A new receiver for GPS and other GNSS improves time-synchronization accuracy in areas with severe reception conditions, such as among buildings and in mountainous areas.
Furuno plans to begin sales of the new GF-88 series time synchronization GNSS receivers in April 2019, and to deploy it widely in fields such as 4G/5G mobile base stations, financial trading, power grids and data centers.
The GF-88’s new algorithm makes use of multipath signals, those reflected or diffracted from buildings and other structures, which previously inhibited accuracy of time synchronization.
By integrating a new satellite signal selection algorithm developed by NTT into Furuno’s time synchronization GNSS receiver, in addition to signals from satellites in line-of-sight locations, multipath signals can be used to reduce time error, the companies said.
In a real multipath reception test environment, time error was reduced to approximately one fifth of earlier values.
The remarkable result promises to enable time synchronization accuracy close to that obtained in open-sky reception environments with no obstructions, even in environments previously considered poor and unsuitable for accurate time synchronization, such as among buildings or in mountainous areas.
The companies will exhibit the results at Tsukuba Forum 2018 Oct. 25-26, and at ITSF 2018, in Bucharest, Romania, Nov. 5-8.
As a member of the European Quantum Technologies Flagship, Teledyne e2v will collaborate with a team of science and industry experts on the iqClock project to commercialize high-precision atomic clocks. This is one of the first of 20 projects being funded by the European Commission.
According to Teledyne e2v, clocks are a critical component of modern society, especially in scientific and engineering applications where precision time measurement is vital. Teledyne e2v’s role in this project is to build the atomics package including the vacuum and control system. Teledyne e2v’s Quantum Group, which is comprised of more than 30 scientists and engineers, will be taking on the project.
The iqClock consortium is made up of both academic and industrial partners who share the same goal of bringing optical clocks closer to the market. Teledyne e2v’s partners include Chronos and British Telecom in the United Kingdom, Toptica in Germany, NKT Photonics in Denmark and Acktar in Israel. Its academic partners include the University of Amsterdam, University of Birmingham, Nicholas Copernicus University Torun, University of Copenhagen, TU Wien (Vienna) and Innsbruck University.
“Optical atomic clocks are the most precise time-telling tools known to man,” said Ole Kock, technical authority for Quantum Technologies at Teledyne e2v. “The challenge is their size and complexity that restricts them to laboratory use. Now, by using superradient laser technology we can help bring optical atomic clocks into the everyday world.”
According to Teledyne e2v, project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No. 820404.
Spaceopal has launched NAVCAST, a GNSS precise point positioning (PPP) service featuring high-accuracy positioning enhancement for end users worldwide. NAVCAST aims to actively support and to accelerate widespread adoption of Galileo.
NAVCAST provides Galileo and GPS real time orbit and clock corrections based on an algorithm RETICLE (REal-TIme CLock Estimation), developed by the German Aerospace Centre (DLR e.V.).
Galileo and GPS observations, from more than 100 receivers of the worldwide IGS network, are used to estimate the current corrections which are broadcast to registered users relaying on the standard NTRIP protocol.
NAVCAST corrections improve the user error down to the centimeter level, making it attractive for a large number of applications, the company said.
Users can appraise the accuracy levels and convergence times achievable using NAVCAST (Galileo + GPS) corrections combined with a precise point positioning (PPP) engine, on the Spaceopal website. The underlying PPP engine (dual-frequency, ionosphere-free observations) estimates the local troposphere delays and fixes the carrier-phase integer ambiguities.
NAVCAST can be considered as proof of concept and Spaceopal’s contribution to high-accuracy GNSS services. NAVCAST corrections, which are broadcast over the Internet, could be in future via satellite constellation (such as MEO satellites).
From November 2010 until end of June 2017, Spaceopal was the prime contractor responsible for Galileo operations under the Galileo Full Operational Capability (FOC) Operations Framework contract, the company said. Spaceopal GmbH will continue to operate the Galileo satellite fleet under the Galileo Service Operator (GSOp) contract. Spaceopal is actively supporting the completion of the system to expand the services up to full operational capability by 2020.
Galileo satellites GSAT0215, GSAT0216, GSAT0217 and GSAT0218, launched in December 2017, were commissioned for operational use as of Oct. 12, with all signals usable: Open Service, Public Regulated Service and Search and Rescue Service.
This increases the number of Galileo satellites that are available for service provision to 18. Initial operational capability for the constellation was declared on December 15 2016.
The additions to the GNSS almanac include the following:
GSAT0215: space vehicle E21 aka Nicole, occupying slot A03 if the constellation, with its payload running on a phased hydrogen maser (PHM) clock.
GSAT0216: E25, Zofia, slot A07, PHM clock.
GSAT02017: E27, Alexandre, slot A04, PHM clock.
GSAT0218: E31, Irina, slot A01, PHM clock
Each satellite weighs 715 kilo; measures 2.7 x 1.2 x 1.1 meters with a deployed solar array span of 14.67 meters; has onboard power of 1,900 W; and broadcasts navigation signals in 3 bands: E5, E6 and E1. Design life of the new satellites is more than 12 years.
Satellites GSAT-219 (Tara), GSAT-220 (Samuel), GSAT-221 (Anna) and GSAT-222 (Ellen) were launched on July 25 and are currently listed as under commissioning.
Galileo status information
Updated information on the status of the Galileo constellation can be found in the Constellation Status section of the European GNSS Service Centre’s (GSC’s) website.
