receivers supporting the Fugro services. These are only examples, and not all third-party equipment manufacturers are shown. Fugro L-band data reception receiver and positioning/subscription software reside inside the receiver.
Conclusions
Test results confirm decimeter-level position accuracy in real-time navigation with G2, the first real-time combined GPS and GLONASS PPP service. Several examples show how G2 improves availability, robustness, and convergence time compared to GPS-only positioning.
More is better. There is no need to wait for future constellations like Galileo to reap the benefits of access to additional GNSS satellites now.
Tor Melgard is R&D manager at Fugro Seastar in Oslo, Norway. He holds an M.Sc. in electrical engineering from the Norwegian Institute of Technology and wrote his thesis at the Department of Geomatics Engineering, University of Calgary.
Erik Vigen is a senior developer at Fugro Seastar. He received his M.Sc. in Geodesy from the Norwegian Institute of Technology.
Ole Ørpen is senior scientist at Fugro Seastar. He received his M.Sc. from the Norwegian Institute of Technology in electrical engineering.
Jon Helge Ulstein is IT superintendent at Bourbon Offshore Norway AS, a subsidiary of the Bourbon Group, Marseilles, France.
The next-generation GPS ground-control system, known as OCX.
Officials from the Space and Missile Systems Center’s Global Positioning Systems Wing announced today the award of the Next-Generation GPS Control Segment (OCX) contract to Raytheon Company, Intelligence and Information Systems, Aurora, Colorado.
The OCX development contract will be 73 months in duration and with option years for sustainment worth $1,535,147,916. The contract will include development and installation of hardware and software at GPS control stations at Schriever Air Force Base in Colorado and Vandenberg AFB in California, deployment of advanced monitor stations at remote sites, and initial contractor support with sustainment options for five years.
OCX will replace the current GPS Operational Control System, maintaining backwards compatibility with the Block IIR and IIR-M constellation, providing command and control of the new GPS IIF and GPS III families of satellites, and enabling new modernized signal capabilities.
“OCX is urgently needed not only to enable new warfighter capabilities but also to put the new GPS III space vehicles into mission operations,” said Col. Dave Madden, commander, GPSW. “OCX will have a flexible architecture that can rapidly adapt to the changing needs of today’s warfighter and will connect to the Global Information Grid so that warfighters around the globe have immediate access to GPS data and constellation status.”
“OCX will allow AFSPC to effectively and efficiently plan and control full-spectrum precision position, navigation and timing information for all GPS user communities,” Madden said. “OCX will achieve this vision by implementing an incremental development approach that supports the evolving military operational environment, while enabling civil and international users who are employing GPS in innovative applications like transportation.”
The Air Force Space Command’s Space and Missile Systems Center, located at Los Angeles Air Force Base, California, is the U.S. Air Force’s center of acquisition excellence for acquiring and developing military space systems including six wings and three groups responsible for GPS, military satellite communications, defense meteorological satellites, space launch and range systems, satellite control network, space-based infrared systems, intercontinental ballistic missile systems, and space situational awareness capabilities.
As usual with an emerging personal, digital communications technology on the brink of mass appeal (think e-mail, mobile phones), privacy is a major concern and a serious topic of discussion with regard to location-based services (LBS). With the GPS component, LBS worries users and regulators even more. So much so that there was a House Congressional hearing this week on Capitol Hill regarding this very subject. It’s not the first hearing on location privacy and certainly won’t be the last. Following are the people who testified:
Lorrie Cranor, Associate Professor, Computer Science and Engineering and Public Policy, Carnegie Mellon University
Mike Altschul, Senior Vice President and General Counsel, CTIA – The Wireless Association
John B. Morris, Jr., General Counsel, Center for Democracy and Technology
Anne Collier, Co-Director, ConnectSafely.org
Jerry King, Chief Operating Officer, uLocate Communications, Inc.
Tony Bernard, Vice President and General Manager, Useful Networks
If you’d like to read the testimony from each of these folks, you can do so here.
I guess as geospatial-knowledgeable people, most are either on one side of the spectrum or the other. One side being the folks who understand the power of location and are willing to gamble that our privacy will remain within our control while enjoying the benefits of LBS applications. The other side being the folks who understand the power of location and are scared to death that Big Brother will have some amazing new tools to trace our every step and record our every decision.
As you may surmise, I’m on the gambling side of the spectrum. While I don’t typically live on the bleeding edge of technology (I don’t own an iPhone or Blackberry), I do own a lot of technology and have a fairly powerful smartphone with GPS capability. I don’t currently subscribe to any LBS apps on my phone besides the navigation feature. However, I’m seriously excited about the future of LBS applications. Personally, I’m excited about LBS technology because it will help make my life run more efficiently. That’s what I’m in it for. GPS navigation has done that for me and I think some other LBS apps will do that for me too.
