The Hibiki processors can take input from up to four frequency GNSS signals simultaneously.
The Hibiki front-end can process up to four GNSS signals for software receivers. (Image: Lighthouse)
The Japanese Quasi-Zenith Satellite System (QZSS) broadcasts GNSS signals in four frequency bands: L1, L2, L5 and L6. Similarly, GPS and GLONASS broadcast in three bands, and the European Galileo and Chinese BeiDou systems broadcast in four bands.
However, many conventional front-ends process only two bands at the same time, and cannot be used for highly specialized applications such as processing multiple signals with different frequencies at the same time.
The Hibiki front-end processor was designed to answer to this GNSS technology demand and is able to process up to four frequency bands simultaneously, the company said.
Hibiki has a high data transfer rate performance using USB 3.0, stably transmitting signal data to the host computer up to 50 million samples per second. This high sampling frequency is much greater than conventional front-end processors, improving L5 signal-receiving performance and reducing multipath.
Raytheon Company’s GPS OCX program is ready for the U.S. Air Force’s launch of the first modernized GPS satellite later this year.
Raytheon’s GPS Next-Generation Operational Control System, known as GPS OCX, is in its final software development phase. This phase focuses on increasing automation and building controls for both L1C, a civilian GPS signal aimed at increasing international access, and M-code, a military GPS signal with better anti-jam capability.
Once complete, the team will begin integration and testing to keep the program on track for full system delivery in June 2021.
The GPS Operational Control System’s launch and checkout system will control launch and early orbit operations and the on-orbit checkout of all GPS III satellites. (Image: Raytheon)
“Our team has two primary goals this year,” said Dave Wajsgras, president of Raytheon intelligence, information and services. “We will support the U.S. Air Force’s GPS III launch this fall and complete the software build for the full operational system by year’s end.”
GPS OCX is the enhanced ground control segment of a U.S. Air Force-led effort to modernize America’s GPS system. The program is implementing 100 percent of DODI 8500.2 “Defense in Depth” information assurance standards without waivers, giving it the highest level of cybersecurity protections of any DoD space system, Raytheon said.
For protection against future cyber threats, the system’s open architecture allows it to integrate new capabilities and signals as they become available.
Because GPS OCX can manage nearly twice the satellites of the current system, it will increase signal strength in hard-to-reach areas like dense cities and mountainous terrain.
Also, advanced automation will free crews to focus on mission-critical tasks such as updating satellite positions more often.
In this month’s column, we travel along the road of PPP development, examine its current status and look at where it might go in the near future
By Sunil Bisnath, John Aggrey, Garrett Seepersad and Maninder Gill
Innovation Insights with Richard Langley
PPP. It’s one of the many acronyms (or initialisms, if you prefer) associated with the uses of global navigation satellite systems. It stands for precise point positioning. But what is that? Isn’t all GNSS positioning precise? Well, it’s a matter of degree.
Take GPS, for example. The most common kind of GPS signal use, that implemented in vehicle “satnav” units; mobile phones; and hiking, golfing and fitness receivers, is to employ the L1 C/A-code pseudorange (code) measurements along with the broadcast satellite orbit and clock information to produce a point position.
Officially, this is termed use of the GPS Standard Positioning Service (SPS). It is capable of meter-level positioning accuracy under the best conditions. There is a second official service based on L1 and L2 P-code measurements and broadcast data called the Precise Positioning Service (PPS).
In principle, because the P-code provides somewhat higher precision code measurements and the use of dual-frequency data removes virtually all of the ionospheric effect, PPS is capable of slightly more precise (and accurate) positioning. But because the P-code is encrypted, PPS is only available to so-called authorized users.
While meter-level positioning accuracy is sufficient for many, if not most applications, there are many uses of GNSS such as machine control, surveying and various scientific tasks, where accuracies better than 10 centimeters or even 1 centimeter are needed. Positioning accuracies at this level can’t be provided by pseudoranges alone and the use of carrier-phase measurements is required. Phase measurements are much more precise than code measurements although they are ambiguous and this ambiguity must be estimated and possibly resolved to the correct integer value.
Traditionally, phase measurements (typically dual-frequency) made by a potentially moving user receiver have been combined with those from a reference receiver at a well-known position to produce very precise (and accurate) positions. If done in real time (through use of a radio link of some kind), this technique is referred to as real-time kinematic or RTK.
A disadvantage of RTK positioning is that it requires reference station infrastructure including a radio link (such as mobile phone communications) for real-time results. Is there another way? Yes, and that’s PPP. PPP uses the more precise phase measurements (along with code measurements initially) on at least two carrier frequencies (typically) from the user’s receiver along with precise satellite orbit and clock data derived, by a supplier, from a global network. Precision, in this case, means a horizontal position accuracy of 10 centimeters or better.
In this month’s column, we travel along the road of PPP development, examine its current status, and look at where it might go in the near future.
In a 2009 GPS World “Innovation” article co-authored by Sunil Bisnath, the performance and technical limitations at the time of the precise point positioning (PPP) GPS measurement processing technique were described and a set of questions asked about the potential of PPP, especially with regard to the real-time kinematic (RTK) measurement processing technique.