Delivery person uses Galileo on a mobile device to deliver a package. (Photo: GSA)
According the the European GNSS Agency (GSA), more than 100 million devices are using Galileo today.
To keep track of Galileo-enabled devices serving a variety of needs as they become available, visit usegalileo.eu.
The Galileo Initial Services allow the use of Galileo Open Service (OS), which enables a free of charge, global ranging, positioning and timing service for the OS users.
Galileo is interoperable with the GNSS constellations (GPS, GLONASS, Beidou). By offering dual frequencies as standard, Galileo is set to deliver real-time positioning accuracy down to the meter range.
For questions about Galileo, contact the GSC Helpdesk.
Four Galileo satellites were added to constellation in October 2018. (Image: GSA)
The U.S. Federal Communications Commission will vote in November on whether to allow U.S. devices to access Galileo.
The Galileo Order is tentatively on the agenda for the Open Commission Meeting scheduled for Thursday, Nov. 15:
Galileo Order – The Commission will consider an Order that addresses waivers of certain satellite licensing requirements for receive-only earth stations operating with the Galileo Radionavigation-Satellite Service. (IB Docket No. 17-16)
“Enabling the Galileo system to work in concert with the U.S. GPS constellation should make GPS more precise, reliable and resilient for American consumers and businesses alike ,” said FCC Chairman Ajit Pai.
In 2015, the National Telecommunications and Information Administration (NTIA) submitted to the FCC a request from the European Commission to waive certain of the commission’s earth station licensing rules to permit non-federal U.S. receive-only earth stations to operate with Galileo.
The NTIA recommended grant of the requested waivers, and the International Bureau issued a Public Notice seeking comment on the potential public interest benefits and technical issues associated with the waiver request.
The FCC is proposing to waive its licensing requirements for non-federal operations with Galileo signals known as E1 and E5, subject to certain technical constraints, officials said.
The FCC includes conditions to ensure users of satellite-based positioning, navigation and timing services in the United States will benefit from Galileo signals. The systems are interoperable under a 2004 agreement.
Below is a summary of the order; the full text can be downloaded here.
Grant in part the request of the European Commission for waivers of certain of the Commission’s earth station licensing rules to permit non-federal U.S. receive-only earth stations to operate with specific signals of the Galileo GNSS without obtaining a license or grant of market access.
Find that the Galileo GNSS is uniquely situated as a foreign GNSS system with respect to the U.S. GPS, since the two systems are interoperable and radiofrequency compatible pursuant to the 2004 European Union/United States Galileo-GPS Agreement.
Find that there are significant public interest benefits associated with operations of non-federal U.S. receive-only earth stations with the Galileo GNSS, including increased availability, reliability, and resiliency of position, navigation, and timing services in the United States.
Grant the request for operations with the Galileo E1 signal, which is transmitted over the 1559-1591 MHz frequency band.
Grant the request, and a waiver of the non-federal portion of the U.S. Table of Frequency Allocations, for operations with the Galileo E5 signal, which is transmitted over the 1164-1219 MHz frequency band.
Deny the request for operations with the Galileo E6 signal, which is transmitted over the 1260-1300 MHz frequency band, since there is no federal or non-federal allocation for RNSS in the U.S. Table of Frequency Allocations in that band and grant of waiver could constrain our future spectrum management for non-federal operations in the U.S. in spectrum above 1300 MHz, where potential changes in the non-federal allocation are under consideration.
One remark in particular caught my eye as I read the press release précis of the European GNSS Agency’s 2018 GNSS User Technology Report. In point, “Today only around 30 percent of receivers use GPS only.”
“What?!?” methought. Incredulously, I downloaded the 92-page document, so easily done at www.gsa.europa.eu, and scrutinized it closely. Surely the GPS-only installed base out there is wider, vaster and deeper (it’s certainly older!) than could have been overtaken already by the wave of multi-constellation devices.
Yes, they are clearly the future. But is the past already gone? That golden age when GPS was all that anyone lived, positioned and navigated by — vanished into the mist?
Only earlier this very year was I upgraded from an iPhone 3 to an 8, with Galileo onboard for the first time. “Hip, but by the skin of my teeth,” I breathed.
Chart: GSA report
In the fine print on page 20 of the report lay clarification for my consternation. “For the analysis, each device is weighted equally, regardless of whether it is a chipset or receiver and no matter what its sales volume is. The results should therefore be interpreted as the split of constellation support in manufacturers’ offerings, rather than what is in use by end users.”
Of the roughly 500 chipsets and modules tallied by the GSA, 30 percent of those models are GPS-only. That’s a number of quite a different color. See the chart for fuller information.
Better minds than this can take a stab at how many devices in the hands end users on this day are still GPS-only. I’d put it above 50 percent.
The writing’s on the wall for the GPS-only artifact, but a good many of those veterans are still out there, working hard in the marketplace. Their reign as the majority may be limited, especially with the rising global tide of multi-constellation smartphones, but let’s honor them one last time before consigning them to the museum.
The GSA’s report, by the way, is a remarkably good and valuable read. No one can know it all, but this slim volume packs a remarkable and essential density of key facts, trends, issues, markets and more.
The European Space Agency has awarded a new framework contract and two new work orders to Thales Alenia Space in France to upgrade the Galileo Mission Segment, the element of the worldwide Galileo ground segment dedicated to delivering navigation services. Included are upgrades to the Galileo Security Monitoring Centre (GSMC) near Paris, and implementation of a second GSMC in Spain, near Madrid.
