GPS World

Category: GLONASS

  • New type on the block: Generating high-precision orbits for GPS III satellites

    New type on the block: Generating high-precision orbits for GPS III satellites

    Read Richard Langley’s introduction to this article: Innovation Insights: Antennas and photons and orbits, oh my!

    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).
    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)
    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.
    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 Rreilich when using igs20.atx (top) and esa22.atx (bottom), respectively.
    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 now (top) and orbit midnight (bottom), respectively.
    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 Figure 4, 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:Photo:
    whereasPhoto:

    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.
    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.
    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:

    Photo:

    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.
    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.
    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.
    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.
    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.

  • Singular XYZ launches GNSS receiver with network RTK rover

    Singular XYZ launches GNSS receiver with network RTK rover

    Singular XYZ has released the Sfaira One GNSS receiver. The portable size, centimeter-accurate receiver provides users with an entry-level network real time kinematic (RTK) rover.

    Sfaira One is equipped with a GNSS module with 1,408 channels for GPS, BDS, GLONASS, Galileo and QZSS tracking — providing centimeter positioning in harsh environments. It also features advanced RTK and an anti-interference algorithm.

    The GNSS receiver connects via Bluetooth and can be configured to conduct surveying tasks on a smartphone. Additionally, Sfaira One supports SingularPad and SingularSurv software and is also compatible with mainstream field survey or GIS software.

    Sfaira One is IP65 dustproof and waterproof, which makes the receiver suitable for all weather conditions. It has a 4,800 mAh battery life with 16 hours working time and type-C interface that can be charged on-the-go with power bank.

    The Sfaira One GNSS receivers are online at SingularXYZ’s website and are available now.

  • GNSS Almanac: Key stats on GNSS constellations

    GNSS Almanac: Key stats on GNSS constellations

    Image: vasilypetkov/iStock / Getty Images Plus/Getty Images
    Image: vasilypetkov/iStock / Getty Images Plus/Getty Images

    In our October 2021 issue, we celebrated the availability of four global navigation satellite system (GNSS) constellations. Below is the status (as of Feb. 23, 2023) of these four GNSS and their two regional cousins.

    Many thanks to Mohamed Tamazin, Ph.D., Senior GNSS Architect for GNSS Simulation with Orolia — a Safran Electronics & Defense company, who provided or confirmed these data. While the data on GPS and Galileo are easily accessible, those for the other constellations are difficult, in some cases very difficult, to find.

    — Matteo Luccio, Editor-in-Chief

    GNSS Almanac chart 2023

  • CHC Navigation releases GNSS RTK steering system

    CHC Navigation releases GNSS RTK steering system

    Image: CHC Navigation
    Image: CHC Navigation

    CHC Navigation has released the NX510 SE Auto-Steer, an automated steering system that retrofits several types of new and old farm tractors and other vehicles. It can be connected to local real-time kinematic (RTK) networks or GNSS RTK base stations.

    NX510 SE is a guidance controller powered by multiple corrections sources and five satellite constellations: GPS, GLONASS, Galileo, BeiDou and QZSS. It has a built-in 4G and UHF modem that connects to all industry-standard differential GPS and RTK corrections to achieve centimeter-accuracy steering.

    NX510 SE contains GNSS and inertial navigation system terrain compensation technology, which maintains high accuracy in challenging environments and terrain. This makes NX510 SE suitable for ditching, planting and harvesting applications.

    In addition, AgNav multilingual software, operating on a 10.1 in industrial display, supports multiple guideline patterns that include AB line, A+ line, circle line, irregular curve and headland turn.

  • Mil-spec GPS/GNSS antennas released by Pasternack

    Mil-spec GPS/GNSS antennas released by Pasternack

    Image: Pasternack
    Image: Pasternack

    Pasternack has released a series of GNSS antennas that meet military specifications for use in several small form factor and mobile applications.

    The mil-spec GNSS antennas are engineered for environmental performance according to the MIL-STD-810G standard and include multi-standard GPS L1, Galileo E1 and GLONASS options.

    The MIL-STD-810G GNSS antennas are IP67 rated. They are available in passive and active versions and provide coverage from 1,597 MHz to 1,607 MHz. The GNSS antennas feature linear polarization for cross-polarized isolation, nominal gain options of -3 dBic and 10 dBic, and SMA mounts.

    Pasternack’s mil-spec GNSS antennas are available now.

  • First Fix: Arrivals and Departures

    First Fix: Arrivals and Departures

    Matteo Luccio
    Matteo Luccio

    As we begin 2023, GNSS development continues apace, as described in this issue’s annual “Directions” section by representatives of Galileo, GLONASS, and BeiDou. We plan to publish a similar update on the GPS program soon.

    Galileo’s user base now stands at more than 3.5 billion, and the services it provides continue to improve and expand. Beginning early this year, free precise point positioning (PPP) corrections for Galileo and GPS (single- and multi-frequency) will improve real-time user position by up to 10 times. While the discontinuation of Soyuz launch services from the Kourou Space Centre in French Guiana, due to the Russia-Ukraine conflict, delayed the two Galileo launches that had been planned for last year, 2022 was a key year for the development of Galileo Second Generation (G2G) satellites. They will provide, among other innovations, a reconfigurable fully digital navigation payload, point-to-point connection between satellites, and advanced jamming and spoofing protection mechanisms.

