Representatives from organizations involved in the UK Sovereign Satellite Based Augmentation System. (Image: Viasat)
Viasat, a global communications company, has successfully demonstrated the UK Satellite-Based Augmentation System (UK SBAS) during a recent test flight. This demonstration, conducted as part of an ongoing trial funded by the Department for Transport through the European Space Agency (ESA), showcased the potential of UK SBAS to provide accurate GPS data to improve safety and operational efficiency.
“The trial on a sovereign UK SBAS is all about delivering trust. Trust for pilots in their tracking systems to stay safe in challenging conditions. Trust for the aviation industry more broadly so it can rely on data to operate more efficiently,” said Todd McDonnell, president, international government, Viasat.
The test flight, carried out from Cranfield Airport using the National Flying Laboratory Centre’s Saab 340B aircraft, demonstrated the capabilities of a UK-based SBAS to deliver more precise and reliable navigation data. With the UK no longer part of the EU’s European Geostationary Navigation Overlay Service (EGNOS), the trial aims to pave the way for a complementary UK SBAS, specifically designed for critical safety-of-life navigation services across air, land and sea.
UK SBAS operates by merging ground monitoring data with satellite connectivity, which offers positioning accuracy down to a few centimeters. The system aims to significantly enhance safety in aviation by providing pilots with confidence in their onboard instruments, particularly during challenging weather conditions where visibility may be limited.
The successful aviation test marks a crucial step in further trials across various transport applications, including rail, unmanned aerial vehicles, and autonomous road vehicles, said Viasat.
Fully funded by the government through ESA’s Navigation Innovation and Support Program (NAVISP) program, the trial aligns with broader efforts to deliver high-accuracy, high-integrity positioning services to boost efficiency and innovation across the transport network.
GMV and Astroscale UK are collaborating on a new project under the European Space Agency (ESA) collision risk and automated mitigation (CREAM) program. The project aims to transform satellite collision avoidance by using Galileo Signal-in-Space (SiS) capabilities.
As low-Earth orbits (LEO) become increasingly congested, satellite operators face difficulties efficiently carrying out collision avoidance maneuvers. In response, the ESA launched the project to explore alternative paths for late collision avoidance maneuvers. The collaboration uses the Galileo Return Link Service to improve the way satellites respond to collision risks.
Traditionally, communication with satellites for collision avoidance maneuvers has been constrained by the limited availability of ground station access. This limitation forces satellite operators to delay crucial avoidance maneuvers while relying on the final passes of ground stations.
GMV’s solution offers an alternative pathway for late maneuver commanding, designed to reduce the wait time for initiating collision avoidance. The initiative proposes a continuous and reliable communication path by using the Galileo, SiS and its Return Link Service. This approach allows for the seamless relay of collision avoidance maneuver decisions to satellites equipped with onboard Galileo-compatible GNSS receivers.
The Galileo system in this role also opens the door to potential synergy with other space situational awareness (SSA) services, such as the European Space Surveillance and Tracking (EU SST). According to GMV, this strategic collaboration could potentially set the foundation for a globally available collision avoidance service.
The Educational Irish Research Satellite, EIRSAT-1, has successfully launched from Vandenberg Space Force Base, California, on Dec. 1, 2023. Hitching a ride on a SpaceX Falcon 9 launcher, the small satellite has made history as Ireland’s first satellite.
Over the course of six years, EIRSAT-1 was designed, built and tested by students from University College Dublin (UCD) in Dublin, Ireland, participating in the European Space Agency (ESA) Academy’s Fly Your Satellite Program. The program is a hands-on initiative that helps university student teams develop their own satellites according to professional standards. The launch opportunity itself was provided by the ESA.
Throughout the development of the satellite, ESA experts provided training and guidance to dozens of UCD students, the ESA said. The students’ learning journey included test campaigns at ESA Education’s CubeSat Support Facility in Belgium, as well as dedicated spacecraft communications sessions at both ESA Academy’s Training and Learning Centre and the European Space Operations Centre in Darmstadt, Germany. These sessions were designed to teach the procedures for operating Ireland’s first spacecraft.
From low-Earth-orbit (LEO), EIRSAT-1 will carry out three main experiments, which were built from scratch by the students:
GMOD, a detector to study gamma ray bursts, which are the most luminous explosions in the universe and occur when a massive star dies or two stars collide.
EMOD, an experiment to see how a thermal treatment protects the surface of a satellite when in space.
WBC, an experiment to test a new method of using Earth’s magnetic field to change a satellite’s orientation in space.