ESA Director of Navigation Paul Verhoef signed the contract with Thales Alenia Space Senior Vice President of Sales Martin van Schaik on Oct. 17 at ESA’s ESTEC technical centre in Noordwijk, the Netherlands.
“Galileo has already proven to be the highest performing satellite navigation system in the world, even before the constellation is complete,” Verhoef said. “This achievement is the result of the close collaboration between the public sector — the European Commission, the European GNSS Agency and ESA — and our industrial partners throughout Europe.
Contract signing: van Schaik (left) and Verhoef. (Photo: ESA)
“Today I am very happy to announce a continued relationship with Thales Alenia Space in one of the most complex parts of the system, namely the Ground Mission Segment, and thank them for their commitment to the programme.”
The constellation in orbit is only one element of the overall satellite navigation system – the tip of the Galileo iceberg. At the same time as the satellites were being built, tested and launched, a global ground segment was put in place.
Complex System
Establishing Galileo’s ground segment was among the most complex developments ever undertaken by ESA, having to fulfil strict levels of performance, security and safety.
In 2017, responsibility for operating the Galileo ground segment was passed to ESA’s partner organization, the European GNSS Agency (GSA). Nevertheless, ESA continues to be in charge of the maintenance, development and evolution of the ground segment, as well as the development of the space segment.
GSMC upgrade and construction. The first work order contracts Thales Alenia Space as prime contractor to undertake all necessary activities to upgrade the Galileo Mission Segment and the GSMC as part of Galileo’s exploitation phase.
This work includes upgrading Galileo’s system architecture to provide more accurate navigation products for broadcast by Galileo satellites, updating obsolescent elements in the current system, and improving operability linked to the provision of services and enhanced robustness.
This work order also includes the construction of additional uplink stations – tasked with uplinking the latest navigation messages to the Galileo constellation — at the existing Galileo ground station sites of Papeete in French Polynesia and Svalbard in Norway.
A new sensor station — providing a ground-based measurement of Galileo signal quality and precise satellite position — will also be installed at Wallis Island in the Pacific.
Galileo’s Nouméa ground station’s Sensor Station and Uplink Station. (Photo: ESA)
The work order will also augment the capabilities for implementation of the Public Regulated Service (PRS), the single most accurate and secure class of Galileo signals. Encrypted PRS signals will be made available only to authorised governmental users through approved national authorities.
Together, the two GSMCs will ensure the security of the overall Galileo system and manage PRS access and operations.
Security monitoring. The second work order contracts Thales Alenia Space as prime contractor to implement security monitoring functions for Galileo operational assets — including the control centres, service facilities and ground stations.
The integration, qualification, deployment and migration into operational services of the various upgraded segments will be undertaken over the next three years.
ESA has issued these work orders in its role of undertaking the design and development of future upgrades and the technical development of infrastructure as well as overseeing Galileo’s deployment, on behalf of the European Union, Galileo’s owner.
Duro Inertial is a ruggedized version of Swift Navigation’s Piksi Multi dual-frequency real-time kinematic (RTK) GNSS receiver combined with CRL’s SmoothPose sensor fusion algorithm, which fuses GNSS and inertial measurements into a combined solution.
The blending of GNSS and inertial measurements provides a dead-reckoning capability that allows Duro Inertial to provide a highly accurate, continuous position solution during brief GNSS outages and to deliver a robust precision navigation solution in harsh GNSS environments.
Duro Inertial is an evolution of Swift and CRL’s first joint product, Duro. Building on the on-board MEMS inertial measurement unit (IMU) that exists in Duro today, Duro Inertial harnesses CRL’s loosely coupled (LC) sensor fusion algorithm, SmoothPose, to blend GNSS and inertial inputs, providing a smoother, more available and more robust position, velocity and time (PVT) solution, the companies said.
Duro Inertial seamlessly blends CRL’s SmoothPose GNSS+INS algorithms with Swift Navigation’s Starling Positioning Engine to deliver a highly-accurate LC positioning solution even in GNSS / RTK denied environments.
The inertial aiding feature can operate with RTK, autonomous GNSS and satellite-based augmentation system (SBAS) position solutions from Starling. Duro Inertial also inherits the full set of features from Duro and Piksi Multi including the light-weight SBP communication protocol, interoperability with legacy protocols such as NMEA output and RTCMv3 input, compatibility with RTK corrections services such as Skylark, Swift’s Cloud Correction Service and many third-party corrections services, and quad-constellation dual-frequency RTK navigation.
The combination of Duro Inertial’s positioning accuracy and its ruggedized enclosure that protects against weather, moisture, vibration, dust and water immersion makes it suitable for construction, mining, logistics, positive train control, robotics and agriculture applications.
“We are excited to introduce our second collaboration with Carnegie Robotics and build on the success of the Duro ruggedized receiver launched last year,” said Timothy Harris, co-founder and CEO of Swift Navigation. “The combination of Carnegie Robotics’ advanced inertial technology and robotics expertise with Swift’s positioning solution will enable an even broader customer segment to benefit from highly-accurate positioning.”
“Duro Inertial is the culmination of our partnership with Swift over the past two years,” added John Bares, CEO of Carnegie Robotics. “Working together we are able to deliver a consistent and highly-accurate positioning solution to benefit a variety of robotics and industrial applications.”
Duro Inertial is scheduled to be available at for purchase in the fourth quarter and is now available for select customer testing.