Since the 2009 article, we’ve seen a significant amount of research and development (R&D) activity in this area. Many scientific papers discuss PPP and making use of PPP — a search on Google Scholar for “GNSS PPP” delivers nearly 7,000 results, and for “GPS PPP” more than 15,000 results! Will PPP eventually overtake RTK as the de facto standard for precise (that is, few centimeter-level) positioning? Or, in light of PPP R&D developments, should we be asking different questions, such as will multiple precise GNSS positioning techniques compete or complement each other or perhaps result in a hybrid approach?
In almost a decade, have we seen much in the way of positioning performance improvement, where “performance” can refer to positioning precision, accuracy, availability and integrity? Or, to some users, has the Achilles’ heel of PPP — the initial position solution convergence period — only been reduced from, for example, 20 minutes to 19 minutes? From such a perspective, all of this PPP research might not appear to have produced much tangible benefit. Advances have been made from this research and we will explore them here. Also, aside from many researchers working diligently on their own PPP software, there are now a number of well-established PPP-based commercial services — a number that has grown and been affected by the wave of GNSS industry consolidation over the decade. Consequently, there is much more to this story.
This month’s article summarizes the current status of PPP performance and R&D, and discusses the potential future of the technique. In the first part of the article, we will present brief explanations of conventional dual-frequency PPP, recent research and implementations, and application of the evolved technique to low-cost hardware. We will conclude the article with a rather dangerous attempt at near-term extrapolation of potential upcoming developments and conceivable implications.
Conventional PPP
The concept of PPP is based on standard, single-receiver, single-frequency point positioning using pseudorange (code) measurements, but with the meter-level satellite broadcast orbit and clock information replaced with centimeter-level precise orbit and clock information, along with additional error modeling and (typically) dual-frequency code and phase measurement filtering. Back in 1995, researchers at Natural Resources Canada were able to reduce GPS horizontal positioning error from tens of meters to the few-meter level with code measurements and precise orbits and clocks in the presence of Selective Availability (SA). Subsequently, the Jet Propulsion Laboratory introduced PPP as a method to greatly reduce GPS measurement processing time for large static networks. When SA was turned off in May 2000 and GPS satellite clock estimates could then be more readily interpolated, the PPP technique became scientifically and commercially popular for certain precise applications.
Unlike static relative positioning and RTK, conventional PPP does not make use of double-differencing, which is the mathematical differencing of simultaneous code and phase measurements from reference and remote receivers to greatly reduce or eliminate many error sources. Rather, PPP applies precise satellite orbit and clock corrections estimated from a sparse global network of satellite tracking stations in a state-space version of a Hatch filter (in which the noisy, but unambiguous, code measurements are filtered with the precise, but ambiguous, phase measurements). This filtering is illustrated in FIGURE 1, where measurements are continually added in time in the range domain, and errors are modeled and filtered in the position domain, resulting in reduced position error in time.
FIGURE 1. Illustration of conventional PPP measurement and error modeling in state-space Hatch filter, resulting in reduced position error in time.
The result is the characteristic PPP initial convergence period seen in FIGURE 2, where the position solution is initialized as a sub-meter, dual-frequency code point positioning solution, quickly converging to the decimeter-level in something like 5 to 20 minutes, and a few centimeters after ~20 minutes when geodetic-grade equipment is used (at station ALGO, Algonquin Park, Canada, on Jan. 2, 2017). For static geodetic data, daily solutions are typically at the few millimeter-level of accuracy in each Cartesian component.
FIGURE 2. Conventional geodetic GPS PPP positioning performance characteristics of initial convergence period and steady state for station ALGO, Algonquin Park, Canada, on Jan. 2, 2017.
The primary benefit of conventional PPP is that with the use of state-space corrections from a sparse global network, there is the appearance of precise positioning from only a single geodetic receiver.
Therefore, baseline or network RTK limitations are removed in geographically challenging areas, such as offshore, far from population centers, in the air, in low Earth orbit, and so on, and without the need for the requisite terrestrial hardware and software infrastructure. PPP is now the de facto standard for precise positioning in remote areas or regions of low economic density, which limit or prevent the use of relative GNSS, RTK or network RTK, but allow for continuous satellite tracking. These benefits translate into the main commercial applications of offshore positioning, precision agriculture, geodetic surveys and airborne mapping, which also are not operationally bothered by initial convergence periods of tens of minutes.
For urban and suburban applications, RTK and especially network RTK allow for near-instantaneous, few-centimeter-level positioning with the use of reference stations and regional satellite (orbit and clock) and atmospheric corrections. The use of double-differencing and these local or regional corrections allows sufficient measurement error mitigation to resolve double-differenced phase ambiguities. All of this additional information is not available to conventional PPP, limiting its precise positioning performance, but which is considered in PPP enhancements.
Progress on PPP Convergence Limitations
Over the past decade or so, PPP R&D activity can be categorized as follows:
Integration of measurements from multiple GNSS constellations, transitioning from GPS PPP to GNSS PPP;
Resolution of carrier-phase ambiguities in PPP user algorithms — in an effort to increase positional accuracy and solution stability, but foremost in an effort to reduce the initial convergence period; and
Use of a priori information to reduce the initial convergence and re-convergence periods and improve solution stability, making use of available GNSS error modeling approaches.