Following EIRSAT-1’s deployment to orbit, the student team is now working to establish contact with the satellite and start operations from their dedicated ground control facility, also entirely operated by students and located at UCD in Dublin.
The European Union is in the final stages of completing a deal with SpaceX to launch four Galileo navigation satellites in 2024, reported SpaceNews.
In press briefings during the European Space Summit in Seville, Spain, Thierry Breton, the European Commission’s commissioner for the internal market, said that he was “finalizing the discussions” for a pair of Falcon 9 launches, each carrying two Galileo satellites, tentatively scheduled for April and July of 2024.
Brenton also said that the final obstacle to completing the launch contract was negotiating a security agreement to protect sensitive technologies on the Galileo satellites, which previously had been launched from the European spaceport in French Guiana, when those satellites are being prepared for launch from the United States.
The launch contract itself was completed in July, Breton noted, and that the European Commission had approved a European Space Agency proposal to use the Falcon 9 for launching those satellites. He said the European Commission would spend $192 million on the Falcon 9 launches.
During a recent meeting of the European Space Agency (ESA) Council, ESA Director General Josef Aschbacher said that the final decision for using SpaceX to launch the Galileo satellites was in the hands of the Commission.
“We have prepared on the ESA side the contractual arrangements with an external launch company, but whether or not the launch will be decided to take place with SpaceX is not in our hands,” he said. “the European Commission will decide.”
There had been discussions for more than a year about using a non-European rocket, such as the Falcon 9, for launching those satellites because of delays in the Ariane 6, the retirement of the Ariane 5 and the withdrawal of the Soyuz after Russia’s invasion of Ukraine. Those satellites would augment the existing operational Galileo constellation and serve as on-orbit replacements if other satellites fail.
ESA had already contracted with SpaceX for three Falcon 9 launches. The ESA said it chose the Falcon 9 after the loss of the Soyuz, delays in the Ariane 6 and concerns about the Vega C, which remains out of service since a launch failure in December 2022.
SpaceX has signed a deal to launch four of Europe’s flagship navigation and secure communications satellites into orbit, reported The Wall Street Journal. The European Commission and the European Union (EU) member states have yet to give a final approval for the deal, the report added.
SpaceX and the European Space Agency recently signed an agreement for two launches next year, each carrying two Galileo satellites.
The deal states the satellites will be launched from the U.S. on SpaceX’s Falcon 9 rocket.
European space officials said last month they face crucial timing decisions in the coming weeks on the return to flight of Europe’s flagship space launchers following a series of delays.
The European Union Aviation Safety Agency (EASA) has certified Madrid-based European Satellite Services Provider (EESP) as a pan-European communication services provider for Iris data link services.
Iris, a project supported by the European Space Agency (ESA) and various European partners, aims to develop a satellite-based air-ground communication system to enhance air traffic management. By 2028, Iris will enable full 4D trajectory management of airspaces globally, making it a crucial means of communication between cockpit crews and controllers.
The certification of ESSP as an Iris data link services provider comes after more than a year of testing and audits at both ESSP and Inmarsat premises. The certification establishing compliance with relevant regulations and associated industrial standards for data link services.
Additionally, ESSP recently signed a long-term contract with Viasat/Inmarsat to act as the Service Provider for Iris data link services, powered by Viasat’s SwiftBroadband Safety (SB-S) connectivity. As The European Geostationary Navigation Overlay Service (EGNOS) service provider, ESSP is already under a long-term contract with the European Agency for the Space Program (EUSPA), where it carries out EGNOS operations and maintenance.
Iris is scheduled to be fully operational in Europe by 2024, providing services to airlines such as easyJet and ITA Airways. ESSP will lead the commercialization of Iris services, targeting European Air Navigation Service Providers (ANSPs). The service provision consortium includes ESSP SAS, Inmarsat, and SITA, with other communication network providers expected to join in the future.
Wildfires have recently spread across Greece, Italy, Spain, Portugal, Algeria, Tunisia and Canada, causing mass environmental and economic damage as well as human casualties. Scientists have warned that wildfires are becoming more frequent and more widespread.
In response, an upgraded version of the World Fire Atlas from the European Space Agency (ESA) is now available. The atlas provides a detailed analysis and map of wildfires across the globe.
Rising global temperatures and the increased extreme weather has led to a surge in the number of wildfires rapidly consuming extensive areas of vegetation and forested lands.
Considering the severe wildfires, ESA has reopened its World Fire Atlas which offers an insight into the distribution of individual fires taking place at a global scale.