Garmin is now offering a GPS, GLONASS and Galileo watch called Instinct. Instinct is a strong and durable watch with GNSS support, plus built-in 3-axis compass, barometric altimeter and wrist-based heart rate sensor.
The watch includes a built-in sports apps, smart connectivity and wellness data.
Photo: Garmin
“We are thrilled to add Instinct to our adventure watch lineup, an approachable smartwatch that is rugged and reliable,” said Dan Bartel, Garmin vice president of global consumer sales. “Instinct is perfect for those who spend their time outdoors and demand a device built tough to stand up in the elements.”
The Instinct is built to endure challenging environments, constructed to military standards (MIL-STD-810G) for thermal, shock and water resistance (rated to 100 meters) with a fiber reinforced polymer case. The chemically strengthened and scratch-resistant display is readable in direct sunlight, and the fully vented silicone bands include two independent, removable keeper loops to ensure a secure fit.
The multi-GNSS feature helps users track their location in challenging environments. The Garmin Explore app helps plan the trip in advance, and the TracBack feature can navigate the same route back to the starting point.
The built-in heart-rate sensor helps monitor heart rate, steps taken, distance traveled, calories burned and more.
China successfully launched a pair of BeiDou-3 navigation satellites into medium Earth orbits on Oct. 15, according to GB Times.
Four hours after the launch, the two satellites were inserted into their intended orbits, according to the China Aerospace Science and Technology Corporation (CASC). The satellites, numbered M15 and M16, are the 39th and 40th launched as part of China’s Beidou system, following the launch of the first in 2000.
Another pair of BeiDou satellites is expected to be launched in November, according to Richard Langley’s Upcoming Satellite Launches.
Liftoff of the Long March 3B rocket sending the Beidou-3 M15 and M16 satellites into orbit. (Photo: CALT)
For the Oct. 15 launch, a Long March 3B rocket with a Yuanzheng-1 upper stage lifted off from the Xichang Satellite Launch Centre in southwest China at 04:23 universal time (12:23 local, 00:23 Eastern).
The China Academy of Launch Vehicle Technology (CALT), which developed the Long March 3B rocket, reported that data logging and active tracking equipment was placed aboard for tests to determine to altitude and timing for future parachute landings for boosters.
Expended rocket boosters frequently land in or near populated areas downrange of Xichang. The trial phase of parachute booster landings is expected in 2019.
At the GPS World Leadership Dinner and Awards Ceremony in Miami on Sept. 27, 120 VIPs from the international GNSS/PNT community gathered to honor recent significant achievements in four fields: Satellites, Signals, Services and Products.
The honorees, so voted by a panel of their peers, appear below. Also, see our article here.
Their remarks upon receiving the awards will appear in the December issue, along with Future Visions for 2019 by the executive officers of GPS, GLONASS, Galileo and BeiDou.
Rounding out the evening after speeches, dinner and good conversation among good friends, old and new, was the Smart City Jam! We attempted to replicate, on the carpet of the 14th-floor banquet room, an obstructed urban environment, replete with malicious jammers. And we challenged all comers to “autonomously” navigate to a goal in this hostile environment with remote-controlled rock-crawlers. Details on this as well coming up in December.
Multi-GNSS will open up PPP to a much wider range of applications.
By Francesco Basile, Terry Moore, Chris Hill, Gary McGraw and Andrew Johnson
INNOVATION INSIGHTS by Richard Langley
ARE WE THERE? In a multi-GNSS world, that is. We’ve asked that question from time to time in this column over the years. So, are we there yet? That depends. One definition of “multi” is more than one. In this sense, we were in a multi-GNSS world as long ago as 1996. In that year, we had two fully populated constellations of satellites: GPS and GLONASS. Unfortunately, the full GLONASS constellation was short-lived. Russia’s economic difficulties following the dissolution of the Soviet Union hurt GLONASS, and by 2002 the constellation had dropped to as few as seven satellites. But GLONASS was reborn, and by Dec. 8, 2011, a full 24-satellite constellation was again operational.
But another meaning of “multi” is many, implying more than two. In the late 1990s, the first satellites to host transponders for satellite-based augmentation systems were launched. So, by the mid-2000s, even though GLONASS was still undergoing its rejuvenation, we were already in a three-constellation world. And receivers then on the market provided the necessary raw measurement data to yield positioning solutions from this system of systems with potentially more continuity and greater accuracy than those obtained using GPS alone.
And so in July 2008, we featured the article “The Future is Now: GPS + GLONASS + SBAS = GNSS.” And then in June 2010, we had “GPS, GLONASS, and More: Multiple Constellation Processing in the International GNSS Service.” In the introduction to that article, we asked that same question: Are we there yet? We concluded that, for early adopters of GPS plus GLONASS data and products, we were. With Galileo test satellites in orbit and an early version of the BeiDou system operational, it was already clear that by the end of the current decade, it wouldn’t just be the early adopters who would be benefiting from multi-GNSS but virtually all users of satellite-based positioning and navigation.
Although we aren’t quite there with fully operational Galileo and BeiDou constellations, we are getting pretty close. And so researchers are looking hard at how to make the best use of multiple-constellation observations in a variety of positioning and navigation scenarios. In this month’s column, a team of such researchers examines the potential benefit of combining GPS and Galileo observations for improving precise point positioning in urban environments, following the advice we read in the Book of Ecclesiastes: “Two are better than one.”