Unlike relative positioning, which makes use of measurements from the user receiver as well as the reference receiver, PPP only relies on measurements from the user site. This situation results in weaker initial geometric strength, and so the addition of more unique measurements is welcome. To make use of measurements from all four GNSS constellations (GPS, GLONASS, Galileo and BeiDou), user-processing engines must account for differences in spatial and temporal reference systems between constellations and numerous equipment delays between frequencies and modulations. The former can be done so that any number of measurements from any number of constellations can be processed to produce one unique PPP position solution. The latter requires a great deal of calibration, especially for heterogeneous tracking networks and user equipment (antenna, receiver and receiver firmware), most notably for the current frequency division multiple access GLONASS constellation.
FIGURE 3 shows typical multi-GNSS float (non-ambiguity-fixed) horizontal positioning performance at multi-GNSS station GMSD in Nakatane, Japan, on March 24, 2017. As with all modes of GNSS data processing, more significant improvement with additional constellations can be seen in sky-obstructed situations.
FIGURE 3. Typical conventional multi-GNSS PPP float horizontal positioning accuracy for station GMSD, Nakatane, Japan, March 24, 2017 (G: GPS, R: GLONASS, E: Galileo and C: BeiDou).
Related to multi-constellation processing is triple-frequency processing afforded by the latest generation of GPS satellites and the Galileo and BeiDou constellations. More frequencies mean more measurements, although with the same satellite-to-receiver measurement geometry as dual-frequency measurements. Again, additional signals require additional equipment delay modeling, in this case especially for the processing of GPS L1, L2 and L5 observables.
For processing of four-constellation data available from 20 global stations in early 2016, FIGURE 4 shows the average reduction of float (non-ambiguity-fixed) horizontal error from dual- to triple-frequency processing of approximately 40% after the first five minutes of measurement processing. In terms of positioning, this result, for this time period with a limited number of triple-frequency measurements, means a reduction in average horizontal positioning error from 43 to 26 centimeters within the first five minutes of data collection.
FIGURE 4. Average dual- and triple-frequency static, float PPP horizontal solution accuracy for 20 global stations. Data collected from tracked GPS, GLONASS, Galileo and BeiDou satellites in early 2016.
PPP with ambiguity resolution, or PPP-AR, was seen as a potential solution to the PPP initial solution convergence “problem” analogous to AR in RTK. Various researchers put forward methods, in the form of expanded measurement models, to isolate pseudorange and carrier-phase equipment delays to estimate carrier-phase ambiguities. These methods remove receiver equipment delays through implicit or explicit between-satellite single-differencing and estimate satellite equipment delays in the network product solution either as fractional cycle phase biases or altered clock products.
FIGURE 5 illustrates the difference between a typical GPS float and fixed solution (for station CEDU, Ceduna, Australia, on June 28, 2017). Initial solution convergence time is reduced, and stable few-centimeter-level solutions are reached sooner. For lower quality data, ambiguity fixing does not provide such quick initial solution convergence. Fixing is dependent on the quality of the float solution; and, for PPP, the latter requires time to reach acceptable levels of accuracy. Therefore, depending on the application, PPP-AR may or may not be helpful.
FIGURE 5. Typical float (red) and fixed (pink) GPS PPP horizontal solution error at geodetic station CEDU, Ceduna, Australia, on June 28, 2017.
To consistently reduce the initial solution convergence period, PPP processing requires additional information, as is the case for network RTK, in which interpolated satellite orbit, ionospheric and tropospheric corrections are needed since double-differenced RTK baselines over 10 to 15 kilometers in length contain residual atmospheric errors too large to effectively and safely resolve phase integer ambiguities. For PPP, uncombining the ionospheric-free code and phase measurements from the conventional model is required, to directly estimate slant ionosphere propagation terms in the filter state.
In this form, the model can allow for very quick re-initialization of short data gaps by using the pre-gap slant ionospheric (and zenith tropospheric) estimates as down-weighted a priori estimates post-gap — making these estimates bridging parameters in the estimation filter. Expanding this approach, external atmospheric models can be used to aid with initial solution convergence.
FIGURE 6 illustrates, for a large dataset, that applying a spatially and temporally coarse global ionospheric map (GIM) to triple-frequency, four-constellation float processing can reduce one-sigma convergence time to 10 centimeters horizontal positioning error from 16 to 6 minutes. If local ionospheric (and tropospheric) corrections are available and AR is applied, PPP (sometimes now referred to as PPP-RTK) can produce RTK-like results with a few minutes of initial convergence to few-centimeter-level horizontal solutions.
FIGURE 6. Averaged horizontal error from 70 global sites in mid-2016 using four-constellation, triple-frequency processing.
PPP Processing with Low-Cost Hardware
As the impetus for low-cost, precise positioning and navigation for autonomous and semi-autonomous platforms (such as land vehicles and drones) continues to grow, there is interest in processing such low-cost data with PPP algorithms. For example, it has been shown that with access to single-frequency code and phase measurements from a smartphone, short-baseline RTK positioning is possible. It has also been shown that similar smartphone data can be processed with the PPP approach. From the origins of PPP, it may be argued that single-frequency processing and many-decimeter-level positioning performance is not “precise.” But we will avoid such semantic arguments here (but see “Insights”), and focus on the use of high-performance measurement processing algorithms to new low-cost hardware. We are currently witnessing great changes in the GNSS chip market: single-frequency chips for tens-of-dollars or less; and boards with multi-frequency chips for hundreds-of-dollars. And these chips will continue to undergo downward price pressure with increases in capability, and be further enabled for raw measurement use in a wider range of applicable technology solutions. There are now a number of low-cost, dual-frequency, multi-constellation products on the market, with additional such products as well as smartphone chips coming soon.