Through its interactive dashboard, users can compare the frequency of fires between countries as well as analyze the evolution of each wildfire taking place over time. The atlas was first available in 2019 and it supported both European civil protection agencies and firefighters.
The dashboard uses night-time data from the sea and land surface temperature radiometer (SLSTR) on board the Copernicus Sentinel-3A satellite. Working like a thermometer in the sky, the sensor measures thermal infrared radiation to take the temperature of Earth’s land surfaces which is used to detect the fires.
Data from the Copernicus Sentinel-3B satellite will be added to the atlas in December.
Over the previous seven years, data from the World Fire Atlas show a substantial number of fires detected in Portugal, Italy, Greece, France and Spain.
Data also shows that Canada has experienced 11,598 fires during the first seven months of this year alone. This is a 705% increase compared to fires detected over the same period of the previous six years. Canada is currently battling the country’s worst wildfire season on record, with more than 10 million ha of land burned, which is said to increase in the coming weeks.
From left to right: Sylvain Loddo, director of the Galileo ground segment program at ESA, Ennio Guarino, head of the EGNOS and Galileo programs at ESA, Lionel Salmon, director of cybersecurity for information systems at Thales, and Alexandra Porez, director of cybersecurity for satellite systems at Thales. (Image: Thales)
Thales and the European Space Agency (ESA) will be working together on the cybersecurity aspects of the Galileo Second Generation (G2G) program.
Under the partnership, Thales’ scalable and flexible architecture, and security equipment will enable the G2G program to strengthen its ability to detect and respond to new cyberthreats. The end-to-end solution Thales proposed will contribute to the development of greater security and resilience of satellites.
In addition, Thales Alenia Space has partnered with the ESA to design and build the G2G ground mission segment, as well as support system engineering and technical assistance activities. The company also will provide six of the 12 satellites of the constellation.
The second-generation ground mission segment is designed to generate and connect the navigation services to the Galileo satellites and to keep the satellites synchronized with a common time reference. The first version will arrive in time for the launch of the first second-generation satellites and for the validation of the system’s in-orbit capabilities. The second version will be responsible for the missions of both the first- and second-generation Galileo satellites.
The new ground mission system, which includes several major technological innovations, will provide more than four billion users worldwide with improved performance in terms of positioning, navigation and synchronization.
Contract ceremony in Madrid, Spain, on June 22. (Image: GMV)
GMV has been awarded a major contract by the European Space Agency (ESA) to develop the ground control segment for the in-orbit validation (IOV) system of the Galileo Second Generation (G2G). The primary objectives of G2G are to introduce new services and technologies; improve existing services and technology; increase the accuracy and robustness of the system; strengthen security; and reduce the system’s maintenance costs.
The ground segment will be responsible for controlling the two new second-generation satellite platforms, which are currently in the design and production phase. A total of 12 satellites are expected to be launched over the next three years. The new ground control system is scheduled to come into operation in 2
025, coinciding with the launch of the first satellite of this second generation.
The new contract signed between GMV and ESA is worth over €200 million. This includes the contracting of core G2G activities, for a value of around €155 million. These activities will be carried out over a period of 42 months, from mid-2023 until the end of 2026, with options for extension until 2028.
Galileo currently serves more than four billion users worldwide, delivering global positioning, navigation, and clock synchronization services with a positioning accuracy of up to 20 cm.
On May 31, the European Space Agency (ESA) announced the main procurement batch of Galileo Second Generation (G2), initiated in summer 2022, has been finalized. The system is now ready for its on-orbit validation development phase.
Following the opening session of the European Navigation Conference (ENC), Javier Benedicto, director of navigation for the ESA, invited Thales Alenia Space, Airbus Defence and Space, and Thales Six GTS to sign contracts commencing system engineering support for the next generation of Europe’s navigation satellite system.
Satellite-building contracts were awarded in May 2021 to Thales Alenia Space and Airbus Defence and Space to create two independent families of satellites amounting to 12 G2 satellites in total. Separate contracts were also awarded to Safran Electronics and Defence-Navigation and Timing and Leonardo to provide the ultra-precise atomic clocks carried aboard.
Employing electric propulsion for the first time, and hosting a higher-strength navigation antenna, the G2 satellites will incorporate six (rather than four) enhanced atomic clocks as well as inter-satellite links to communicate and cross-check with one another. They will be controllable with an increased data rate to and from the ground and will operate for 15 years on orbit.
In addition, G2’s fully digital payloads are being designed to be easily reconfigured on orbit, enabling them to respond to the evolving needs of users with novel signals and services.