Over the years, precise point positioning (PPP) has been applied to many real-time applications that require sub-decimeter-level accuracy over a wide area or on a global scale. It is currently a standard in scenarios characterized by open-sky conditions, where a receiver is likely to have continuous track of GNSS satellites. On the other hand, PPP’s typically long convergence time means the technique has not been widely used in constrained and transient signal environments associated with urban areas. Analysis with both simulated and real data has shown that, once Galileo reaches final operational status, the PPP convergence time will be cut by more than half when using both GPS and Galileo observations. Accordingly, multi-GNSS will open up PPP to a much wider range of applications.
To begin, we assessed the positioning performance of GPS and Galileo signals, alone or used together, in open-sky conditions. A Simulink-based software simulator was used to simulate 24-hour-long observation sessions from 10 static (fixed location) receivers spread worldwide, which were then processed with the POINT software (developed by the University of Nottingham and three other British universities) in static (receiver assumed fixed) PPP mode with an elevation cutoff angle of 10° and with carrier-phase ambiguities estimated as real or floating-point values. For each station, the simulator was run 55 times to provide a sufficient number of data points to characterize the general behavior of the processing algorithms; therefore, a total of 550 points were considered.
For better GPS-Galileo interoperability, PPP results based on the ionosphere-free (IF) combination between GPS L1 and L5 and Galileo E1 and E5a observables were considered.
The metrics used to define the positioning performance are the errors in the north, east and down components of the position once all of a daily file has been processed and the time these errors take to converge below 10 centimeters.
The open-sky condition always guarantees excellent geometry and signal continuity even considering only one constellation.
PPP Results. TABLE 1 shows the root mean square (RMS) of the errors and convergence times of the three components of position for the different configurations for the 550 points considered. Both single- and dual-constellation systems are able to provide a sub-decimeter-level accuracy after a few tens of minutes. On average, positioning with Galileo E1-E5a IF performs better that GPS L1-L5 IF: the Galileo solution is more accurate and converges faster than the GPS solution.
TABLE 1. Comparison between GPS-only, Galileo-only and GPS plus Galileo PPP results. RMS of the positioning errors and convergence times for the stations considered.
The reason for this behavior is the assumed lower noise on Galileo pseudoranges. It is well known that the quality of the pseudoranges affects the convergence time of the PPP solution.
For this reason, one would expect some improvements by employing the Galileo Alternative BOC (AltBOC) modulated E5 signal. Thanks to its very large signal bandwidth of at least 51 MHz, Galileo E5 is characterized by excellent rejection properties of both long-range and short-range multipath. However, as shown in Table 1, when comparing the PPP solutions obtained using the Galileo E1-E5 IF and E1-E5a IF combinations, they have nearly the same performance. The reason for this apparent contradiction can be found in the use of the IF combination with E1. Given that E1 represents the dominant source of error in the IF combinations, its noise is amplified by a factor of 2.34 in the IF combination with E5 and by a factor of 2.26 when combined with E5a. Also, the smaller errors (with respect to E1) in E5a are amplified by 1.26, while the one in E5 is amplified by 1.34. Therefore, depending on the noise level in the Galileo pseudoranges, there might be instances where the noise in the E1-E5 IF combination is close to the one in the E1-E5a IF combination.
The number and the geometry of the observed satellites also affect the convergence time. For this reason, when using the two systems together, the time the vertical errors take to go below 10 centimeters was reduced by 50 percent with respect to the GPS-only case and by 18 percent with respect to the Galileo-only case.
URBAN ENVIRONMENTS
The poor signal visibility and continuity associated with urban environments, together with the slow (re)convergence time of PPP, usually make the technique unsuitable for land navigation in cities. However, as demonstrated in the previous section, using a dual-constellation not only improves the visibility conditions, but also reduces the PPP convergence time. Therefore, it might be possible to extend the applicability of PPP to land navigation in certain urban areas.
To assess the positioning performance of two-constellation GNSS in these constrained environments, we analyzed the signal availability and geometry of five different simulated sites in the neighborhood of the University College London (UCL) campus. We adopted building boundaries, which determine the minimum elevation angles above which GNSS signals can be received due to building obstruction. FIGURES 1 and 2 illustrate the location and the building boundaries for each site. FIGURE 3 shows the junction (site B) between Gower Street (site A) and University Street (site C).
FIGURE 1. Locations of the urban sites that are considered in the analysis.FIGURE 2. Building obstruction masks controlling satellite visibility for each site.FIGURE 3. Google Map image showing the junction (site B) between Gower Street (site A) and University Street (site C) in the midst of the University College London main campus.
When processing data from multi-constellation GNSS, the differences between the system time of the different constellations need to be considered. For this reason, when GPS and Galileo are used simultaneously for precise positioning, the Kalman filter state vector (in general) includes the three position components, the receiver clock offset, and the GPS-Galileo Time Offset (GGTO) — whether or not a predicted value might be available in a navigation message from one of the constellations. On the other hand, in PPP processing, the multi-constellation precise products used are based on the same system time, and therefore, in theory, it is not necessary to estimate the GGTO. However, existing intersystem biases may affect the PPP performance, and so it is advisable to estimate them in the Kalman filter.
Traditionally in PPP, the state vector also includes the residual zenith wet tropospheric delay and the carrier-phase ambiguities. Therefore, the minimum number of satellites required for GPS plus Galileo PPP is six. The geometry conditions are also an important factor for assessing the GNSS positioning performance. For land navigation, the horizontal dilution of precision (HDOP), which provides information about the achievable horizontal precision (and, assuming a bias-free solution, accuracy), is particularly relevant. For many land applications, such as precision agriculture and urban positioning, horizontal accuracy is more critical than vertical accuracy. Assuming that the ranging error in the carrier phase is 20 centimeters, to have decimeter-level horizontal accuracy HDOP needs to be no larger than 5. In most cases, HDOP values as small as 2 are desired.
