Global Positioning News

Category: Opinions

  • Expert Advice: Managing the GPS Constellation for Today’s Needs

    Expert Advice: Managing the GPS Constellation for Today’s Needs

    John Lavrakas
    John Lavrakas

    In a recent editorial in GPS World’s Survey & Construction e-newsletter entitled “No Joy in Surveyville,” Eric Gakstatter lamented the performance of the GPS constellation for surveying. He is not alone. In June, the Australian Broadcasting Company reported that farmers in Australia were experiencing major problems with GPS because two satellites had been removed from service.

    For many, GPS is at its best performance ever, with 29 satellites in orbit and user range errors at their lowest levels in years. Yet for others, GPS performance falls short of expectations. What is the real issue here? Is it the number of usable satellites in the constellation — or have the demands of the user community grown?

    Today’s Performance

    Let’s first take a look at the performance relative to the current constellation. The GPS Standard Positioning Service (SPS) Performance Standard identifies 24 nominal orbital slots for a 24-satellite constellation. In this article, I refer to these as the 24 primary slots. My source material for the slot allocations is the U.S. Coast Guard operational advisories.

    Examining GPS performance over the past three years with respect to satellites in the key orbital slots, we see some interesting trends. Figure 1 presents the average number of satellites on orbit as well as the average number of healthy satellites in the 24 primary slots. A healthy satellite is one that has not been removed from service either due a scheduled outage (satellite and clock maintenance) or from an unscheduled anomaly (for example, degraded clock operation or problems with the spacecraft bus).

    Figure 1. Average number of healthy satellites on orbit (blue) and average number of healthy satellites in the 24 primary slots (red).
    Figure 1. Average number of healthy satellites on orbit (blue) and average number of healthy satellites in the 24 primary slots (red).

    The number of usable (healthy) satellites grew from 26 to about 28 on average, but this has not changed substantially in the past three years. It has varied between 27 and 29 satellites, with no significant upward or downward trend over this period. The number of satellites in primary slots, however, does show a noticeable trend, growing steadily through 2003 until late 2004 when it leveled off, after which it began to decrease. This trend recurs in Figure 2, where we view the same metric in half-year increments.

    Figure 2. Average number of healthy satellites in primary slots, shown in half-year increments.
    Figure 2. Average number of healthy satellites in primary slots, shown in half-year increments.

    The reduced number of filled primary slots stems from unscheduled outages. Scheduled outages have no significant impact on number of satellites usable since the operators typically remove a satellite from service for only a few hours, and such maintenance is performed on the order of once a month per satellite. Unscheduled outages, however, can last days and may require significant effort on the part of the satellite operators to resolve.

    The SPS Performance Standard states that 24 operational satellites must be available on orbit with 0.95 probability (averaged over any day). We see this figure has been met at the 100 percent level over the past three and a half years.

    The SPS Performance Standard further states that at least 21 satellites in the 24 nominal plane/slot positions must be set healthy and transmitting a navigation signal with 0.98 probability (yearly average). This figure is met.

    Figure 3 presents the monthly availability of the primary 24-satellite constellation (blue plot), that is, the percent of time over a month that there is a full set of 24 usable satellites in their primary slots. Here we see a marked trend, showing a steady growth from the beginning of 2003 up to the end of 2004, followed by a reduction, but still above the 95 percent level. Figure 3 also shows the monthly availability of 21 or more satellites assigned to the 24 primary slots (red plot), which has been at 100 percent over the past three and a half years.

    Figure 3. Monthly availability of the primary 24-satellite constellation (bue) and of 21 or more satellites assigned to the 24 primary slots (red).
    Figure 3. Monthly availability of the primary 24-satellite constellation (bue) and of 21 or more satellites assigned to the 24 primary slots (red).

    So What’s the Problem?

    If the U.S. government is meeting its commitments, why do users see degraded performance?

    Part of the issue is that the government manages the constellation to a set of metrics that is not up with the times, so to speak. The SPS Performance Standard has a legacy dating from prior to May 2000 when the government imposed Selective Availability, the intentional degradation of the positioning and timing accuracy for civilian users.

    Surveyors back then were considered eccentrics, as it were, living off the crumbs that fell from the table of the basic service. They took advantage of carrier phase tracking, but were content to post-process the data. Work that took days and weeks prior to GPS could now be done in hours. Well, those days are gone, and the push is now to get work done in minutes.

    The familiar adage “Give GPS users a yard and they’ll want an inch” has a corollary: “Give GPS users a process that takes them hours and they’ll want it done in minutes — or seconds!” Users have found they can do their processing much faster, as long as the constellation performance is well above the levels set in the SPS Performance Standard. This has indeed been the case since 2000.

    The GPS program has placed into orbit more satellites than originally anticipated. The general thought was that 27 satellites were sufficient to support the 24 satellite constellation. With 28, 29, and even 30 satellites in orbit, GPS has exceeded expectations, yet now the new expectations are that the government will sustain this level of performance.

    Improvements Happen

    The U.S. Air Force has made significant improvements in GPS operations as well in recent years. The satellite operators have become more user-focused. Prior to taking a satellite offline for maintenance, operators examine the effect of its removal to users worldwide. Also, they have adjusted operational procedures such that anomalies that once took hours to correct are now resolved within minutes. These improvements have directly benefited users, yet despite this, the mindset of the GPS operators is still to provide the service identified in the SPS Performance Standard, which is not the same as day-to-day service that users have come to expect.

    The Presidential Policy on National Space-Based Position, Navigation and Timing (PNT), issued in December 2004, provides high-level guidance on what service users can expect. Among its goals for space-based PNT, the policy states:

    • provide uninterrupted availability of positioning, navigation, and timing services
    • meet growing national, homeland, economic security, and civil requirements, and scientific and commercial demands.

    What “availability” is assumed in the first goal? Is it availability for users employing the 5-degree mask angle (as defined in the SPS Performance Standard), or is it the more stringent demand of mask angles at 10 degrees or higher? Is it availability of four satellites in view to support the generic user or that of six satellites in view to support receiver autonomous integrity monitoring with fault detection and exclusion?

    What “demands” mentioned in the second goal are to be met? Are they the demands of the precision farmer and surveying community? Or perhaps the tighter requirements of the urban user?