There are 28 Galileo satellites on orbit, making it the most precise satellite navigation system —providing meter-level accuracy to more than four billion users around the globe. There are 10 Galileo satellites due to be launched, after which the first of the G2 satellites with enhanced capabilities are expected to join the constellation in the next few years.
While I’m likely preaching to the choir here, GNSS cannot work unless we have an accurate description of the orbits of the satellites and the behavior of their atomic clocks. The accuracy with which this information is provided to a receiver or data processing software is the most important component of the error budget of GNSS positioning, navigation and timing and constitutes most of what is known as the signal-in-space (SIS) range error.
Each GNSS satellite broadcasts a description of its orbit or ephemeris along with the offset of its active clock from the system’s time standard in a navigation message decoded and used by the receiver. These data are predictions of the orbit and clock offset as computed by the system’s ground control segment and uploaded to each satellite. A recent assessment by U.S. Space Systems Command of the GPS SIS error averaged across all active satellites for a one-week period was about 50 centimeters, root-mean-square. While this is entirely adequate for many GNSS uses, it falls short of the required accuracy for high-demanding applications such as surveying, geodesy, atmospheric sensing, reference frame studies and tectonic monitoring. Which is why various organizations both private and public compute very accurate orbits and clocks and provide these to users. These computations, using data from global receiver networks, are very exacting and model the tiniest effects on the (primarily) carrier-phase measurements these receivers provide.
These effects include the offset in the electrical phase centers of a GNSS satellite’s transmitting antenna from the satellite’s center of mass and how that varies with the direction of the signal from the satellite to a receiver on Earth. Furthermore, this behavior must be calibrated and modeled for each radio frequency that the satellite transmits. Another effect that must be accounted for are the perturbations caused by non-perfect yaw-steering of a satellite’s solar panels. These panels continuously track the Sun but they have difficulty keeping up at orbit noon and midnight. Accurate models of the actual yaw angle are very important for high-precision GNSS orbits. As if these model requirements were not enough, the effect of solar radiation pressure on satellite orbits must also be modeled. While they don’t have (rest) mass, photons have energy and this can be imparted to satellites when they impinge on them. While a single photon has a negligible effect, the billions upon billions of photons making up sunlight do have a noticeable effect on a GNSS satellite’s motion and must be accounted for by orbit models.
One organization producing precise orbits for GNSS satellites – arguably the most precise in the world – is the International GNSS Service (IGS), a voluntary federation of more than 200 agencies, universities and research institutions across the globe. Several of these organizations each produce precise orbits, which they submit to the IGS to establish orbit products. One of these organizations is the Navigation Support Office (NSO) at the European Space Agency’s European Space Operations Centre. In this quarter’s Innovation column, a team of NSO engineers discusses how they have improved the orbit modeling of the GPS III satellites by around a factor of two with estimated orbit errors of about 2 centimeters or less. Wizardry? Not really – just rocket science.
To produce GNSS satellite orbit ephemerides and clock data with high precision and for all constellations, the Navigation Support Office of the European Space Agency’s European Space Operations Centre (ESA/ESOC) continually strives to keep up and improve its precise orbit determination (POD) strategies. As a result of these longstanding efforts, satellite dynamics modeling and GNSS measurement procedures have progressed significantly over the last few years, especially those developed for the European Galileo satellites. Because the accuracy of ESA/ESOC’s GNSS orbits has reached such a high level (about 1 to 3 centimeters), introducing a completely new type of GNSS satellite into the processing is not as easy as it used to be. New spacecraft models – the first and foremost being a model for a satellite’s response to solar radiation pressure (SRP) – are needed for the “newcomer” so that the quality of the overall multi-GNSS solution does not suffer. Just as important are spacecraft system parameters, or metadata, such as the location of the satellite antenna’s electrical phase center and the satellite attitude law.
In this article, we show the efforts we have made at ESA to bring the quality of our orbit estimates for the GPS Block III satellites up to par with those for Galileo and the earlier GPS satellite blocks. We report on the results from on-ground and in-flight determinations of the Block III transmit antenna phase center characteristics up to 17 degrees from the antenna boresight direction. Moreover, we take advantage of the non-zero horizontal offsets of the transmit antenna from the spacecraft’s yaw axis to estimate the satellite yaw angle during Earth eclipse season and present a simple analytical formula for its calculation. Finally, we describe the development and validation of improved radiation force models for the Block III satellites.
We start, however, by giving a brief overview of the GPS Block III program.