TABLE 2 gives an overview of the visibility and geometry conditions at the selected sites. A dual-constellation (GPS and Galileo) receiver placed at one of the two road junctions will always, or almost always, see at least six satellites with an HDOP better than 5. At sites A and C, these minimum requirements for signal availability and geometry are met for more than 75 percent of the day. Obstructions due to high buildings, such as at site E, allows us to have at least six satellites for only 13 percent of the time.
TABLE 2. Percentage of epochs in 24 hours for which dual-constellation GNSS meets the minimum visibility (number of satellites, N) and geometry requirements (horizontal dilution of precision, HDOP).
From our preliminary study, it seems clear that high-accuracy positioning in urban environments is possible, but only in some areas where buildings are relatively short, providing good signal availability and geometry. Things can slightly improve by considering additional systems, such as GLONASS and BeiDou, and by exploiting the non-line-of-sight (reflected) signals. However, it is well known that an additional obstacle for PPP in urban environments is signal discontinuity. Indeed, when a GNSS receiver loses lock on the carrier, the positioning filter needs to be reinitialized, meaning that further tens of minutes are required before reconvergence.
To test the reconvergence time of PPP in transient signal environments, a pedestrian carrying a multi-GNSS receiver was simulated to be walking along the path in FIGURE 4. The receiver was simulated to be located for the first half hour of the simulation in the front yard of UCL’s Wilkins Building (where the simulation begins and ends), before starting to move. This is to allow the initial convergence of the PPP filter.
FIGURE 4. The measured trajectory of the simulated pedestrian kinematic test.
FIGURE 5 shows the visibility for a given GNSS satellite. Only the epochs when the receiver is moving are considered. Therefore, the first 30 minutes, when the receiver is static, are not included in the plot. Data gaps due to building obstructions are visible, with the largest being about 12 minutes and the average less than 2 minutes. As a consequence, the carrier-phase ambiguities need to be estimated all over again; and, as previously mentioned, this process usually requires tens of minutes before reconvergence.
FIGURE 5. Satellite availability during the kinematic test.
FIGURE 6 shows the HDOP and the number of visible satellites for the kinematic test, while FIGURE 7 shows the RMS, over 50 simulations, of the horizontal components of the positioning error when GPS L1 and L2 and Galileo E1 and E5, linearly combined into the IF combination, are processed in kinematic PPP mode with the POINT software. At the beginning of the kinematic test, when the HDOP is well below 5, the horizontal error is at the centimeter level, while, after 33 minutes from the beginning of the simulation, building obstructions don’t permit a converged solution below the 20-centimeter accuracy level.
FIGURE 6. Horizontal dilution of precision and number of visible satellites for the kinematic test.FIGURE 7. RMS of the position errors for the kinematic test.
This short example clearly demonstrates that two-constellation PPP has, in theory, the potential to precisely navigate ground vehicles in some urban environments; however, it is too sensitive to signal discontinuity. Slow solution reconvergence to the few decimeter/centimeter level still represents the main limitation to the use of PPP for high-accuracy applications in cities. Nonetheless, GPS plus Galileo PPP easily enables sub-meter-level horizontal accuracy for most of the simulations we have carried out. After signal loss, it only took a few tens of seconds to have a horizontal accuracy of better than a meter.
SMOOTHED CORRECTIONS
As an alternative to ambiguity-fixing methods aimed to improve the (re)convergence time, we propose a method that mitigates the effect of the ionosphere and which thereby reduces the reconvergence time of the PPP solution after initial convergence has been achieved. In this new approach, while the two-frequency carrier phases are linearly combined in the traditional IF combination, the uncombined pseudoranges are corrected by a pre-smoothed ionospheric delay (via a Hatch filter), computed using the geometry-free combination of two-frequency pseudoranges.
Once the Hatch filter has converged, ideally we have IF pseudoranges with lower noise than the traditional ones. Therefore, in case the PPP filter needs to restart, we can obtain a quicker reconvergence thanks to the lower noise on the ionosphere-corrected pseudoranges. Indeed, provided that the signal gap is not very large, the ionosphere smoothing filter doesn’t need to be restarted from the raw values.
It is possible to predict the ionospheric delay computed from two-frequency carrier-phase measurements using a linear fitting model from previous measurements within a sliding time window. As an example, high-rate data recorded on July 25, 2017, from station DAEJ in Daejeon, Republic of Korea, were used to analyze the ionosphere prediction error.
In FIGURES 8 and 9, the RMS of the prediction errors for different time windows have been plotted against the data gap length. The prediction error depends on both the time latency of the observation and the elevation angle of the satellite. It increases with the data gap length, but larger time windows can damp the divergence of the error. A time window of 120 seconds was used both for satellites above and below 30° elevation angle. In this case, the error for a 5-minute prediction is about 4 centimeters for a satellite above 30° and 7 centimeters for satellites with a low elevation angle. These values are much smaller than the noise in the pseudorange measurements and can, therefore, be neglected.
FIGURE 8. RMS of the prediction errors vs. data gap length for satellite elevation angles greater than 30°.FIGURE 9. RMS of the prediction errors vs. data gap length for satellite elevation angles less than than 30°.