    This policy also states that the government will improve the performance of space-based positioning, navigation, and timing services. This implies that the service identified in the SPS Performance Standard will need to be changed to accommodate these improvements.

    To some extent, the U.S. government can only go so far in meeting user needs with the current system. The maximum number of satellites today’s operational control system can support is 30. As of the time of this article, GPS had 29 operational satellites on orbit, although at any given time not all are usable, due to necessary maintenance or unscheduled downtime. Whenever satellites are set unusable, the satellite operators look at the resulting performance, comparing it to the SPS Performance Standard. This is where the issue lies. The SPS Performance Standard assumes a generic user with only a 5-degree mask angle, yet this one assumption no longer represents a significant class of GPS users: the precision users.

    If there are 29 satellites available, why is a reduction to 27 such a big issue? Today’s GNSS users are more demanding than the users were even five years ago. Accuracy is the thing, and real-time accuracy is the most important thing. Today GNSS is used in precision applications such as agriculture, surface mining, and seismic drilling. To get the needed accuracy, users of GNSS exclude low-elevation angle satellites to mitigate the effects of the atmosphere. They set the mask angle in their receivers to 8, 10, even 12 degrees. This higher mask angle reduces the number of available satellites to the users, and correspondingly the dilution of precision and associated positioning error goes up, as illustrated in Figure 4.

    Figure 4. Picture of DOP performance over various mask angles for June 5, 2006.
    Figure 4. Picture of DOP performance over various mask angles for June 5, 2006.

    Is GPS Properly Managed?

    So the issue becomes, is the U.S. Air Force managing the constellation in the best interests of all of its users? Is keeping older satellites in orbit the best policy, and asking the satellite operators to do the best they can with the constellation provided to them? Or is it better to expend taxpayer dollars to replace the older, yet still operational, satellites with newer satellites?

    From a user’s perspective, the newer satellites are better — far better than the older satellites in range accuracy, health, and resistance to integrity failures. The increased reliability produced by their redundant systems also acts as insurance against longterm failures of GPS.

    Today’s users do expect more from GNSS. While today they have but one choice, in the future they will have at least two others, as Galileo and GLONASS come online. So it is important for the U.S. government to continue to adapt GPS operations to support its current user base.

    Recommendations

    There is no easy solution to the problem of ensuring that GPS continues to meet today’s user’s needs, since the field of users and applications is becoming more diverse and demanding. For many, the preferred answer is to launch more satellites, keeping the level at 30 satellites, but there are significant cost implications with this approach.

    On the other hand, relying on an aging constellation to remain operational is also fraught with peril. Many satellites are on their final legs, with key components on a single point of failure. The clocks onboard the satellites are not as stable as they used to be, and require considerable attention from the satellite operators. The cost of losing satellites is significant for certain sectors in our economy, the sectors that employ precision GNSS. In the distant future this problem will be eliminated through the diversity of fully operational Galileo and GLONASS constellations, but for now the issues are immediate and real.

    The U.S. government can and should take several steps to better address the increasing demands on GPS:

    • The government should update the SPS Performance Standard to accommodate other classes of users and bring the metrics up to date with respect to current performance.
    • The satellite operators should refine their assessments of user impact to include a view of how special classes of users are impacted. In particular this should include precision users and aviation applications. This involves using higher mask angles in their assessments and incorporating receiver autonomous integrity monitoring (RAIM) availability.
    • The decision-making authorities in GPS should continue to support an aggressive program to replace aging satellites.

    Such improvements will continue to benefit GPS users worldwide, and help ensure the U.S. government’s goal of providing the best PNT service available.

    John W. Lavrakas is a consultant in satellite navigation. He has spent the past 26 years in GPS, working in satellite command and control, user operations, GPS receiver development, and satellite navigation performance analysis. Contact him at [email protected].

  • NDGPS Heads Toward the Budget Chop Block

    Nationwide Differential GPS (NDGPS) heads toward the budget chop block. Its 2007 allocation has been scaled back to zero.  As in aught, nought, zot.

    NDGPS, also referred to as the Coast Guard differential system, made its first NDGPS broadcast in the early-to-mid-90s. After years of industry talk about real-time DGPS, NDGPS was the first, mostly reliable source of real-time corrections that was free of charge (via your tax dollars).

    Support from the mapping and non-aviation navigation markets pushed the government (the Department of Transportation took the lead) to continue the build-out of the NDGPS network. Each year, more sites were added, and NDGPS became a solid resource for folks wanting a free source of DGPS corrections that delivered meter-level accuracy. They just had to buy the hardware (beacon receiver and antenna) to use it. Today, more than ten years later, there are more than 80 broadcasting sites — and more planned — spread out over the continental United States, Alaska, and Hawaii, providing free DGPS coverage used by hundreds if not thousands of users on a daily basis.

    A successful program?

    Up to and including FY 2006, Congress and the President allocated a substantial annual budget for NDGPS improvements, operations, and maintenance. For example, the FY 2006 budget came to approximately $10 million — half of what was originally requested. Almost $100 million more is needed to complete the network build-out. After that, about $9.2 million is needed annually to operate and maintain the system.

    However, unlike years past, this years budget (FY2007) for NDGPS improvements, operations and maintenance is………………….$0 (zero). The rumor mill says that since there’s no money to operate and maintain, some sites may actually be shut down. That’s not the case according to the Department of Transportation, however.

    “The FY06 budget provides approximately $10 million for the NDGPS program.  These funds will be used to operate and maintain the existing NDGPS system through October 1, 2007”, says Steven Kulm, Director, Office of Pubic Affairs for the DOT’s Federal Railroad Administration.

    So that means no money for new sites that were planned for FY 20007, and no money for developing new technologies such as High Accuracy (HA)-NDGPS.

    Wow, what went wrong?

    The problem facing NDGPS today, in my opinion, is the lack of a “killer app.” In other words it’s a neat tool and serves hundreds (if not thousands) of people on a daily basis, but if it disappeared tomorrow, life would go on. Therefore, when it comes to cutting the budget during tight times, programs like NDGPS are prime targets.