I realize (and appreciate) that not everyone is like me. I’ve got four kids (ages 12-18), an active spouse, and a fair number of extended family members in the Portland, Oregon, metro area. I’m a family-oriented person so I’m active in my kids’ lives and also try to keep in regular touch with family members who are local. I also am involved in some community activities that require meetings, etc. All of this is on top of my work schedule, which can be hectic. To manage my schedule, I enter my commitments (business as well as personal) in my smartphone calendar. In turn, it tells me where I need to be and when I need to be there. Without it, I’d be in trouble. There’s no way I could commit to memory where I need to be and when.
My smartphone takes care of helping me manage my schedule and how to get where I need to be (via GPS). I’m happy with it. It’s very good when I set up all my appointments and addresses in advance.
However, I know I’m missing a lot of opportunities to meet up with my family and friends…if I only knew where they were (or they know where I am) at a particular point in time. For example, maybe I’m on the other side of town watching my kid’s basketball game. My sister could be in the same high school watching her kid, but on a different court. If I knew that at the time, I would certainly make the effort to go sit with her, even if it was just for a few minutes. Yes, I could call her and ask her where she is at, but she’s not the only one. There’s probably 25-30 people like her I stay in touch with and it is not practical to call each one. That number would grow substantially if I include business contacts who I’d be interested in meeting up with if the opportunity arose (think airports and conferences).
This sort of dynamic people-connecting is what LBS can bring to the table. Of course, LBS can offer many other services (coupons at nearby restaurants, etc.) but the people-connecting opportunity, for me, is the big one.
What does people-connecting entail?
Essentially, you’ll need three items:
your current location.
some way to communicate with people.
software to make it all work.
A mobile phone w/GPS fits that bill. It doesn’t necessarily need to be a smartphone, but a more powerful phone will offer more powerful features. Also, you’ll need some kind of data plan and possibly text messaging. Lastly, you’ll need some sort of software on the phone to make it all work together.
Following is a sample phone screen of a people-connecting software application.
Back to the original topic of this column: Privacy
Of course, a major concern by regulators and potential users is how personal location information will be used by the LBS application software. Will this be just another way that your personal information will be collected and sold to spammers? In addition to spammers, do you really want your family/friends knowing where you are 24/7? These are not unreasonable concerns.
I don’t worry about privacy with LBS applications and I’ll tell you why.
There is a lot of hyper-sensitivity about privacy with LBS applications (House Congressional Hearing this week on the subject) so I think LBS software vendors are well aware that a line has been drawn in the sand and a sort of zero-tolerance policy has been established. Secondly, leading LBS companies were involved with CTIA (The Wireless Association) in developing a document titled “Best Practices and Guidelines for Location-Based Services” so they are intimately aware of the privacy issue.
There are two guiding principles in the Best Practices guidelines mentioned above:
LBS providers must inform users about how their location information will be used, disclosed, and protected so that a user can make an informed decision whether or not to use the LBS or authorize disclosure.
Once a user has chosen to use an LBS, or authorized the disclosure of location information, he or she should have choices as to when or whether location information will be disclosed to third parties and should have the ability to revoke any such authorization.
Read the entire CTIA Best Practices guideline here.
The Final Analysis
One consideration I will give when subscribing to a LBS app in the future is to make sure I subscribe either through my wireless service provider (Sprint, AT&T, Verizon, etc.) or by an established, reputable LBS app provider. This kind of due diligence is no different from when you consider purchasing an application for your personal computer. Common sense tells you not to download an app from Nigeria. You’ll need to practice the same diligence when selecting an LBS application.
I also wouldn’t consider an LBS application where I don’t have the opportunity to control my personal network of people who are granted access to my current whereabouts. In fact, I’d want the ability to shut off broadcasting my location altogether. Again, I don’t think that any mainstream LBS application is not going to have these features due to the h
igh-profile sensitivity to privacy.
I know the LBS applications are already available to accomplish the people-connecting that I want. But, like I wrote earlier, I don’t live on the bleeding edge of technology. I live a step back from the edge. I wasn’t the first to join Facebook (although I’m glad I eventually did) and I won’t be the first to run a people-connecting LBS application, but there’s no doubt in my find that it will eventually be an important tool for me and, most likely, you too. The upside is just too big to ignore.