To process data from such products with a PPP engine, modifications are required to optimally account for single-frequency measurements in the estimation filter, optimize the measurement quality control functions for the much noisier code and phase measurements compared to data from geodetic receivers, and optimize the stochastic modeling for the much noisier code and phase measurements. The single-frequency measurement model can be modified to either make use of the Group and Phase Ionospheric Calibration linear combination (commonly referred to as GRAPHIC) or ingest data from an ionospheric model. Due to the use of low-cost antennas, as well as the low-cost chip signal processing hardware, code and phase measurements suffer from significant multipath and noise at lower signal strengths; therefore, outlier detection functions must be modified. Also, the relative weighting of code and phase measurements must be customized for more realistic low-cost data processing.
FIGURE 7 compares the carrier-to-noise-density ratio (C/N0) values from ~1.5 hours of static GPS L1 signals collected from a geodetic receiver with a geodetic antenna, a low-cost receiver chip with a patch antenna, and a tablet chip and internal antenna, as a function of elevation angle. Received signal C/N0 values can be used as a proxy for signal precision. The three datasets were collected at the same time in mid-September 2017 in Toronto, Canada, with the receivers and antennas within a few meters of each other. The shading represents the raw estimates output from each receiver, while the solid lines are moving-average filtered results.
FIGURE 7. Carrier-to-noise-density ratios of ~1.5 hour of static GPS L1 signals from a geodetic receiver with a geodetic antenna, a low-cost receiver chip with a patch antenna, and a tablet chip and internal antenna, as a function of elevation angle.
Keeping in mind the log nature of C/N0, the high measurement quality of the geodetic antenna and receiver are clear. The low-cost chip and patch antenna signal strength structure is similar, but, on average, 3.5 dB-Hz lower. And the tablet received signal strength is lower still, on average a further 4.0 dB-Hz lower, with greater degradation at higher signal elevation angles and much greater signal strength variation.
The PPP horizontal position uncertainty for these datasets is shown in FIGURE 8. Note that reference coordinates have been estimated from the datasets themselves, so potential biases, in especially the low-cost and tablet results, can make these results optimistic. Given that only single-frequency GPS code and phase measurements are being processed, initial convergence periods are short and horizontal position error reaches steady state in the decimeter range. The geodetic and the low-cost results are comparable at the 2-decimeter level, whereas the tablet results are worse, at the approximately 4-decimeter level. Initial convergence of the geodetic solution is superior to the others, driven by the higher quality of its code measurements. The grade of antenna plays a large role in the quality of these measurements, for which there are physical limitations in design and fabrication. While geodetic antennas can be used, this is not always feasible, given the mass limitations of certain platforms or the cost limitations for certain applications.
FIGURE 8. Horizontal positioning error (compared to final epoch solutions) for geodetic, low-cost and tablet data processed with PPP software customized for single-frequency and less precise measurements.
Comments Regarding the Near Future
The PPP GNSS measurement processing approach was originally designed to greatly reduce computation burden in large geodetic networks of receivers by removing the need for network baseline processing. The technique found favor for applications in remote areas or regions with little terrestrial infrastructure, including the absence of GNSS reference stations. Given PPP’s characteristic use of a single receiver for precise positioning, various additional augmentations have been made to remove or reduce solution initialization and re-initialization interval to near RTK-like levels. But, to what end?
This question can be approached from multiple perspectives. From the theoretical standpoint, there is the impetus to maximize performance — millimeter-level static positioning over many hours, and few-centimeter-level kinematic positioning in a few minutes — by augmenting PPP in any way necessary. There is the academic exercise of maximizing performance without the need for local or regional reference stations – apparent single-receiver positioning, or truly wide-area augmentation. In terms of engineering problems, we can work to do more with less, that is, decimeter-level positioning with ultra-low-cost hardware, or the same with less, that is, few-centimeter-level positioning with low-cost hardware. And from the practical or commercial aspect, the great interest is for the implementation of evolved PPP methods for applications that can efficiently and effectively make use of the technology.
In terms of service providers, be it regional or global, commercial or public, there is momentum to provide enhanced correction products that are blurring the lines across the service spectrum from constellation-owner tracking to regional, terrestrial augmentation. A public GNSS constellation-owner, through its constellation tracking network, can provide PPP-like corrections and services. A global commercial provider with or without regional augmentation can provide similar services. The key is providing multi-GNSS state-space corrections for satellite orbits, satellite clocks, satellite equipment delays (fractional phase biases), zenith ionospheric delay and zenith tropospheric delay at the temporal and spatial resolution necessary for the desired positioning performance at reasonable cost, that is, subscription fees that particular markets can bear.
Given these correction products, PPP users have a greater ability to access a wide array of positioning performance levels for various new applications, be it few-decimeter-level positioning on mobile devices to few-centimeter-level positioning for autonomous or semi-autonomous land, sea and air vehicles. PPP can be used for integrity monitoring and perhaps safety-of-life applications where low-cost is a necessity and relatively precise positioning for availability and integrity purposes is required. For safety critical and high-precision applications, such as vehicle automation, PPP can be used alongside, or in combination with, RTK for robustness and independence with low-cost hardware. Such a parallel and collaborative approach would require a hybrid user processing engine and robust state-space corrections from a variety of local, regional and global sources, as we are seeing from some current geodetic hardware-based commercial services.