    Now, I’m guessing the DOT may say that PTC (Positive Train Control) may be the killer app for NDGPS because it reportedly will save the railroad industry “billions” each year. That may be so, but if it was that easy to sell, then no one would be hacking away at the relatively puny NDGPS budget.

    Putting the pressure on NDGPS is the Federal Aviation Administration’s (FAA) Wide Area Augmentation Service (WAAS) program. Like NDGPS, WAAS is a free government service providing corrections to improve accuracy and reliability of GPS positioning. The FAA began developing WAAS in the mid-90s and it was declared operational in July 2003.

    WAAS has two things going for it that NDGPS doesn’t.

    • A killer app. The future of aviation navigation is based squarely on GPS, and WAAS is an integral part of that program. The FAA is banking so heavily on GPS and has so many initiatives based on GPS, that there is no way it can back its way out of the program…and it shouldn’t. There are so many valuable uses for GPS in aviation that it boggles the mind. Some applications are focused on efficiency (e.g. better traffic control and throughput) and others are safety-of-life driven (e.g. situational awareness such as ADS-B). Because of this, the FY 2007 budget for WAAS is close to $100 million.

    • WAAS is easy to use for non-aviation folks like us. It’s a no-brainer and it comes standard on every GPS receiver you purchase today. Whereas NDGPS requires the use of additional hardware (a 300khz receiver) and a separate antenna, no additional hardware or software is required to use WAAS. Using WAAS is virtually automatic. NDGPS receivers will never reach that level of simplicity. Good quality NDGPS antennas, by their nature, are bulky and I have serious doubts that any company will attempt to design an NDGPS receiver-on-a-chip (some have tried and failed), especially at this late stage of the game. With those two strikes, consumer GPS units will never incorporate NDGPS technology. And thus NDGPS will never achieve mass-market status like WAAS already has.

    Availability

    The foothold that NDGPS-supporters are hanging onto is the issue of the correction availability. Whereas WAAS is satellite-based and is dependent on line-of-sight between the user and the broadcast satellite, NDGPS broadcasts corrections on the 283-325khz band and does not require line-of-site between the user and the transmitter. It can even be received inside some buildings. However, signal propagation is interrupted by rough terrain and in metro areas, and ambient radio interference can interrupt the signal too. Although it’s not a true national service, there are certainly areas where the NDGPS can be received and WAAS can’t. The reverse can be stated also.

    Accuracy

    The NDGPS vs. WAAS debate in the surveying/mapping community has been a lively one this past year. This is due largely to major GPS manufacturers introducing professional mapping GPS receivers that use WAAS to achieve meter-level accuracy. Although NDGPS has the capability of being more accurate than WAAS, meter-level accuracy seems to be good enough for most mapping applications given the additional expense and equipment overhead required to use NDGPS. Think about it: Garmin discontinued their NDGPS receiver product line and now every GPS product they sell is WAAS-enabled.

    Fate

    Clearly, from a funding perspective WAAS has won the battle for now and has NDGPS back on its heels. The next twelve months (or less) will determine the direction of the NDGPS program.

    “At this time, the U.S. Department of Transportation is deliberating how to administer the program in the future.  We anticipate the Administration’s proposed FY08 budget (to be released in early 2007) will provide guidance on what the future of NDGPS will be,” says Kulm.

    The challenge for NDGPS-supporters in the next few months is to not let the purse-string holders forget about NDGPS and its niche applications. If it’s business-as-usual in FY 2007 and the purse-string holders don’t feel the pain, what are the odds they’ll throw money at NDGPS in FY 2008?

    — Eric Gakstatter

  • No Joy in Surveyville

    I have something to say about our mainstay — GPS — and its troubles of late. So many people in the survey/mapping community have asked me when this problem is going to be resolved. The problem is there aren’t enough healthy satellites for surveying community to use.

    I think the GPS decision makers don’t believe there is a problem because
    with a clear sky, you still get a 3D position, anytime, anywhere in the world.
    Herein lies the problem: we don’t usually work in clear-sky environments.
    Neither do consumer GPS users. My Honda Odyssey GPS navigation system doesn’t
    get a GPS position fix nearly as much as it used to.

    (Just to clarify: I’ve spent the past 16 years in the GPS survey/mapping
    industry using many brands of GPS equipment and software. My first ten years
    in GPS were spent as a product manager and the last six years as a GPS user
    and consultant. I’m a non-partisan advocate for the GPS user community.)

    The fact is that GPS has suffered more outages to key satellites (or satellites
    in key orbital slots) in the past year than it has since GPS was declared operational
    in the early 90’s.

    PRN 25 has behaved like a legacy Jaguar automobile for the past 6+ months:
    an hour shut down for maintenance for every hour it has spent operational.
    Of course I’m exaggerating, kind of, but I’m sure you understand
    my point if you’ve been using RTK with any frequency for the past 6+
    months. I reported last month that the next GPS satellite launch (scheduled
    for September ’06) would most likely replace PRN 25 according to the
    Chief GPS Liaison at the USCG, Doug Louden. But I’m beginning to doubt
    this, given the precarious slot that PRN 30 occupies and its surprisingly rapid
    decline in health. PRN 30 is on its last clock from what I understand. It isn’t
    as old (9.5 yrs) as PRN 25 (14 yrs), but it’s still two years past its
    design life.

    Other GPS satellites are significantly past their design life and heading
    for failure. You can’t use RTK a full day with the current GPS constellation
    even with every satellite healthy. Take one out that’s in a key slot
    like PRN 25 or PRN 30 and it gets really ugly.

    I think it’s not unreasonable to ask the GPS JPO for a healthy, reliable
    constellation of GPS satellites.

    Delays in launching replacement satellites have been so significant that GPS
    users have quit asking me about new launches. It looks like only one will be
    launched in 2006 and only one will be launched in 2007 — if the schedule
    doesn’t slip further. This is a far cry from the “three-a-year” launch
    plan laid out a few years ago.

    I don’t believe the GPS JPO thinks there is a problem, but you only
    have to look at the new product introductions to understand that there is.  For
    years, most survey-grade GPS manufacturers ignored GLONASS and thought it would
    go away, and GPS would fill the bill. However, in the last 12 months, the number
    of survey-grade GPS manufacturers offering GPS/GLONASS products has at least
    doubled.