I didn’t plan it this way, but my coverage of GPS 24+3 turned out to be a three-part series, with this column being part three. One reason it turned into a three-part series is because I’m learning more about it along the way, but its mostly because details weren’t released all at once.
The good news is that I (along with help from others….thank you) was able to generate an almanac that simulates 24+3 reasonably well. The idea behind doing this is that I could compare the satellite visibility plots in satellite visibility software using both the original almanac (I chose January 1, 2010) and a GPS 24+3 modified version of the same almanac. For those plots, I could present to you what you can realistically expect the improvement to be with the 24+3 satellite configuration.
A quick note before diving into the 24+3 configuration. At the end of this column is a brief discussion about solar activity and GNSS/GPS. Last week, there was a solar event and some users have voiced concerns about that. I’ve addressed those in a section at the end of this article.
24+3
You can view my first two columns relating to the 24+3 configuration by following these links:
I’d like to update you on some bits of information that I’ve learned about 24+3 since my last column. I asked the HQ Air Force Space Command some questions about 24+3 and they kindly responded.
EG: Will the satellites (SVN24, SVN26) remain healthy during their repositioning journey?
HQ AFSC: Yes. The satellites will be set unhealthy for the initial Delta-V, but will return to healthy status approximately 24 hours after initiation of the Delta-V. Initial Delta-V for SVN24 was accomplished on 13 Jan 10 and returned healthy on 14 Jan 10. SVN 24 will take up to a year to reach its final destination. Initial Delta-V for SVN 49 was accomplished on 21 Jan 10 and will arrive at its expanded position in Jun 10. Initial Delta-V for SVN26 will begin early Feb 10.
EG: Why the two-year timeframe to realize the benefits when all repositioning will be complete in 12 months?
HQ AFSC: The two-year timeframe is a conservative estimate which takes into account potential operational necessities which could extend the time required for completion. We must take a disciplined approach to cover possible failures and ensure continuity of coverage during the transition.
We will be adding GPS IIF vehicles to the constellation and older vehicles may fail during the transition timeframe. As vehicles are added and removed, the current plan is subject to change in order to provide the best service to all civil and military users. Some of these decisions could require additional time to complete the expanded constellation. However, benefits will likely be realized well in advance of 24 months.
EG: What is the reasoning behind using SVN49 as a key component of the 24+3 configuration since it won’t benefit a significant portion of the civilian user community, namely aviation and marine navigation as well as other SBAS (WAAS) and DGPS users? In my understanding, the FAA’s and the Coast Guard’s user bases are primarily single-frequency pseudo-range, users who won’t be able to use SVN49.
HQ AFSC: SVN49 was selected because it is a brand-new satellite with four good clocks. Although issues with SVN49’s navigation signals may make it unusable for all civil use, it could still put out a valid set of signals for military use. The Air Force team is continuing to work “open book” with civil and industry GPS experts to determine the possible outcome of SVN49. Although SVN49 is not currently healthy, GPSW and 50th SW are actively working a mitigation that may allow setting the vehicle healthy in the future. As a mitigation in case we are unable to set SVN49 healthy, SVN30 will be rephased to the same slot following a successful launch and on-orbit checkout of IIF-1. We expect to have either SVN30 or SVN49 healthy and broadcasting from the expanded slot within a 24-month timeframe. At this time, no decisions have been made and no options have been ruled out regarding SVN49.
Satellite Visibility Plots
As promised, I’ve (with help) been working on creating an almanac that simulates the 24+3 constellation. My goal was to be able to show you what the benefit to you will be with the new GPS 24+3 satellite configuration.
The method I used was to modify an almanac from January 1, 2010. The reason I chose that day is because it was before the satellite repositioning began. The first satellite began its repositioning journey on January 13, 2010.
Within the almanac, I adjusted the position of three of the satellites in the almanac to reflect the new orbit locations they are going to assume.
SVN 24 is moving from slot D5 to slot D2F
SVN 26 is moving from slot F5 to slot F2F
SVN 49 is moving from slot B5 to slot B1F
Following is a graphic I’ve published before that illustrates the satellite repositioning:
Using the original January 1, 2010, almanac to plot a satellite visibility chart and then using the 24+3 modified almanac to plot another chart for the same location, I was able to generate the following comparisons between the current GPS satellite configuration and the 24+3 satellite configuration. Please note the following:
A 15-degree elevation cutoff was used to account for obstructions (terrain, buildings, trees).
The modified almanac does not take into account the other three satellites that are being slightly repositioned (SVN46, SVN55, SVN56) so the modified almanac represents a worst-case scenario.