GPS BLOCK III
The U.S. Space Force GPS Block III (previously referred to as Block IIIA) is a series of 10 satellites being procured by the United States to bring new future capabilities to both military and civil positioning, navigation, and timing (PNT) users across the globe. Designed and manufactured by defense contractor Lockheed Martin (LM), the satellites are reported to deliver three times better accuracy, 500 times greater transmission power, and an eightfold enhancement in anti-jamming functionality over previous GPS satellite blocks. At ESA/ESOC, we are paying particular attention to this new tranche of satellites as they are the first to broadcast L1C, a new common signal interoperable with other GNSS, including Galileo.
At the time of this writing, there are six GPS III space vehicles (SVs) in orbit. The first one – nicknamed “Vespucci,” in honor of Italian explorer Amerigo Vespucci – lifted off atop a SpaceX Falcon 9 rocket from Cape Canaveral Air Force Station, Florida, in December 2018, and entered service on January 13, 2020. An additional four SVs are expected to be launched soon, before moving on to an updated version called GPS IIIF (“F” for Follow On). The first Block IIIF satellite is projected to be available for launch in 2026.
In view of the growing number of GPS III SVs in orbit, and soon to be joined by IIIFs, accurate spacecraft models and metadata information are becoming more and more important in order to maximize PNT accuracy.
SATELLITE ANTENNA PHASE CENTER PARAMETERS
GNSS signal measurements refer to the electrical phase center of the satellite transmitting antenna, which is neither a physical nor a stable point in space. The variation of the phase center location as a function of the direction of the emitted signal on a specific frequency is what we call the phase center variation (PCV). The mean phase center is usually defined as the point for which the phase of the signal shows the smallest (in a “least-squares” sense) PCV.
Figure 1: Ground-calibrated GPS Block III transmit antenna phase center variations (PCVs). (All figures provided by the authors).
The point of reference for describing the motion of a satellite, however, is typically the spacecraft center of mass (CoM). The difference between the position of the mean phase center and the CoM is what we typically refer to as the satellite’s antenna phase center offset (PCO). Both PCO and PCV parameters must be precisely known — from either a dedicated on-ground calibration or one performed in flight — so that we can tie our GNSS carrier-phase measurements consistently to the satellites’ CoM.
On-Ground Calibrations. Like for previous GPS vehicles, the Block IIR and Block IIR-M satellites, LM has fully calibrated the GPS III transmit antennas prior to launch at their ground test facilities. Antenna offset parameters for all three carrier signals (L1, L2 and L5) were posted on the U.S. Coast Guard Navigation Center (NAVCEN) website (www.navcen.uscg.gov) shortly after each satellite launch. In December 2021, NAVCEN released the PCOs for SV number (SVN) 78, along with updates to the first four satellites (see Table 1). About ten months later, in October 2022, the antenna pattern for each satellite and signal frequency were published (see Figure 1).
Table 1: Ground-calibrated GPS Block III transmit antenna PCOs in millimeters. (Image: GPS World staff)
The December 2021 offsets are referred to as predicted values at the end of year one on orbit. They differ from the previous ones by several centimeters in both vertical (Z) and horizontal (X and Y) directions. Particularly surprising are the X- and Y-PCOs, which were initially reported to be close to zero. The differences in the horizontal PCOs have generated uncertainty and debate, especially within the International GNSS Service (IGS) about which values to adopt for the new antenna model release (igs20.atx). Testing of the two different PCO datasets in our software demonstrated that the non-zero values as given in Table 1 are the significantly more accurate ones. We will return to this later in this article.
Combined Ground- and Space-Based Tracking. In this part of this article, we discuss the combination of dual-frequency tracking data from geodetic-quality GPS receivers in low Earth orbit (LEO) with those from a global receiver network on the ground to determine the phase center parameters of the GPS Block III transmit antennas. The LEO-based measurements were taken by the GNSS receivers on board the ocean altimetry satellites Sentinel-6 Michael Freilich and Jason-3. The 1,336-km altitude of both of these missions enables the estimation of the GPS satellite antenna PCVs from 0 up to 17 degrees from boresight while GPS receivers on Earth can only see the satellites up to a maximum angle of 14 degrees. The 14-degree limit is also referred to as the GPS satellites’ edge of Earth (EoE) angle.
For the modeling of the PCVs we follow the approach of the IGS using piece-wise linear functions of the boresight angle and constraining the PCV values to between 0 and 14 degrees to have zero mean. Furthermore, we employ fully normalized spherical harmonic expansions of degree 8 and order 5 to solve for the azimuth- and elevation-angle-dependent PCVs of the orbiting receiver antennas. The IGS standard antenna phase center corrections from igs20.atx are applied to all terrestrial receiver and GPS Block II transmit antennas.