    Meanwhile, GLONASS is pushing forward with an aggressive launch schedule.
    Three in December ‘06, three in Q3 ’07 and three in Q4 ’07.
    Furthermore, the two new GLONASS satellites that are already in orbit are reportedly
    to be declared operational in the next 60 days. Granted, only time will tell
    if they can honor that schedule, but even launching half of what they plan
    would be significant.

    So, I’d like to raise my hand from the back of the classroom and say
    the Survey, Construction and GIS industry segments aren’t happy with
    the way GPS is performing these days.

    — Eric Gakstatter

  • Letters to the Editor – July 2006

    Flex Power

    See “Potential Problems for Users of Modernized GPS Signals in Mixed-Mode Operations” by Eric Gakstatter in The System news section, page 15, June issue.

    Dear Mr. Gakstatter,

    The Navstar GPS Joint Program Office (JPO) appreciates receiving inquiries regarding GPS signals, because it often helps us improve the signal documentation. We are particularly glad to field questions from GPS World because of your ability to “spread the word.”

    We have entered a new era. During the 27 years since the first GPS satellite was launched in 1978, there have been only three navigation signals. When the first Block IIR-M satellite was launched on 25 September 2005, the number of navigation signals doubled with the addition of L2C and M codes on L1 and L2. Block IIF satellites will add L5, and Block III satellites will provide L1C. From the original three signals the number will grow at least to eight, not including the fact that L1C, L2C, L5, and the M codes each have two separate components, a pilot carrier and data. Further complicating the mix is the need to multiplex more than the two original signals on L1 and L2 while maintaining a constant total amplitude but permitting power to be shifted from one signal to another, that is, flex power. Also, instead of only two fixed message structures, now there are many, and the new ones are more flexible than before.

    All of these changes are designed to improve performance, and they will. However, it is inevitable that in the process some confusion will result. For example, IS-GPS-200D addresses the phase relationship between L2P(Y) and L2C in one of the notes in Table 3-III (and similarly in paragraph 3.3.1.5) by stating: “The two carrier components on L2 [L2P(Y) and L2C] may not have the phase quadrature relationship. They may be broadcast on the same phase.” It was expected that this would inform users of the two possible phase relationships. However, only when receivers actually observed a phase change during on-orbit testing of the first IIR-M did the full implication of these few words sink in.

    The purpose of such phase flexibility is to optimize a satellite’s power efficiency throughout its life and for each of its signal configurations. Unlike the past where the C/A and P(Y) phase relationship was defined and fixed, the multitude of new signals and their components may take on different phase relationships from time to time and from satellite generation to generation. This flexibility is needed to achieve the best overall results from signal modernization, but it may impose new tasks on user equipment and on differential messaging services.

    For example, the JPO was not aware that L2C user equipment would employ cross-mode phase measurements between legacy (code-aided cross correlation) phase measurements and direct L2C phase measurements. In a common mode system, a very rare phase shift should cause little or no problem. (One question to commercial users is whether the satellite should be taken out of service briefly during such a phase change.) If cross-mode measurements are used, then the phase relationships must be known. This can be done by messaging, such as having the information in an almanac. However, it probably is better and certainly faster for the user equipment to monitor these relationships.

    The JPO has had a long-standing process for dealing with such issues. It is called the Interface Control Working Group (ICWG). Information about the ICWG, including how to join, is available at: http://gps.losangeles.af.mil/engineering/icwg/. The JPO has long wanted more commercial participation so these types of questions can be discussed and resolved and the specifications improved. We invite you and your readers to participate, especially during this time of rapid signal modernization. Thank you.Respectfully, best regards,

    — Mark C. Crews

    U.S. Air Force, Navstar GPS Chief Engineer

  • Potential Problems for Users of Modernized GPS Signals in Mixed-Mode Operations

    PRN 17, the first IIR-M satellite launched in September 2005, began broadcasting the second GPS civil signal, L2C, in December 2005. PRN 17 is the first in the new generation of GPS satellites with a new feature called flex power. According to the U.S. Air Force, flex power adds the capability for the Department of Defense to increase power on both P- and M-code (both military) signals to defeat low-level enemy jamming.

    When flex power was enabled for testing (for a very short period of time), a problem was observed by certain GPS users. This problem was associated with the definition of the phase relationship between L2C and legacy L2 P/Y. In this scenario, users who are operating L1/L2/L2C GPS equipment, in conjunction with legacy L1/L2 GPS equipment, could have a problem maintaining carrier-phase ambiguity resolution with any modernized satellite operating in modes where signal phase relationships are changing or are unknown.

    This is not just a flex power issue, but a potential issue with any new modernized GPS signal if provisions are not included to inform users in real time of signal phase relationships. This is potentially a long-term problem because there will be a mixed set of modernized/legacy signals for an extended period of time, as well as a mixed set of modernized/legacy user equipment. The important thing is that these potential problems can be fixed by broadcasting appropriate data in the GPS navigation messages in a timely manner.

    This fix to this potential problem would slightly change the GPS user interface specifications and add bits for defining the phase relationship between the modernized and legacy signals. This data would have to be added to both the L1 and L2C signals since, for the time being, there is no data on the L2C signals. For L1C, (in the draft L1C specification) the phase relationship between L1C and L1 C/A has been defined. For L2 and L2C interoperability during modernization, a similar parameter to provide the phase relationship between the L2 P/Y and L2C is needed for mixed equipment processing. (Refer to Section 3.5.4.6 subframe 3, page 7 signal phase of the newly released Draft IS-GPS-800 L1C specification dated April 19, 2006.)

    Another possible solution is for L2C-capable receivers in a network to track both L2C and L2 P/Y simultaneously, to directly measure the phase difference between the two phases. However, the drawback is that the more robust L2C signal will be tracked at times when the legacy L2 P/Y cannot &#151 the main reason for implementing L2C in the first place.

    — Eric Gakstatter
    Contributing editor of the Survey & Construction newsletter

  • Potential Problems for Users of Modernized GPS Signals inMixed-Mode Operations

    When the new flex power feature aboard PRN 17, the first IIR-M GPS satellite,
    was enabled for testing (for a very short period of time), a problem was observed
    by certain GPS users.