Near-future trends should also include more low-cost, multi-sensor integration with PPP augmentation. Optimized navigation algorithms and efficient user processing engines will be a priority as the capabilities of low-cost equipment continue to increase and low-cost integrated sensor solutions are required for mass-market applications. Analogous to meter-level point position GNSS, lower hardware costs should drive markets to volume sales, PPP-like correction services, and GNSS-based multi-sensor integration into more navigation technology solutions for various industry and consumer applications.
Clearly, the future of PPP continues to be bright.
SUNIL BISNATH is an associate professor in the Department of Earth and Space Science and Engineering at York University, Toronto, Canada. For over twenty years, he has been actively researching GNSS processing algorithms for a wide variety of positioning and navigation applications.
JOHN AGGREY is a Ph.D. candidate in the Department of Earth and Space Science and Engineering at York University. He completed his B.Sc. in geomatics at Kwame Nkrumah University of Science and Technology, Ghana, and his M.Sc. at York University. His research currently focuses on the design, development and testing of GNSS PPP software, including functional, stochastic and error mitigation models.
GARRETT SEEPERSAD is a navigation software design engineer for high-precision GNSS at u-blox AG and concurrently is completing his Ph.D. in the Department of Earth and Space Science and Engineering at York University. His Ph.D. research focuses on GNSS PPP and ambiguity resolution. He completed his B.Sc. in geomatics at the University of the West Indies in Trinidad and Tobago. He holds an M.Sc. degree in the same field from York University.
MANINDER GILL is a geomatics designer at NovAtel Inc. and concurrently is completing his M.Sc. in the Department of Earth and Space Science and Engineering at York University. His M.Sc. research focuses on GNSS PPP and improving positioning accuracy for low-cost GNSS receivers. He holds a B.Eng. degree in geomatics engineering from York University.
FURTHER READING
• Comprehensive Discussion of Technical Aspects of Precise Point Positioning
“Precise Point Positioning” by J. Kouba, F. Lahaye and P. Tétreault, Chapter 25 in Springer Handbook of Global Navigation Satellite Systems, edited by P.J.G. Teunissen and O. Montenbruck, published by Springer International Publishing AG, Cham, Switzerland, 2017.
• Earlier Precise Point Positioning Review Article
“Precise Point Positioning Using IGS Orbit and Clock Products” by J. Kouba and P. Héroux in GPS Solutions, Vol. 5, No. 2, October 2001, pp. 12–28, doi: 10.1007/PL00012883.
“Precise Point Positioning for the Efficient and Robust Analysis of GPS Data from Large Networks” by J.F. Zumberge, M.B. Heflin, D.C. Jefferson, M.M. Watkins and E.H. Webb in Journal of Geophysical Research, Vol. 102, No. B3, pp. 5005–5017, 1997, doi: 10.1029/96JB03860.
“Reduction of PPP Convergence Period Through Pseudorange Multipath and Noise Mitigation” by G. Seepersad and S. Bisnath in GPS Solutions, Vol. 19, No. 3, March 2015, pp. 369–379, doi: 10.1007/s10291-014-0395-3.
“Global and Regional Ionospheric Corrections for Faster PPP Convergence” by S. Banville, P. Collins, W. Zhang and R.B. Langley in Navigation, Vol. 61, No. 2, Summer 2014, pp. 115–124, doi: 10.1002/navi.57.
“A New Method to Accelerate PPP Convergence Time by Using a Global Zenith Troposphere Delay Estimate Model” by Y. Yao, C. Yu and Y. Hu in The Journal of Navigation, Vol. 67, No. 5, September 2014, pp. 899–910, doi: 10.1017/S0373463314000265.
“External Ionospheric Constraints for Improved PPP-AR Initialisation and a Generalised Local Augmentation Concept” by P. Collins, F. Lahaye and S. Bisnath in Proceedings of ION GNSS 2012, the 25th International Technical Meeting of the Satellite Division of The Institute of Navigation, Nashville, Tennessee, Sept. 17–21, 2012, pp. 3055–3065.
“Integer Ambiguity Resolution on Undifferenced GPS Phase Measurements and Its Application to PPP and Satellite Precise Orbit Determination” by D. Laurichesse and F. Mercier, J.-P. Berthias, P. Broca and L. Cerri in Navigation, Vol. 56, No. 2, Summer 2009, pp. 135–149.
“Resolution of GPS Carrier-phase Ambiguities in Precise Point Positioning (PPP) with Daily Observations” by M. Ge, G. Gendt, M. Rothacher, C. Shi and J. Liu in Journal of Geodesy, Vol. 82, No. 7, July 2008, pp. 389–399, doi: 10.1007/s00190-007. Erratum: doi: 10.1007/s00190-007-0208-3.
“Isolating and Estimating Undifferenced GPS Integer Ambiguities” by P. Collins in Proceedings of ION NTM 2008, the 2008 National Technical Meeting of The Institute of Navigation, San Diego, California, Jan. 28–30, 2008, pp. 720–732.
• Precise Positioning Using Smartphones
“Positioning with Android: GNSS Observables” by S. Riley, H. Landau, V. Gomez, N. Mishukova, W. Lentz and A. Clare in GPS World, Vol. 29, No. 1, January 2018, pp. 18 and 27–34.