Figure 2: GPS Block III transmit antenna PCVs as a function of boresight angle. The gray shaded area indicates the angular range that is inaccessible from the ground but relevant to high altitude LEO missions such as Sentinel-6 Michael Freilich or Jason-3.
The estimated Block III antenna PCVs are depicted in Figure 2. The estimates for the five individual antennas match each other to within 0.4 millimeters root-mean-square (RMS) (see Figure 2, top). The agreement among the PCVs that we get when processing the tracking data from each LEO receiver’s antenna separately is at the sub-millimeter level, too (see Figure 2, middle). Overall, the level of consistency suggests that the PCVs are of very good quality and that a block-specific representation is sufficient for precise applications. Comparison of the final block-specific PCV estimates against the values from the current IGS antenna model and from the ground calibrations shows strong agreement (RMS = 0.7 millimeters) between 0 and 14 degrees from boresight (see Figure 2, bottom). Beyond the 14-degree limit, the differences compared to the IGS standard are up to three centimeters, underlying the urgent need for an update of the igs20.atx file.
Applying the extended PCV corrections as part of the POD process to the GPS LEO receiver data shows significant improvement in the post-fit carrier-phase residuals when compared to the PCV corrections from the IGS legacy model. It removes a previously existing boresight angle-dependent trend and leads to a more than 20% reduction in the computed residual RMS (see Figure 3).
Figure 3: Post-fit residuals of GPS III carrier-phase data from Sentinel-6 Michael Freilich when using igs20.atx (top) and esa22.atx (bottom), respectively.
YAW MODELING
Figure 4: Yaw turn maneuver of GPS Block III satellite SVN 78 near orbit noon (top) and orbit midnight (bottom), respectively.
GNSS satellites cannot follow an ideal yaw-steering whenever the Sun elevation angle relative to the orbital plane (the so-called beta angle) gets too low and the yaw rate required to keep the satellite solar panels pointing towards the Sun exceeds the maximum satellite yaw rate. The strategies on how GNSS satellites perform rate-limited yaw-steering are different for each type of spacecraft and only partly documented for public users. Continuous knowledge of GNSS spacecraft yaw attitude, however, is important for kinematic and dynamic reasons. Errors in yaw are known to affect the modeling of transmit antenna phase center’s position, carrier-phase wind-up, and radiation pressure forces. On the other hand, when the mean antenna phase center location is offset from the spacecraft’s Z-axis, the satellite yaw state can be estimated instantaneously from the tracking data of a global receiver network. The approach behind this is commonly referred to as “upside down” or “reverse kinematic precise point positioning” (RPP). The horizontal antenna offset vector can be viewed here as a kind of rotating lever arm whose length determines the accuracy of the yaw angle estimates. Since the Block III X-offset is just 7 centimeters, one should not expect the same RPP accuracy as for other GNSS satellites like those of the GPS IIF or GLONASS-M series, which have an X-offset that is six (GPS IIF) or even eight (GLONASS-M) times larger.
Nonetheless, with more than three hundred ground stations, kinematic RPP works reasonably well even for GPS III as we can see from Figure 4, which shows the estimated yaw angle of SVN 78 while passing orbit noon and orbit midnight with a Sun elevation angle of almost zero degrees. The plots suggests that Block III satellites — unlike previous Block IIA and IIF SVs — perform their yaw slews near noon and near midnight in the same way and at the same yaw rate. In this respect, the yaw turn behavior is similar to that of the IIR/IIR-M satellites. However, with a maximum yaw rate of 0.10 degrees per second, the Block III satellites rotate only half as fast as those of the IIR/IIR-M family. What is also different is the start time of the yaw maneuver. As can be seen from Figure4, the maneuver does not start when the required yaw rate exceeds the physical limit but already a couple of minutes before.
The RPP analysis has led to the development of a simple yaw model for the Block III satellites. For a Sun elevation angle β below β0 = 4.780 degrees, the yaw angle can be approximated with an RMS accuracy of about 8 degrees by the following formula: whereas
is a modified Sun elevation angle, SIGN(β0, β) a FORTRAN function returning the value of β0 with the sign of β, and η is the satellite’s argument of latitude with respect to orbit midnight. The agreement between estimated and modelled yaw angles is illustrated in Figure 5.