    PRN 17, the first IIR-M satellite launched in September 2005, began broadcasting
    the second GPS civil signal, L2C, in December 2005. PRN 17 is the first in
    the new generation of GPS satellites with a new feature called flex power.
    According to the U.S. Air Force, flex power adds the capability for the Department
    of Defense to increase power on both P- and M-code (both military) signals
    to defeat low-level enemy jamming.

    When flex power was enabled for testing (for a very short period of time),
    a problem was observed by certain GPS users. This problem was associated with
    the definition of the phase relationship between L2C and legacy L2 P/Y. In
    this scenario, users who are operating L1/L2/L2C GPS equipment, in conjunction
    with legacy L1/L2 GPS equipment, could have a problem maintaining carrier-phase
    ambiguity resolution with any modernized satellite operating in modes where
    signal phase relationships are changing or are unknown.

    This is not just a flex power issue, but a potential issue with any new modernized
    GPS signal if provisions are not included to inform users in real time of signal
    phase relationships. This is potentially a long-term problem because there
    will be a mixed set of modernized/legacy signals for an extended period of
    time, as well as a mixed set of modernized/legacy user equipment. The important
    thing is that these potential problems can be fixed by broadcasting appropriate
    data in the GPS navigation messages in a timely manner.

    This fix to this potential problem would slightly change the GPS user interface
    specifications and add bits for defining the phase relationship between the
    modernized and legacy signals. This data would have to be added to both the
    L1 and L2C signals since, for the time being, there is no data on the L2C signals.
    For L1C, (in the draft L1C specification) the phase relationship between L1C
    and L1 C/A has been defined. For L2 and L2C interoperability during modernization,
    a similar parameter to provide the phase relationship between the L2 P/Y and
    L2C is needed for mixed equipment processing. (Refer to Section 3.5.4.6 subframe
    3, page 7 signal phase of the newly released Draft IS-GPS-800 L1C specification
    dated April 19, 2006.)

    Another possible solution is for L2C-capable receivers in a network to track
    both L2C and L2 P/Y simultaneously, to directly measure the phase difference
    between the two phases. However, the drawback is that the more robust L2C signal
    will be tracked at times when the legacy L2 P/Y cannot — the main reason
    for implementing L2C in the first place.

  • L2C — not just vanilla GPS anymore

    Welcome to the second edition of GPS World’s Survey & Construction e-newsletter. My name is Eric Gakstatter ([email protected]). I’ve spent the past 16 years in the GPS survey/mapping industry using many brands of GPS equipment and software. My first ten years in GPS were spent as a product manager and the last six years as a GPS user and consultant. I’m a non-partisan advocate for the GPS user community.

    This subject of this month’s column is L2C. It’s not just about vanilla GPS anymore. GPS modernization weighs heavily in future of satellite surveying. What does L2C bring to the table? When do the new features become useful enough to start making equipment purchasing decisions? While some of the answers may be obvious, others may surprise you.

    First of all, I’ll preface this column by noting that L2C is only a small part of the Global Navigation Satellite System picture that includes L5, GPSIII,  Glonass (Russia’s satellite system) and Galileo (Europe’s satellite system). Discussing all of the GNSS components is too much for one column so I’ll be chipping away at all of them in the coming months.

    Last September (’05) was a big step for GPS modernization when the first IIR-M satellite was launched…starting the next phase of GPS with a second civilian signal (L2C). Currently, there is one civilian signal (L1 C/A). L2 was designed for military use…although civilian manufacturers have been very resourceful in developing codeless techniques for utilizing L2; therefore making dual frequency receivers (L1/L2) very useful for the user community.

    Basically, L2C can be viewed as an add-on feature to the existing L2 band. In practical terms, L2C will help in two areas:

    • It will allow for user receivers to more accurately correct for the error that is generated when the GPS signal passes through the Earth’s ionosphere. L2C provides manufacturers with a new code, enabling them to address the ionospheric delay in a more direct manner than the codeless techniques used by today’s dual frequency receivers. It will also open the door for non survey-grade GPS manufacturers to design survey-grade dual frequency receivers at a much lower R&D cost with fewer patent blocks.
    • Contrary to popular belief, the L2C signal is weaker (not stronger) than L1 C/A code. The idea that L2C will “punch through the trees” with a stronger signal is incorrect. What L2C does offer is a more robust code structure with improved error-correcting methods that will allow it to be used more effectively in marginal satellite signal conditions than what we experience today. Just how much it will help will only be known when the satellite constellation is in place and the receivers are developed to optimize it.

    These two enhancements will result in more competition in the survey-grade GPS receiver marketplace because survey-grade dual frequency receivers will be easier to design. With increased competition, it’s reasonable to expect more competitive prices. With L2C, you can also expect GPS to perform better in weaker satellite conditions.

    With the benefits of L2C to the survey/mapping market clearly established, when are we going to be able to use it? Well, it’s going to be awhile. The publicized year of 2010 is not realistic as this point. Educated speculation says that 2012 is more likely. Don’t forget that it’s not just a matter of tossing satellites into orbit. There are control and management systems on the ground that need to be developed, tested and rolled-out to make use of the new signal. After the hype generated last September when the first IIR-M satellite was launched, the delays in the follow-up IIR-M launches have been disappointing. For example, the launch scheduled for 1Q 2006 has been pushed out until September…a full year after the first IIR-M.

    From the launch schedule, you can see it’s a bit early to start making equipment purchasing decisions based on L2C. Yes, I think that manufacturers will do their best to exploit a partial constellation of L2C satellites and perhaps there will be some innovative
    developments in that area, but just note that by the time there is a minimum constellation of L2C satellites in orbit, there will be another two or three generations of receivers that will have been introduced to the market.

    I’m at the American Congress on Surveying and Mapping annual conference this week in Orlando. Look for my report on conference news in next month’s issue.

  • WAAS Clarification

     

    The FAA’s announcement (reported in March GPS World) that WAAS in the northeastern United States and eastern Canada may be significantly inhibited by relocation of WAAS-broadcasting satellite AOR-W before the new PanAmSat becomes fully operational in fall 2006 caused unease in some surveying organizations. Based on tests completed last year, before anyone knew that AOR-W would relocate to 142W longitude, these organizations replaced legacy GPS mapping units using post-processing and the Coast Guard NDGPS with high-performance WAAS-enabled mapping receivers.