For Black History month in February, the Free-Lance Star of Fredericksburg, Virginia, profiled a woman few of us know about — Gladys West.
Capt. Godfrey Weekes, then-commanding officer at the Naval Surface Warfare Center Dahlgren Division, described to the newspaper the “integral role” played by West.
Gladys West’s work helped develop the Global Positioning System. (Photo: U.S. Navy)
“She rose through the ranks, worked on the satellite geodesy and contributed to the accuracy of GPS and the measurement of satellite data,” he said. “As Gladys West started her career as a mathematician at Dahlgren in 1956, she likely had no idea that her work would impact the world for decades to come.”
West collected data from the satellites, focusing on information that helped to determine their exact location as they transmitted from around the world. Data was entered into large-scale super computers that filled entire rooms, and she worked on computer software that processed geoid heights (precise surface elevations).
As a girl growing up in Dinwiddie County, Virginia, Gladys knew she didn’t want to work in the fields or a tobacco factory like her parents did.
“I was ecstatic,” she said of her career. “I was able to come from Dinwiddie County and be able to work with some of the greatest scientists working on these projects.”
Jim Colvard, technical director at NSWC Dahlgren from 1973 to 1980, knew West as a student in his graduate program and as a professional employee. “She was an excellent student and a respected and productive professional,” he wrote in an email. “Her competence, not her color, defined her.”
Gladys West, at Dahlgren with Sam Smith in 1985, looks over data from the Global Positioning System she helped develop. (Photo: U.S. Navy)
While it sounded like good news at first, once the real results were plucked from the slurry, they resemble nothing so much as the same old uncertainties.
About the future of GPS III, I mean.
Additional money allocated by Congress to the Department of Defense budget — $80 billion on top of $549 billion for FY18 and $85 billion added to the $562 billion previously set for FY19 — alleviated longstanding worries about sufficient funding for GPS. The satnav system has always been at the mercy of raiding by other military programs, over budget and cash-strapped. At least that pressure will be off, we thought.
But money on paper does not always lead, expeditiously or at all, to boots on the ground or birds in the sky. The funds come with an enthusiasm for reorganizing everything. To streamline it, effectivize it, make it more…businesslike.
In the case of the Space and Missile Defense Command, this means eliminating three top-level decision-making positions, and designating someone other than the secretary of the Air Force as responsible for space budget prioritization.
Congress stopped short of its initial idea, which was to establish a whole new department for military space activities, separate from and equal to the Air Force. But there’s no doubt that the 2018 National Defense Authorization Act strongly rebukes the current way of doing space things in the Pentagon and the Air Force — while proffering more money to do them.
Like many announced initiatives to drain the swamp elsewhere, this one has just drawn in more murky water. It may take four or five years, according to some with Pentagon insight, to figure out new procedures, lines of command and the actual fulcrums of decision-making. In the meantime, matters will — you guessed it — slow down.
All this as the GPS III launch schedule and OCX next-generation ground-control readiness slide rightward, and military GPS user equipment can count on at least a decade to even partially update. Mind you, Increment I of the new M-code cards is not yet complete. Once it is, the three major contractors who have developed them will compete to sell their varying versions to the different branches of the military, the different arms of those branches, and the different weapon systems (716, by GAO’s count) operated by those arms.
The pursuit of increased resilience in space, clearly destined to be a contested domain in the event of large-scale international conflict, may actually inhibit itself in the near term.
As noted below, the previous wielder of this space has relinquished satnav matters to take up, as he says, “some unfinished business with life.” I owe him a great debt. He gave me my start here.
Glen Gibbons Retires
Glen Gibbons announced his retirement from active leadership of Inside GNSS magazine at the end of last year, when he wrote he was “promoting myself to Editor Emeritus.” Gibbons was editor of GPS World from 1989 to 2005, and editor and publisher of Inside GNSS from 2006 through 2017.
In 2003, he received the U.S. Institute of Navigation’s Norman P. Hays Award for inspiration and support contributing to the advancement of navigation. GPS World joins all those around the industry and the international GNSS community in recognizing and thanking him for his many years of coverage of and service to the field of positioning, navigation and timing.
The U.S. Air Force has awarded a GPS III satellite launch contract to SpaceX. This is the third GPS III launch contract awarded; the previous two also were awarded to SpaceX.
SpaceX will receive a $290,594,130 firm-fixed-price contract for launch services to deliver three GPS III missions (1 base and 2 options) to the intended orbit using two Evolved Expendable Launch Vehicles (EELVs).
A SpaceX Falcon 9 rocket lifts off from Space Launch Complex 4E at Vandenberg Air Force Base, California, Jan. 14, 2017. (Photo: SpaceX)
The launch contract provides the government with a total launch solution for the GPS III mission, including launch vehicle production, mission integration, launch operations and spaceflight certification. The launches will take place from Cape Canaveral Air Force Station or Kennedy Space Center, Florida.
The GPS III missions are planned to launch between late 2019 and 2020.
“The three GPS III missions will deliver sustained, reliable GPS capabilities to America’s warfighters, our allies and civil users,” the U.S. Air Force said in a statement. GPS provides positioning, navigation and timing service to civil and military users worldwide.
In a second launch services contract, United Launch Alliance has been awarded a $351,839,510 firm-fixed-price contract for launch services to deliver Air Force Space Command (AFSPC)-8 and AFSPC-12 satellites to the intended orbit.