Figure 5: Differences between yaw angle estimates and yaw angle models.
Fourier Series for Radiation Force Modeling. The most critical component determining the shape of a GNSS satellite’s trajectory is SRP – the force caused by the impact of solar photons hitting the satellite’s surfaces. A satellite’s sensitivity to SRP can be characterized by the variation of the cross-sectional area to mass ratio (A/M) of the satellite body as it orbits Earth and the Sun. The greater the change in A/M, the higher the sensitivity. From this perspective, the Block III spacecraft can be considered the most sensitive in GPS history.
Based upon LM’s tried-and-true A2100 bus, the satellite is much more elongated than previous generations. With an estimated size of 7.5 meters squared, the X-side is almost twice as large as the Z-side. Depending on the elevation angle of the Sun relative to the orbital plane, the body’s cross-sectional area exposed to sunlight varies between 4.0 and 8.5 meters squared (See Figure 6). With a nominal on-orbit weight of approximately 2,160 kilograms, this results in a change of A/M of 0.0021 meters squared per kilogram. For comparison, the corresponding values for the previous GPS SVs are 0.0015 (IIF), 0.0017 (IIR), and 0.0013 (IIA) meters squared per kilogram.
Figure 6: Size of GPS satellite body’s cross-sectional area exposed to sunlight.
Given the size and shape of Block III spacecraft, an appropriate radiation force model is considered mandatory to achieve the highest orbit accuracy possible. With that said, we empirically derived a set of background force models for the first five GPS III satellites. Our approach rests on dynamical long-arc (9-day) fitting to precise orbit data spanning up to three years and the following low-order Fourier functions of the Earth-spacecraft-Sun angle ε to represent the radiation force in the satellite body-fixed system:
The Fourier coefficients (XS1, XS2, XS3, YC2, ZC1, ZS2 and ZS4) are iteratively adjusted together with initial epoch state, a constant Y-axis bias, and 1‐cycle per revolution along‐track parameters to best fit the orbit data in a least-squares sense. All individual 9-day arc solutions are rigorously combined on a normal equations level to form a robust set of Fourier model coefficients for each satellite or group of satellites.
ORBIT OVERLAP TESTS
Figure 7: Impact of horizontal antenna PCOs and Fourier force model on day-boundary orbit overlap errors.
To investigate the effect of the transmit antenna PCOs and the Fourier force models on the satellite orbits, we use our ESA/IGS processing strategy to generate dynamic 24-hour-arc solutions spanning January 2020 to December 2022, first with zero PCO and the non-zero horizontal offsets from Table 1 and no a-priori radiation force model, then with the non-zero offsets and the additional Fourier model in the background. The direct comparison of the generated orbits reveals significant differences for the Block III satellites of about 0.1 meters (3D).
To demonstrate the improved performance of the non-zero offsets and the Fourier model, we take the orbits for successive days and look at the midnight epoch where they overlap. The difference in the orbit position, subsequently referred to as “overlap error,” gives us a worst case estimate of the satellite orbit accuracy. Comparison of the overlap errors provides evidence that the Block III orbits are much more accurate when using the non-zero rather than the zero X and Y PCOs. The overall 3D overlap RMS reduces from 49.5 millimeters (with zero PCOs) down to 32.3 millimeters (with non-zero PCOs). Results for the Sun elevation regions below 45 degrees, in particular, show significant improvement (see Figure 7).
Use of the Fourier model has additional positive impact on the overlaps. Comparing the orbits produced with and without the a-priori radiation force model, we see a decrease in the 3D overlap error RMS from 32.3 to 29.7 millimeters averaged over all satellites. The orbit component that benefits most from both the improved antenna phase and the advanced force modeling is the one normal to the satellite orbital plane (across track). The SVs improving the most are SVN 75 and SVN 78, though significant improvements can be seen for all other satellites too (see Table 2).
Table 2: Day-boundary overlap RMS errors of GPS III spacecraft orbits in millimeters.
EMPIRICAL PARAMETER ESTIMATES
Another means of assessing the quality of spacecraft models is the size and variability of the five-plus-three empirical dynamic radiation pressure parameters that we still estimate on a daily basis for each GNSS satellite in addition to its a-priori force model. Introducing the non-zero PCO and Fourier models into the POD turned out to reduce the size of the empirical parameters and their dependency on the satellite-Sun geometry to a great extent as the example in Figure 8 demonstrates.
Figure 8: Impact of horizontal antenna PCOs and Fourier force model on empirical once-per-revolution acceleration term BC.