    The FAA notice doesn’t tell the full story, however. Two new WAAS broadcasting satellites launched last fall. PanAmSat (133W) began broadcasting in test mode with corrections full-time this March, and Telesat (107W) is scheduled to begin the same mode on or around April 1, 2006. The FAA announcement does not take into account either of these broadcasting satellites.

    If these test signals are considered, there will be no degradation in WAAS visibility. In fact, users in the northeastern United States and eastern Canada will enjoy dual WAAS satellite coverage. WAAS satellite visibility in central and western North America has improved in the past 60 days with the new test signals and relocation of AOR-W.

    However, the FAA won’t certify the accuracy/reliability of the new satellites until after extensive testing. Until then, non-aviation receivers may use the signals at their discretion &#151the same mode WAAS operated in prior to its July 2003 commissioning. Also, non-aviation WAAS receivers may not be configured to use the new test signals; check with the manufacturer.

    &#151 Eric Gakstatter, Editor of GPS World’s new Survey &#38 Construction E-Newsletter

  • Welcome to GPS World’s Survey & Construction Newsletter

    Hello, and welcome to the first issue of GPS World’s Survey & Construction Newsletter. You are encouraged to forward this email to your colleagues, and they in turn are encouraged to sign up for their own — free — subscription.

    I’m Eric Gakstatter ([email protected]), your editor on this resource for the survey and construction communities. I’ve spent the past 16 years in the GPS survey/mapping industry using many brands of GPS equipment and software. My first ten years in GPS were spent as a product manager and the last six years as a GPS user and consultant. I’m a non-partisan advocate for the GPS user community.

    The first subject I’ve selected to discuss with you is the FAA’s WAAS program because of the recent and significant changes that have taken place in this program.

    WAAS UP?

    WAAS is one of the most widely misunderstood GPS technologies of today in the survey/mapping marketplace. Recent WAAS broadcasting satellite launches and a WAAS broadcasting satellite relocation along with vague press releases have further muddied the waters. In the interest of panic-relief for survey/mapping WAAS users, a more prudent, in-depth explanation is warranted.

    Recently, I was on a construction site for a project I’m involved with. It was a park-like setting with a lot of drainage and irrigation being laid. From the beginning, I knew the job superintendent was very comfortable with technology. The younger, lean fellow spoke efficiently, carried a laptop and seemed on top of his game when questioned by various owner’s reps and sub-contractors….he even carried a hand-held GPS mapping receiver that he used to map various structures installed throughout the project.

    Of course, I had to talk with him about his thoughts and perceptions of GPS. A part of the conversation went as follows:

    Me: How accurate has that unit been for you?

    Him: Very accurate. Do you see that little airplane on the screen (he points at the screen)?

    Me (looking at his screen and trying to figure out what he’s talking about): Oh, yes.

    Him: That means there’s an AWACS airplane flying near here sending me corrections. You know, the military airplane with the big antenna on it?

    I sighed deeply as the image my mind had built of this young, high-tech construction superintendent faded away.

    That “little airplane” on the screen he was referring to was an indicator that his GPS receiver was using corrections from a Federal Aviation Administration (FAA) Wide Area Augmentation System (WAAS) satellite.

    WAAS is perhaps one of the most widely misunderstood GPS technologies of today in the survey/mapping marketplace and the above conversation is a typical example.

    Further complicating this is the recent announcement by the FAA that WAAS in the northeastern US and Eastern Canada may be significantly affected by the relocation of AOR-W (the US east coast WAAS broadcasting satellite) before the new PanAmSat will be considered fully operational in Fall 2006. See the FAA announcement at http://gps.faa.gov/programs/waas/non-aviationUsers.htm.

    The FAA announcement set off a panic in some organizations that had recently implemented a significant number of high performance, WAAS-enabled mapping receivers to replace legacy GPS mapping units that used post-processing and the Coast Guard NDGPS system. These organizations based their decisions on performance tests completed last year before anyone knew that AOR-W was going to relocate to 142W longitude instead of 98W longitude as originally announced.

    The rest of the story…

    First of all, two new WAAS broadcasting satellites were launched last Fall. One of them (PanAmSat at 133W longitude) began broadcasting in test mode with corrections full-time this month (March). It is expected that the other (Telesat at 107W longitude) is scheduled to begin broadcasting test mode with corrections full-time on or around April 1, 2006. The FAA announcement does not take into account either of these broadcasting satellites.

    If these test signals are considered, there will be no degradation in WAAS visibility. In fact, users in the Northeastern US and Eastern Canada will enjoy dual WAAS satellite coverage. For example, in Montreal, Quebec the two new WAAS broadcasting satellites will be visible at ~28 degrees and ~12 degrees above the horizon. Before the AOR-W relocation, it was the only WAAS satellite visible and it was at ~36 degrees above the horizon.

    WAAS satellite visibility in the central and western US, Mexico and western Canada has improved dramatically in the past 60 days with the new test signals and relocation of AOR-W. For example, in Portland, Oregon, WAAS satellite POR is visible at ~12 degrees above the horizon. The two new WAAS broadcasting satellites and relocation of AOR-W to 142 degrees west longitude now means that three additional WAAS satellites will be visible at ~32 degrees, ~36 degrees, and ~35 degrees above the horizon in the Pacific Northwest.

    The caveat is that the FAA won’t certify the accuracy/reliability of the new WAAS broadcasting satellites for quite some time after extensive testing. Until that time, non-aviation receivers are free to use the test mode signals at their discretion. This is the same mode that WAAS was operating in prior to it’s July 2003 commissioning. Also, your non-aviation WAAS receiver may or may not be configured to use the new test signals. You should check with the manufacturer of your unit.

    Well, I didn’t have the heart to tell him then there weren’t any AWACS airplanes sending him corrections (although I did tell him later). It’s just one more example I’ve encountered of the misinformation floating around about WAAS among survey/mapping professionals. There is not enough space in this issue to debate the advantages/disadvantages of WAAS for survey/mapping usage, but don’t be so quick to dismiss the technology before you fully investigate it’s performance and consider the recent developments.