The original almanac is the first plot. The modified 24+3 plot is directly below it.
Portland, OR USA (N45 41, W122 11) Original Almanac:
Portland, OR USA (N45 41, W122 11) 24+3 Almanac:
Miami, FL USA (N25 46, W80 11) Original Almanac:
Miami, FL USA (N25 46, W80 11) 24+3 Almanac:
Tokyo, Japan (N35 42, E138 30) Original Almanac:
Tokyo, Japan (N35 42, E138 30) 24+3 Almanac:
London, England (N51 30, W000 07) Original Almanac:
New Dehli, India (N28 54, E77 13) Original Almanac:
New Dehli, India (N28 54, E77 13) 24+3 Almanac:
Rio De Janeiro, Brazil (S22 27, W42 43) Original Almanac:
Rio De Janeiro, Brazil (S22 27, W42 43) 24+3 Almanac:
Bangkok, Thailand (N13 49, E100 28) Original Almanac:
Bangkok, Thailand (N13 49, E100 28) 24+3 Almanac:
Perth, Australia (S31 49, E116 10) Original Almanac:
Perth, Australia (S31 49, E116 10) 24+3 Almanac:
A Quick Note about Solar Activityand GNSS/GPS
I’ve read media reports and I’ve been asked about a solar event that occurred last week (Thursday, February 12) and what possible effect it had on GPS operations.
I consulted with Joe Kunches of the NOAA Space Weather Prediction Center to understand how significant of an event it was.
“There was some activity but I would not think it would have an impact on GPS,” stated Kunches.
I asked him at what point would GPS operations be affected.
“As for flares (Radio Blackouts on the NOAA Scales), I’d say 10 to 20 times stronger than last week (R3 to R4 and above) would be sufficient to affect GPS on the dayside, but not for long,” said Kunches.
So, although there were media reports about the solar event last Thursday, if you had trouble with your GPS it wasn’t due to solar activity.
However, solar activity is a serious issue for GPS users, especially those using high-performance L1 receivers (sub-meter). You can be sure that I’ll will be covering this subject in-depth as we move further into the current solar cycle.
If you haven’t seen the announcement regarding my Webinar this Thursday (February18, 10 a.m. Pacific Time, 1800 hrs GMT), you might be interested. The title is “GPS for GIS – 101.” It’s a beginner’s (and refresher’s) guide to using GPS for GIS data collection. I’ve invited Craig Greenwald as Guest Commentator.
I ran across an opinion piece entitle “The creative Destruction of GIS” by Ed Parsons, a Geospatial Technologist at Google, published online by GIS Development. I have to admit that when I first scanned the piece, my face crinkled up with the “whhhhaaaaat?” sort of look.
For a common frame of reference, you might want to click here and browse/read the article first before reading the rest of my column. I suppose its taboo to reference narrative in a competitive publication, but I was never one to follow the rules especially if I think the reader will benefit from the interaction.
First off, I don’t know Ed and have never met him. He seems like a knowledgeable gent and from his bio, one can see he’s spent a fair amount of time in the geospatial industry. A friend of mine asked for my thoughts on his piece and I’ve seen it referenced in one or two other places so I thought I’d give it a whirl.
After reading the article more thoroughly, it’s truly a sheep in wolf’s clothing (as opposed to the popular “wolf in sheep’s clothing”). Yes, its bark is worse than its bite. My guess is that an editor got a hold of the piece and inserted the eye-catching title “The creative destruction of GIS” and subtitles “The end of GIS?” and “The post-GIS future”.
No Chicken Little, the GIS sky is not falling.
To his credit, Ed paints a pretty good picture of the infancy of web publishing and Location-Based Services (LBS). But before we go there, let’s set the record straight about GIS. I think Ed wrote it just right:
“…GIS, as described previously, is and will remain the domain of specialists,…”
and
“GIS as we recognise it today will not disappear.”
I agree. The GIS industry, as we know it, is not going anywhere soon. It’s going to continue its moderate and healthy annual double-digit growth rate (save a few economic speed bumps along the way). The same software makers, in general, will dominate the GIS market and serve as a platform on which GIS’s are built for: fed/state/local government, natural resources, utilities, armed forces, transportation, engineering/surveying, yada, yada, yada.
“What do these broad trends mean for the future of GIS? The focus of the industry is moving away for the creation/ maintenance of geospatial information to its exploitation.”