This is the fourth competition under the current Phase 1A procurement strategy. Both launch service contract awards strike a balance between meeting operational needs and lowering launch costs through reintroducing competition for National Security Space missions, according to Los Angeles Air Force Base, which made the announcement.
“The competitive award of these two EELV launch service contracts directly supports Space and Missile Systems Center’s mission of delivering resilient and affordable space capabilities to our nation while maintaining assured access to space,” said Lt. Gen. John F. Thompson, U.S. Air Force Program Executive Officer for Space and SMC commander.
SpaceX won two previous GPS III launch contracts, one awarded in March 2017 and one in April 2016.
Wingtra has officially launched the WingtraOne PPK high-precision mapping drone. Wingtra said its drone, which features vertical take-off and landing, is designed to set a new benchmark for large-scale surveying and mapping applications.
WingtraOne PPK offers large area coverage, ultra-high accuracy and brilliant image resolution. It features an advanced PPK module and high-quality cameras like the 42-megapixel full-frame camera Sony RX1RII, it is now possible to reach down to 1-centimeter absolute accuracy in aerial mapping.
To prove this accuracy claim, the Wingtra team performed test flights in a gravel quarry. The process was documented and is now explained in a white paper on the company website.
Conventional drone mapping on centimeter accuracy requires ground control points (GCPs) to correct the final map. Besides requiring additional surveying equipment and being extremely time consuming, setting up GCPs might be downright risky or just not possible in the area of interest.
More advanced solutions achieve similar levels of accuracy by using GPS correction technology for the georeferencing of the aerial imagery: namely RTK (real-time kinematics) or PPK (post processed kinematics).
RTK requires real-time base station connectivity and corrects GPS signals during the flight, while PPK corrects them after the flight and therefore offers greater robustness and consistency.
Moreover, PPK is independent from base stations or base station networks. It is highly reliable, accurate and time saving to use, Wingtra said. Neither special flight preparations nor intensive post-processing steps are required to achieve down to 1-cm accurate aerial maps.
Swift Navigation, a San Francisco-based tech firm that is building centimeter-accurate GPS technology for autonomous vehicles, has released the latest firmware upgrade to its flagship product, the Piksi Multi GNSS module.
Firmware update 1.4 is the fourth improvement since Piksi Multi began shipping one year ago.
Duro – Piksi enclosure.
The firmware release also enhances Duro, the ruggedized version of the Piksi Multi receiver housed in a military-grade, weatherproof enclosure designed specifically for outdoor deployments.
The upgrade is available at no cost to Piksi Multi and Duro users and provides full support for GLONASS, in addition to the GPS satellite constellation. Access to dual constellations greatly improves availability, reliability and range between GNSS base and rover devices, the company said.
According to Swift Navigation, the firmware release also adds NMEA GGA output capability to existing NTRIP (Networked Transport of RTCM via Internet Protocol), enabling Piksi Multi and Duro to seamlessly position by sending and receiving data from CORS (Continuously Operating Reference Station) base stations over the Internet.
GLONASS + GPS support. The new firmware provides full and reliable integer ambiguity resolution for GLONASS (G1/G2) + GPS (L1/L2C) for use with Swift Navigation products and most third-party base stations.
RTCM 1230 and 1033 interoperability. Thisallows Piksi Multi and Duro to communicate with many third-party industry-standard receivers.
NTRIP NMEA GGA support. This enables network RTK solutions and virtual base network (VBN) services.
Additional Fundamental Improvements
Full position and velocity covariances now published for advanced users for use in autonomous systems.
Carrier phase reacquisition was improved by seconds.
Fix reliability and availability was enhanced for extremely precise positioning accuracy in SPP mode was increased when RTK is not available.
“The 1.4 firmware release is a step change improvement for our customers deploying Piksi Multi and Duro,” said Fergus Noble, CTO of Swift Navigation. “The addition of a second GLONASS satellite constellation enhances reliability and centimeter-accurate positioning in challenging environments, better supporting ground applications in precision agriculture, robotics and autonomous vehicles. Best of all, our customers benefit from new features delivered as a software update, at no additional cost and with no changes to their Piksi Multi or Duro hardware, underscoring Swift’s commitment to continuous improvements in our product lines.”
Geneq has launched the SXblue Premier GNSS receiver, which is available in a submetric version (GNSS) or centimetric version (real-time kinematic, RTK).
The new SXblue Premier GNSS receiver is equipped with the Pacific Crest Maxwell 6 Trimble technology with BD910 (GNSS version) and BD930 (RTK version) OEM boards, delivering 220 channels to acquire and track GNSS signals from all constellations in view. It makes effective use of GPS, GLONASS, Galileo, BeiDou, QZSS and SBAS signals for outstanding highly precise positioning.
The SXblue Premier is small and light weight, and rugged for field work. It is equipped with dual mode for Bluetooth V2.1 and Bluetooth V4.0, ensuring the unit’s wireless communication with any Android or Windows terminal. With its two models, the user will have large efficiency and flexibility on the field either with SBAS corrections or RTK reference networks.
In addition, SXblue Premier can be configured for Wi-Fi hotspots, allowing users to connect and access a web management platform. It also can be used as a data link, providing a quick connection to the internet to receive corrections from reference station (CORS) networks so that it can process RTK measurements.