  • Galileo, GLONASS, and GPS

    By Javad Ashjaee, President & CEO, Javad Navigation Systems

    The launch of the Galileo test satellite marks another very significant and exciting day in the history of navigation. Just as competition between GPS manufacturers rapidly enhanced technology and performance, the competition between the constellation providers should do the same.

    In 1984 a high-end GPS receiver weighed more than 100 pounds, performed poorly, was very difficult to operate, and came with a price tag of about $200,000. Thanks to competition, a high-end GPS receiver now weighs less than one pound, performs significantly better and with greater ease, and costs less than $5,000.

    The benefits of the competition Galileo will bring are already apparent. I believe that the Galileo project had a significant role in removing Selective Availability (SA) from the GPS signal, or at least expedited it. In fact, the removal of SA was announced at the first conference discussing Galileo. The Galileo project also proved to be a positive influence on the GPS modernization project and encouraged faster development of GLONASS satellites.

    Undoubtedly, competing with GPS was the main reason and force behind the creation of GLONASS. GLONASS, in turn, helped the removal of SA from GPS and encouraged the Galileo project. With Galileo, history may very well repeat itself.

    Galileo Advantages. Galileo has several advantages over its historical counterparts. Galileo benefits from more than 30 years of experience with GPS and 20 years with GLONASS. It should be, and most probably will be, more advanced than GPS and GLONASS from the get go. Galileo also has the luxury of international support in general and that of U.S. GPS officials in particular. In contrast, GLONASS was a project of the Cold War and was, at best, ignored by the West. I witnessed current U.S. support for GLONASS, however, a few months ago when coordinating the meeting between U.S. Department of Defense and Russian officials to discuss common GPS and GLONASS signals. With a solid support system and history on its side, Galileo has a promising future.

    That is not to say that Galileo goes unchallenged. GPS and GLONASS started mostly as military projects. The two recent wars clearly demonstrated the significance of such navigation systems as an essential military component, forcing military powerhouses to compete further and faster. Galileo’ motivation, however, is partly military, partly commercial, and partly pride. Satisfying the commercial motivation is a factor that, in my opinion, could hinder the timely progress and success of Galileo. The one-country support of GPS and GLONASS seems to have expedited the decision-making process. Galileo has scattered decision-making centers but seems to be resolving this disadvantage quickly.

    Galileo Opportunities. From the technical side, I do not see any fundamental problem for the interoperability of GPS, GLONASS, and Galileo. GPS and GLONASS signal structure differences are much more drastic than those of GPS and Galileo. Yet we were able to circumvent GPS-GLONASS technical issues relatively easily. The amazing opportunity is the fact that, with Galileo on the horizon, all existing receivers will be obsolete in a few years, and the challenge of competing to make the best user equipment is enormous. I am excited to begin this new round of competition as soon as technical issues and specifications are finalized.

    The competitive, historical, and technical aspects of Galileo help create the face of the future of navigation. GPS is already an established system. It will continue to improve and serve as the backbone of navigation systems for many years to come. GLONASS, too, is halfway there. Russia has strong motivation to complete and support it, and with the price of oil at the current level, it will prove to be a strong competitor sooner than we may think. The recent launch of three GLONASS satellites is a good signal. Galileo is starting solidly. Will China create a fourth navigation system? It has the need to support its military operation and markets and has the financial might to pay for it. I cannot imagine a fifth system, at least not in this century. Multiple navigation systems operating independently help increase public awareness and confidence and open and expand markets quickly.

    A key factor in the success of GPS was the cooperation between GPS authorities and manufacturers. Technical details were readily available to all GPS manufacturers worldwide. In 1983, during development of the first generation of GPS receivers at Trimble, I had daily phone conversations with then Captain Karl Kovach at the GPS Control Segment to ask questions and give him feedback on GPS satellite performances that I was observing. Availability of cost-effective high-quality receivers brings the fruits of the system to the world.

    A gentleman from a GLONASS-related institute recently described the status of GLONASS as “ constellation of 14 satellites and many thousands of Javad receivers.” Generously giving me credit for my past activities, he also pointed to the importance of open, unbiased cooperation within industry. I hope Galileo’ commercial structure and objectives allow all of us to compete to develop user equipments of the highest quality and lowest cost.

    For the recent launches of Galileo and three GLONASS satellites, congratulations to all of us civilian users!

  • Expert Advice: Unhealthy, Unappreciated, Incompletely Understood: The State of Our System

    By Jules G. McNeff

    The Defense Science Board recently released the long-awaited report of its Task Force on the Future of the Global Positioning System. The Task Force conducted its deliberations during the latter half of 2004 and early 2005, a period of significant behind-the-scenes activity bearing on GPS. These activities included international negotiations and agreement, national policy discussions on GPS management, and considerations affecting GPS governance. After a lengthy Department of Defense internal review process, the report was approved for public release in October.

    The Task Force itself represented a remarkable confluence of talent, including experts in GPS design, in military, civilian, and scientific applications of GPS, and in the inner workings of military, government, and industry operations. The insights and guidance of its co-chairs, Dr. James Schlesinger and Dr. Robert Hermann, with their unique combination of experience and personal credibility, lent enormous gravity to the undertaking. Their product illuminates in many ways the critical role GPS plays in our world. As an opening premise, proved throughout the report, it notes that “GPS is vital to the United States and to the DoD because, as a fundamental information system, it provides a common thread of precise position and time throughout our national security and economic infrastructures.”

    One can remember many previous boards and committees that issued recommendations for GPS, and may have built a semblance of awareness but didn’t lead to tangible action. The result, noted from the beginning by the co-chairs, was that the apparently healthy GPS program wasn’t really all that healthy and that the malaise affected virtually all aspects of the program. They urged and the Task Force responded with discussion and findings specifically intended to be actionable and to address the underlying causes of the malaise, which are rooted in long-standing institutional factors that will require reinvigorated leadership and persistent follow-up to correct.