Multi-Frequency Combinations. The method introduced in the previous section allows users to be free from the constraint of IF observables and, therefore, to look for multi-frequency combinations aimed to minimize the noise on the pseudoranges. The next-generation GNSS satellites will broadcast open signals over three frequencies. The triple-frequency, geometry-preserving combination aimed to reduce the noise, instead of mitigating the ionosphere, can be used for positioning purposes.
TABLE 3 summarizes the assumed values for the ratios ni between the noise on different GPS and Galileo pseudoranges and the ones on L1/ E1. FIGURE 10 shows a color map of the noise amplification factor associated with different linear combinations between GPS L1, L2 and L5. The x-axis is α3, the coefficient multiplying the pseudorange on L5 in the combination, while the y-axis is the ionosphere amplification factor of the triple-frequency combination with respect to L1, q. The noise for this combination can be as little as 0.57 times the noise on L1, while the corresponding ionosphere amplification factor is 1.49. Once the smoothed ionosphere correction has converged, we can potentially have an IF pseudorange 81 percent less noisy than the L1-L2 IF, and, therefore, a much faster reconvergence.
TABLE 3. Assumed noise, ni, on GPS and Galileo pseudoranges, i, and their ionospheric delay, q, with respect to L1/ E1.FIGURE 10. Geometry-preserving surface in the space q-α3-n (ionosphere amplification factor – L5 pseudorange multiplier – noise amplification factor) for GPS L1-L2-L5 combinations.
Similar conclusions can be drawn by considering Galileo signals. Using triple-frequency combinations with E1, E5a and E5b, we can obtain 81 percent less noise than E1-E5a IF, while a reduction of the noise in the IF pseudorange up to 90 percent was observed using E5 alone. Triple-frequency combinations involving E5 don’t bring such large improvements with respect to using E5 alone. Indeed, a maximum of 16 percent less noise can be registered when combining E1, E5a and E5 with respect to the E5 uncombined case. TABLE 4 illustrates the minimum noise amplification factor for each triple-frequency combination and its ionosphere amplification factor.
TABLE 4. Minimum noise achievable through GPS and Galileo triple-frequency pseudorange combinations and their ionospheric delay with respect to L1/ E1.
The noise associated with the ionosphere-corrected multi-frequency pseudorange combination is as large as meter level before converging to centimeter level. For this reason, a proper weighting method, which considers the varying noise on the ionosphere correction, needs to be employed.
To test the benefit of the new approach for the reconvergence time, three hours of simulated GPS and Galileo data from a static site in La Misere, Seychelles, were processed with the POINT software in kinematic mode. After 90 minutes, the PPP filter was forced to restart to simulate reconvergence. The multipath time constant was set to 5 seconds, which is a typical value for kinematic multipath. The performance of the traditional L1- L2 IF combination was compared with the triple-frequency pseudorange combination, corrected by the smoothed ionosphere delay coming from the Hatch filter.
FIGURE 11 shows the precision (RMS error over 50 simulations) of the horizontal components after filter restart. The new approach has much faster reconvergence than the traditional PPP method based on the IF combination. Indeed, while the traditional method takes about 11 minutes to have a horizontal error below 10 centimeters, using the low-noise combination, this accuracy is achieved after 171 seconds. Even better performance can be achieved considering the Galileo E5 signal (see FIGURE 12).
FIGURE 11. RMS error of the horizontal position components of static site using GPS data after filter restart.FIGURE 12. RMS error of the horizontal position components of static site using Galileo data after filter restart.
The E1-E5 IF combination requires 10 minutes for the horizontal convergence, while using E5 with the Hatch filter we have the horizontal solution converged in about 30 seconds. It is worth noticing that in the presence of static multipath, the proposed weighting method may lead to an overly optimistic weighting of the pseudorange measurements in the PPP filter and to a slower reconvergence of the positioning solution. Indeed, the long correlation time in the static multipath, of the order of a few minutes, makes it hard to filter out by the Hatch filter, hence the corrected measurements have larger errors than expected.
The effect of static multipath in the new configuration is visible in FIGURE 13, where the reconvergence of the horizontal component for the L1-L2 IF combination is compared with the new approach. In this case, the time constant of the simulated multipath was set to 1 minute. In this scenario, the triple-frequency low-noise combination corrected by the smoothed ionosphere combination quickly converges below 20 centimeters; however, it takes significantly longer than the L1-L2 IF combination to reach the 10-centimeter accuracy level.
FIGURE 13. RMS error of horizontal position component of static site using GPS data after filter restart with 1-minute multipath time constant.
Also, the new method was tested with the kinematic simulation as in the previous section. Here, the GPS triple-frequency combined pseudorange and Galileo E5 pseudorange (both corrected with the smoothed ionosphere) are processed in kinematic PPP mode with the POINT software. FIGURE 14 compares the RMS of the horizontal errors with the IF configuration. Less than a minute after the receiver lost lock on the satellites, the solution reconverged below the 20-centimeter level, while it took less than 30 seconds to go below 50 centimeters.
FIGURE 14. RMS error of the horizontal position components of kinematic trajectory using GPS and Galileo data and the smoothed ionosphere approach after filter restart.
CONCLUSIONS
In this article, we described a comparison that we carried out between GPS-only, Galileo-only and GPS plus Galileo PPP. Results based on simulated open-sky conditions demonstrated that Galileo performs better than GPS thanks to an assumed lower E1-E5a IF noise with respect to L1-L5. Two-constellation PPP enables faster (re)convergence compared to the single constellation case.