With its internal memory using an 8-GB solid state disk, SXblue Premier provides enough storage space for field data collection or raw data recording for a high data sampling rate.
Multiple compatible software programs — including FieldGenius, Carlson, Collector for ArcGIS — will meet the users’ diverse need, making SXblue Premier more powerful and flexible.
The European GNSS Agency (GSA) has officially launched the equipment for dual frequency Galileo, GPS and EGNOS project (EDG²E) with a consortium led by Thales. The four-year project intends to develop a dual-frequency multi-constellation receiver, enabling enhanced navigation capabilities, support standardization and certification preparation, and facilitate the expected increase in air traffic, both in Europe and globally.
The prototype EDG²E receiver use GPS and Galileo signals as well as those from the European SBAS multi-constellation EGNOS. The project aims to achieve a prototype demonstration by 2021. At the end of the EDG²E project, the first SBAS dual-frequency GPS+Galileo receivers for aviation will be ready for final development and use in the aviation sector and in other safety-critical applications. Fully achieved receivers are foreseen to be installed in commercial aircraft by 2025.
EGNOS, certified for use in aviation since February 2011, is developing its own next generation, called EGNOS V3, to further enhance performance by complementing both the EU Galileo and the US GPS satellite navigation constellations.
“EGNOS v3 will provide aviation users with an increased quality of services, better accuracy and extended coverage area among other key performance indicators” said Jean-Marc Pieplu, GSA head of the EGNOS Services Programme. “Fundamental Element Programme is a medium that supports development of terminals and antennae fostering use of E-GNSS in all domains. In this perspective,EDG²E is an important step for GSA as it will contribute to availability of high technology products on the aviation market, taking benefit of Dual Frequency Multi Constellation feature offered by EGNOS v3.”
The consortium includes Thales, Thales Alenia Space and ATR, as well as contributions from Dassault Aviation and the French Civil Aviation Authority.
Feature image courtesy of the European Space Agency (ESA).
In a notice advisory to NAVSTAR Users (NANU), the U.S. Coast Guard Navigation Center announced that starting March 7, after 22:00 Zulu hours, GPS will transition satellite SVN34 (PRN18) into the broadcast almanac for all satellites.
The almanac transition, one satellite at a time, will require approximately 24 hours to complete.
Also, on approximately March 8, SVN34 will resume transmitting L-band utilizing PRN18. SVN34/PRN18 will be unusable until further notice.
Future NANUs will notify users of any changes to the above stated status.
Contact the navigation center for more information:
Results from new tests of the Satellite Time and Location (STL) service, using equipment configurations with a differential source and with a more accurate OCXO clock, show timing accuracy of 160 nanoseconds.
The STL service uses a signal from the low-Earth orbit (LEO) Iridium constellation.
In 2016, Satelles demonstrated sub-microsecond timing using a stand-alone TCXO-based receiver (see “Innovation: Navigation from LEO,” July 2017 GPS World).
New testing employed three different configurations of equipment, services and environment, including a Stanford Research Systems (SRS) rubidium vapor frequency reference, based on the PRS10 module, and a Satelles Evaluation Kit (EVK2) STL receiver, comprising a Maxim RF chip, Xylinx Spartan-3 FPGA, TI dual-core DSP chip, and internal OCXO (oven-controlled crystal oscillator) or external clock.
Parameters and equipment for the three tests are:
Optimal. Outdoor antenna, Rubidium clock powered on for months prior to data collection, receiver configured in static mode with a known location, and high-quality antenna.
Sub-optimal. Indoor antenna, Rubidium clock powered on six hours prior to data collection, receiver configured in static mode with an unknown location, and low-quality antenna.
Three independent receivers collecting data, receiver on-board OCXO, indoor antenna, receiver configured in static mode with an unknown location, low-quality antenna. Tests performed: 10 days with no local reference station running; 10 days with local reference station, 20-kilometers away from test receivers, providing timing corrections to STL ground segment.
See Figure 1 for more extensive test results. Also see a previous article.
FIGURE 1. OCXO timing result with base station.
The 66-satellite Iridium LEO constellation transmits overlapping spot beams, which provide location-specific data that changes every few seconds.
Air Force Issues GPS III Follow-on Contract
The U.S. Air Force Space Command released its request for proposals to build 22 new GPS III satellites, called the GPS III Follow-On Phase 2 contract.
The contract will be awarded to a single bidder, and has an estimated dollar value of $10 billion including all options.
Phase 2 is planned as a single, predominantly fixed-price incentive-type contract awarded via full and open competition for production of 22 GPS III satellites. Deadline for proposals is April 16. Delivery of the first satellite is to be in 2026.
Phase 1 contracts awarded in May 2016 to Boeing, Northrop Grumman and Lockheed Martin (builder of the first 10 GPS III satellites) “determined that viable, low-risk, high-confidence sources exist to conduct a full and open competition for Phase 2, the production of 22 GPS III SVs [space vehicles] starting in the FY19 timeframe.”
BeiDou’s Long March
On Feb. 12, BeiDou-3 28 and 29 were launched into medium-Earth orbits, following the launch of a pair of BeiDou satellites on Jan. 11. The satellites form part of a third phase of BeiDou deployment, taking BeiDou coverage from regional to covering the countries along the Belt and Road initiative by the end of 2018, and global by 2020.