    The Task Force noted a general lack of awareness of GPS role in the national infrastructure on the part of senior leaders in all areas of government. Although many people are aware of narrow aspects of GPS performance in individual applications, very few are truly aware of the breadth of GPS contributions to the national security and economy, nor of the enabling effects GPS has on critical national infrastructures. The Task Force viewed raising that awareness level among the nation’s senior leadership as key to addressing some of the other problems facing the program. Many of the other problems derive from lack of or misplaced management attention that allowed the components of the system to become unsynchronized.

    Unbalanced attention to satellites at the expense of operational control functions and user equipment, annual diversion of funding from GPS to other programs, and delays among all the services in programming funds to equip military forces with improved user equipment are all symptoms of incomplete understanding of the role of GPS in military missions in general. Delays in making new signal capabilities available to users and reluctance to incorporate civil information sources into GPS constellation management are symptoms of incomplete understanding of the role of GPS in domestic and international civil infrastructures. Dilution of and uncertainty about policy and operational authority and responsibility for GPS are symptoms of insufficient appreciation among the most senior leaders for the critical importance stable, coherent policies and clear lines of communication represent to the consistent operation of GPS as a national resource and international utility.

    This was a full slate of weighty issues, and the Task Force members addressed each in their discussions and findings. The report of their efforts has now been published (www.acq.osd.mil/dsb/reports/2005-10-GPS_Report_Final.pdf) and is being briefed at the highest levels of the Defense Department and in other government offices.

    The recommendations it contains can serve as a prescription to make the Global Positioning System more healthy, robust, and vibrant for all of its military and civilian users and applications around the world. But the prescription will only be effective if the report’s messages are received, understood, accepted, and acted upon by those charged with the responsibility to maintain GPS viability. One cannot overemphasize the importance of firm and systematic implementation of the recommendations coupled with focused, dedicated, and persistent follow-up. Otherwise, this uniquely capable and credible Task Force will have seen its efforts wasted and its findings will become just another report.

    Given the undeniable importance of GPS to both the national and international security and economy, that would be a monumental tragedy.

    Jules G. McNeff served for several years in the Office of the Assistant Secretary of Defense and was responsible for DoD navigation systems policy and overall management and oversight of the GPS program. He subsequently worked at SAIC and NASA before joining Overlook Systems Technologies as vice president for strategies and programs. He became a charter member of the Editorial Advisory Board of GPS World in 1990.

  • The View From Here: Mapping Harmony

    By Glen Gibbons

    The Global Positioning System has provided more than a few ironies in its relatively short existence: A system so accurate that, until last year, government policy required operators to degrade the quality of the open C/A-code signal. A navigation instrument more accurate than the maps across which navigators plotted their courses. Early GPS-based car guidance systems that displayed vehicle location in the middle of buildings or lakes.

    But, as with so many other aspects of daily life, what may have seemed funny before September 11 is no longer a laughing matter..

    The need for a better correspondence of location information is underscored by the urgency being given to the Federal Communications Commission’s (FCC’s) five-year-old mandate for enhanced 911 (E911) services. E911 provides mobile telephone users with the same automatic location information (ALI) of emergency calls now en-joyed by users of wireline phones at fixed sites. The benefits of ALI for getting police, firefighters, and ambulances to an emergency quickly are obvious..

    The first phase of E911 implementation — identifying the nearest cell site from which a call comes — only covers less than half of the U.S. population. Implementation of Phase II, which requires much more accurate real-time positioning, was scheduled to begin October 1. Last month, however, the FCC granted extensions to five national wireless carriers for initiating their Phase II plans. The agency still expects carriers to provide all mobile phone users with E911 coverage by the end of 2005..

    Three wireless carriers will employ handset-based assisted-GPS techniques in providing ALI that must be twice as accurate (50 meters versus 100 meters) as the “network-based” positioning that the other carriers have selected. (This should prove interesting in the marketplace. Because the E911 capability imposes no direct cost on customers, why would consumers choose non-GPS equipment and carriers offering substantially less accurate service?).

    Little of the E911 delay stems from unavailability of GPS technology. Upgrading software at switching servers is the primary cause for postponements sought for handset-based systems. Even with the lower accuracy standards, however, carriers with network-based solutions pleaded for more time to get their positioning technology to work..

    After the communications and positioning kinks are worked out of the E911 systems, public safety and commercial location-based service providers will still face an operational dilemma. That is the mismatch between positioning techniques and mapbases and differences among maps discussed earlier. Cartographers have long understood that variations among coordinate systems and datums can make the same latitude/longitude mean different things to different people. But until GPS came along, navigation and tracking techniques were so much cruder that such cartographic variations disappeared inside the error ellipse of the positioning systems..

    Under Phase II, emergency call centers (public safety answering points or PSAPs, in FCC parlance), public safety agencies, and E911 callers need to be on the same page. Use of proprietary mapbases with incompatible grid designs in either paper or electronic format is a recipe for disaster. It will create coverage ambiguities near PSAP boundaries (Which agency should handle the call?) and lead rescuers tens or even hundreds of meters away from injured or imperiled callers. Yet a distinctive reference grid seems like a much less important proprietary feature for competing map vendors than the other information and cartographic design built into their products..

    The Public X-Y Mapping Project has proposed one solution to this mishmash of maps: adoption of a U.S. National Grid (USNG) for Spatial Addressing. The USNG would effectively match up with the Military Grid Reference System (MGRS), taking advantage of that public domain systemyy?s use of the Universal Transverse Mercator (UTM) grid. MGRS is one of the most common datums residing within GPS receivers and could be made the default mode for E911 calls, according to Jules McNeff, one of the mapping project’s principals and a well-known GPS advocate.

    Agreement between civilian and military mapping standards in these days of homeland security concerns probably wouldn’t be a bad idea. And the benefits, of course, would carry over into the commercial realm of value-added location-based services, too..

    The interagency Federal Geographic Data Committee’s standards working group recently recommended adoption of USNG as a preferred national standard. “Effective implementation of USNG on maps and in GPS receivers is the single most important thing [that we] can do to improve emergency response operations nationwide almost immediately,” says McNeff. Readers interested in exploring the USNG proposal can find more details on-line at and.

    Whether it’s USNG or another universal reference system, GPS manufacturers, public safety agencies, commercial service providers, mapmakers, and the general public have a common interest in achieving a GPS-friendly national spatial standard.