Reading the above quote in his piece is one part where my face got a little crinkly. I beg to differ with that statement and here’s why…
If my kids asked me which career has significant upside in the next ten years, I’d say GIS makes the list. What’s my claim based on? [Big sigh]….there is so much ground that GIS hasn’t covered yet; the industry is just beginning to mess around with 3D, the much-needed surveying+GIS collaboration has really just begun, and soooooo many paper records are still waiting to be entered into a GIS. The list is very extensive. On top of that, data collection technologies like remote sensing and GPS are improving substantially so better quality data will continue to be introduced. Of course, GIS is the engine and data is the fuel. We’ve got some pretty good engines available to us, but we are still lacking for fuel. Data is the name of the game so the idea that the industry is moving away from data creation and maintenance is a little absurd to me.
In my opinion, data is the future of GIS. I’ll use the GPS analogy to illustrate this. When GPS was invented in the early 1970’s, the military creators never envisioned how it is being used today. Never in their wildest dreams did they think that civilian engineers would figure out a way to tap into the signal intended only for military users and be able to design a small GPS receiver that can deliver cm-level positioning in real-time. Similarly, ten years from now GIS will be used in ways we can’t envision today. The difference and what makes GIS more dynamic than GPS is that fundamentally, the GPS infrastructure hasn’t changed much in the last 15 years. Yes, there are new satellites and the L2C signal, but the change has been relatively small. Mind you, the user side of GPS has changed a lot.
That’s not so for GIS. The GIS infrastructure (GIS software tools) has changed significantly as well as tools for the user side of GIS…and they will continue to change. In the coming years, new GIS tools will be developed, new data will be harness and new GIS applications will be developed. Just think about it…the United States, one of the most developed nations in the world, doesn’t even have a complete land record (parcel) database in a GIS yet and many local governments are still years away from it. Hi-resolution elevation contours? Not a chance. Just yesterday, I was searching for 2 foot contours of a 150 acre parcel (a park) in a major metropolitan area. Nothing. The best they made available to the public were 100’ contours in a pdf map. Mind you, I’m sure they had much better data in-house so part of the problem is data sharing policy but I really doubt they have 2 foot contour lines of the parcel. Just think of the analysis (eg. drainage) that could be done if 6” contours were available for every park, open space and developed area within a country. The good news is that it’s just a matter of time before that kind of detailed data is generated and available.
Regarding web publishing and Location-Base Services (LBS), these are two areas that will go crazy. I’ve written quite a bit about LBS in the past. It’s a blank slate at this point and there’s a ton of entreprenurial energy being expended to find the sweet spots. Rest assured there will be many. But that’s not much to do with the future of GIS as we know it. Yes, serving GIS data to the public in a usable format is valuable and growing, but that’s not what Ed is writing about. What Ed’s piece really needs is to be separated from the GIS discussion. Web publishing and LBS for Joe Consumer is a huge topic and worthy of Google chasing after those segments, not GIS.
With apologies to James Bond, Ian Fleming, and, well, just about everybody else. Here is a grab from my mail bag. The message was subject-lined: GPS Spy Applications.
“I recently suspected my wife of cheating, having been involved with gps as a land surveyor since 1995, I used and application called mobile-spy.
“In order to install the application onto an iPhone you have to “jailbreak” the phone. Once its installed it will forward all text, url’s, and a gps location every 30 minutes if it has satellite availability. To make a long story short, I caught my wife in a pretty precarious spot, or spots. It’s my opinion that she was sneaking out and meeting someone at various spots on our normal routes, little hidden offroad trails if you know what I mean. Well I tested and retested the phones gps and the data from the mobile-spy website where I purchased the software, which is actually sold under the name “retina-x” and they make there money by giving you access to these logs through mobile-spy.com.
“However, my wife contests that all this data is wrong, of course, and she’s never been anywhere near these places. On the other hand, I have a ton of evidence saying she WAS at these locations. She says she’s read an article on AT&T that shows evidence that the gps in the iPhone is faulty and gives out bogus locations. As I said, I tested this a couple of times and it seemed to work perfectly.
“In good faith we’ve agreed to let me take the iPhone and perform more in depth tracking over a span of a few weeks. I am not really a writer but I’ll definitely keep detailed logs of my observations. Have you guys already had this particular issue come up before? If so, I’d love to know anything you can tell me because the way it stands I am getting a divorce unless this application can be proven wrong! My email is [email protected]
Cell phone is XXX.XXX-XXXX, I don’t check voicemails, so if I don’t answer just send me a text with your name and number. I look forward to hearing from you soon.”
Sleep was what I wanted, you know what I got. Wide awake, staying up late, wishing I was not.
Survey editor Eric Gakstatter posed these questions to the GPS Wing; their answers follow.