NARROW-LANE AMBIGUITY FRACTIONALS
Integer ambiguity resolution — that is, resolving the unknown cycle ambiguities of double-differenced carrier-phase data to integer values — is considered indispensable to GNSS satellite POD and commonly results in a factor of two improvement in orbit precision. Of particular importance is the narrow-lane ambiguity that results from combining the carrier-phase measurements from a pair of GNSS frequencies. One of the intermediate steps in the ambiguity resolution algorithm is the fixing of the double-differenced narrow-lane ambiguities to integer values. For reliable fixing, the fractional part of the difference between the integer and decimal (float) values should be as close as possible to zero and follow a symmetrical distribution. The “tailedness” of the distribution curve may be characterized by its kurtosis — the larger the kurtosis, the fewer values are in the tails of the distribution and the more peaked is the distribution. In other words, the larger the kurtosis, the closer the “fractionals” cluster around zero, the more ambiguities can be resolved with higher confidence, and the more accurate the resolved solution. Moreover, as satellite orbit and antenna phase center errors do not cancel out completely through double-differencing, the narrow-lane kurtosis may also be considered as an indicator for the accuracy of the satellite force and phase center models that were used. The results in Figure 9 show that the non-zero horizontal PCOs bring a major improvement and that the Fourier force model does give some additional benefit.
Figure 9: Impact of horizontal antenna PCOs and Fourier force model on fractional part of double-differenced narrow-lane ambiguities.
CONCLUSIONS
Adding a new GNSS satellite type to high-precision multi-GNSS solutions requires detailed knowledge and understanding of the satellite type. Key issues are the transmit antenna phase center parameters, the satellite’s attitude, and the radiation pressure forces acting on its surfaces.
In this article, improved antenna phase center, attitude, and radiation pressure models for the current series of GPS Block III spacecraft have been developed using multiple years of in-flight orbit and tracking data. A number of internal metrics such as post-fit carrier-phase residuals, day-boundary orbit differences (overlaps), empirical acceleration parameters, and carrier phase ambiguity statistics have been used to gauge the models’ performances. Overall, the results underscore the importance of the models for GPS III orbit determination. This applies primarily to the radiation force and the antenna phase center model, or more precisely, the horizontal (X and Y) offsets of the phase center model whose existence has been neglected for years in the analysis of GPS III data.
Comparison of the overlap statistics suggest that orbits generated based upon updated (non-zero) phase center corrections and ESA/ESOC’s new (Fourier-based) radiation pressure model in the background are better by almost a factor of two. The average overlap RMS errors calculated across all current Block III SVs and for each orbital component (radial, along track and across track) dropped from 21 , 28 and 35 millimeters down to 14, 21 and 16 millimeters, respectively.
More relevant when it comes to processing GPS data recorded on board low-flying satellites such as Sentinel-6 Michael Freilich or Jason-3, is the extension of the current IGS Block III antenna PCV model beyond a 14-degree boresight angle. After applying the extended PCV corrections, we reduced Block III carrier-phase residuals by 20% with no or few systematic signatures remaining, unlike the residuals produced with the current IGS antenna model. The IGS is strongly encouraged to adopt the Block III PCV extension into their antenna model to continue to support GPS-based POD of low-Earth-orbiting satellites.
For further details on ESA/ESOC’s solar radiation pressure modeling approach, see our paper “GPS III Radiation Force Modeling” presented at the IGS 2022 Virtual Workshop: click here.
FLORIAN DILSSNER is a satellite navigation engineer in the Navigation Support Office at the European Space Operations Centre (ESOC) of the European Space Agency (ESA), Darmstadt, Germany. He earned his Dipl.-Ing. and Dr.- Ing. degrees in geodesy from the University of Hannover, Germany.
TIM SPRINGER has been working for the Navigation Support Office at ESA/ESOC since 2004. He received his Ph.D. in physics from the Astronomical Institute of the University of Bern in 1999.
FRANCESCO GINI is a satellite navigation engineer in the Navigation Support Office at ESA/ESOC. He received his Ph.D. in astronautics and space sciences from the Centro di Ateneo di Studi e Attività Spaziali at the University of Padova in 2014.
ERIK SCHÖNEMANN is a satellite navigation engineer in the Navigation Support Office at ESA/ESOC. He earned his Dipl.-Ing. and Dr.- Ing. degrees in geodesy from the University of Darmstadt, Germany.
WERNER ENDERLE is head of the Navigation Support Office at ESA/ESOC. He holds a doctoral degree in aerospace engineering from the Technical University of Berlin, Germany.