An analysis of GNSS signal availability, continuity and satellite geometry was also performed to study the feasibility of PPP in urban environments. Preliminary results, based on simulations, showed that dual-constellation (GPS plus Galileo) PPP is possible in urban areas with relatively short buildings in which a satellite minimum availability requirement is met most of the time. However, signal discontinuity still represents the major problem for traditional PPP in urban environments, due to long reconvergence times.
Finally, we proposed a new PPP configuration based on triple-frequency combinations, intended to minimize the noise on the pseudorange and corrected by a smoothed ionospheric delay. This configuration seems to provide faster reconvergence than the traditional PPP with the IF combination if applied to kinematic scenarios. In static applications, the very slow varying multipath error makes the proposed weighting method, based on the error in the smoothed ionosphere correction, overly optimistic. In such cases, the IF combination reconverges more quickly to high-accuracy levels better than 20 centimeters.
ACKNOWLEDGMENTS
The research described in this article was sponsored through a studentship agreement between the University of Nottingham and Rockwell Collins UK Limited. The article is based on the paper “Multi-Frequency Precise Point Positioning Using GPS and Galileo Data with Smoothed Ionospheric Corrections” presented at the 2018 IEEE/ION Position, Location and Navigation Symposium, held in Monterey, California, April 23–26, 2018. All figures attributed to the authors unless otherwise specified.
FRANCESCO BASILE is a postgraduate research student at the Nottingham Geospatial Institute of the University of Nottingham in the United Kingdom. He received his M.Sc. in space and astronautic engineering from the University of Rome – La Sapienza and his B.Sc. in aerospace engineering from the University of Naples – Federico II, both in Italy.
TERRY MOORE is the director of the Nottingham Geospatial Institute where he is the Professor of Satellite Navigation. He is a fellow and the president of the Royal Institute of Navigation (RIN) and also a fellow and a member of council of the Institute of Navigation (ION).
CHRIS HILL is an associate professor in the Faculty of Engineering at the University of Nottingham and a member of the Nottingham Geospatial Institute research group. He holds a Ph.D. in satellite laser ranging and he is a fellow of the RIN.
GARY MCGRAW is a technical fellow with the Rockwell Collins Advanced Technology Center in Cedar Rapids, Iowa. McGraw is a fellow of the ION and is a senior member of the IEEE.
ANDREW JOHNSON is a chief engineer at Rockwell Collions UK in Winnersh, Berkshire, United Kingdom. Johnson has a B.Sc. in electronic and electrical engineering from the University of Surrey in Guildford, United Kingdom.
FURTHER READING
Authors’ Conference Paper
“Multi-Frequency Precise Point Positioning Using GPS and Galileo Data with Smoothed Ionospheric Corrections” by F. Basile, T. Moore, C. Hill, G. McGraw and A. Johnson in Proceedings of PLANS 2018, the Institute of Electrical and Electronics Engineers / Institute of Navigation Position, Location and Navigation Symposium, Monterey, California, April 23–26, 2018, pp. 1388–1398, doi: 10.1109/PLANS.2018.8373531.
“Multi-Constellation GNSS Performance Evaluation for Urban Canyons Using Large Virtual Reality City Models” by L. Wang, P.D. Groves and M.K. Ziebart in Journal of Navigation, Vol. 65, No. 3, July 2012, pp. 459–476, doi: 10.1017/S0373463312000082.
“Potential Benefits of GPS/GLONASS/GALILEO Integration in an Urban Canyon – Hong Kong” by S. Ji, W. Chen, X. Ding, Y. Chen, C. Zhao and C. Hu in Journal of Navigation, Vol. 63, No. 4, October 2010, pp. 681–693, doi: 10.1017/S0373463310000081.
Multi-Constellation Use in Aviation Applications
“Assessment of Alternative Positioning Solution Architectures for Dual Frequency Multi-Constellation GNSS/SBAS” by G. McGraw, B.A. Schnaufer, P.Y. Hwang and M.J. Armatys in Proceedings of ION GNSS+ 2013, the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation, Nashville, Tennessee, Sept. 16–20, 2013, pp. 223–232.
“Undifferenced GPS Ambiguity Resolution Using the Decoupled Clock Model and Ambiguity Datum Fixing” by P. Collins, S. Bisnath, F. Lahaye, and P. Héroux in Navigation, Vol. 57, No. 2, Summer 2010, pp. 123–135, doi: 10.1002/j.2161-4296.2010.tb01772.x.
“Integer Ambiguity Resolution on Undifferenced GPS Phase Measurements and Its Application to PPP and Satellite Precise Orbit Determination” by D. Laurichesse, F. Mercier, J.-P. Berthias, P. Broca and L. Cerri in Navigation, Vol. 56, No. 2, Summer 2009, pp. 135–149, doi: 10.1002/j.2161-4296.2009.tb01750.x.
Hatch Filter
“Combinations of Observations” by A. Hauschild, Chapter 20 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.
“The Synergism of GPS Code and Carrier Measurements” by R. Hatch in Proceedings of the Third International Geodetic Symposium on Satellite Doppler Positioning, Las Cruces, New Mexico, Feb. 8–12, 1982, Vol. II, pp. 1213–1232.
Dilution of Precision
“Dilution of Precision” by R.B. Langley in GPS World, Vol. 10, No. 5, May 1999, pp. 52–59.
Kalman Filtering
“Least-Squares Estimation and Kalman Filtering” by S. Verhagen and P.J.G. Teunissen, Chapter 22 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.
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