    On Nov. 29, 2022, Russia launched the 51st Glonass-M satellite, about 20 years after launching the first one. Currently, 13 of these satellites are operating beyond their guaranteed lifetime, with an average orbit lifetime of more than 10 years. Starting this year, the constellation will be renewed by Glonass-K and Glonass-K2 satellites, which provide CDMA signals to users.

    Currently, 45 BDS satellites are operational in orbits, including 15 BDS-2 satellites and 30 BDS-3 satellites. The constellation says that it has reached a continuity of 99.996% and an availability of 99%, with a global positioning accuracy better than 1.5 meters horizontally and 2.5 meters vertically (95% confidence).

    Tracy Cozzens, who has been a pillar of this magazine for 17 years, is retiring this month. We will miss her journalistic acumen, dedication to clarity and style, attention to detail, and wealth of institutional knowledge. We wish her a well-deserved retirement. At the same time, we welcome aboard Maddie Saines, our new managing editor, who is near the beginning of her career.

    I am pleased to announce that Rob VanBrunt has joined GPS World’s Editorial Advisory Board. In mid-December, the board of directors of Spirent Federal Systems, a provider of PNT test solutions for the U.S. government and contractors, appointed him as the company’s president/CEO-designate, a role he will assume when the onboarding process is complete.

    VanBrunt began his career at Spirent Communications in 1990 as product developer and manager, and then held posts of increasing responsibilities, moving to director and vice president roles focused on management, strategy and mergers and acquisitions. Most recently, he was executive vice president in the Office of Business Excellence. VanBrunt has a B.S. in electrical and electronics engineering from Rutgers University.

    Spirent Communications is a global provider of automated test and assurance solutions for networks, cybersecurity and positioning. In July 2001, the company formed Spirent Federal Systems as a wholly owned subsidiary and U.S. proxy company. Spirent Federal markets and sells Spirent Communications’ products in North America. It also provides value-added features and ongoing customer support.

    On Jan. 1, I lost my beloved mother, Maristella “Mimi” Luccio. She was 87.

    Matteo Luccio | Editor-in-Chief
    [email protected]

  • Directions 2023: GLONASS Renews Its Constellation

    Directions 2023: GLONASS Renews Its Constellation

    On Nov. 29, 2022, Russia launched the 51st Glonass-M satellite, completing a 20-year history that began on Dec. 10, 2003, with the launch of the first one. These satellites have been providing navigation signals in two frequency bands, L1OF and L2OF, to civil users since 2011.The average orbit lifetime for this type of satellite is more than 10 years, and 13 Glonass-M satellites operate beyond their guaranteed lifetime. The last set of seven satellites has been broadcasting the first CDMA civil signal, L3OC, by means of an additional antenna and onboard transmitter.

    Starting this year, the constellation will be renewed by Glonass-K and Glonass-K2 satellites, which provide CDMA signals to users. Furthermore, four Glonass-K satellites will be supplemented with additional Glonass-K satellites and the first Glonass-K2 satellite. The K2 satellite has passed all ground tests and is ready to be transported to the launch site (Figure 1). Table 1 lists the technical characteristics of GLONASS satellites.

    GLONASS image001
    Figure 1. Artist’s rendition of the Glonass-K2 satellite in orbit.
    Table 1. The evolutions of GLONASS satellites.
    Table 1. The evolutions of GLONASS satellites.

    The distinguishing feature of this satellite’s design is its two antenna arrays — one for CDMA signals with phase centers on the geometrical axis of the satellite, and the second for FDMA signals with phase centers shifted by 0.9 m relative to that axis.

    The optical reflector panel center is also located on the satellite’s geometrical axis and passed through its mass center. It seems to be a very interesting scientific task to estimate the satellite flight model parameters by International Laser Ranging Service stations with the objective to improve the accuracy of the navigation signals for both antenna arrays.

    Future GLONASS satellites will have a single antenna array for CDMA and FDMA signals (see Figure 2).

    GLONASS image002
    Figure 2. The evaluations of GLONASS satellites.

    For analogous updates on the other three GNSS constellations, please see:

  • Ukraine hackers target GLONASS for cyberattacks

    Ukraine hackers target GLONASS for cyberattacks

    Photo: EvgeniyShkolenko/iStock/Getty Images Plus/Getty Images
    Photo: EvgeniyShkolenko/iStock/Getty Images Plus/Getty Images

    Ukraine’s hacker underground named GLONASS as one of its top priorities, according to media reports that cite a post on the “IT army” Telegram channel.

    The IT army, formed on Saturday, is a collective of volunteer hackers. “We need to mobilize and intensify our efforts as much as possible,” the IT army posted.

    Besides GLONASS, hackers are focusing on Russian telecom companies and the railway network in Belarus — a key staging area for Russia’s invasion of Ukraine.

    The Belarusian Cyber Partisans, a hacking team focused on Belarus, told Reuters it had disabled railway traffic systems in Belarus. Another target is the electrical grid.

  • Seen & Heard: Driving fish, North Korean tests

    Seen & Heard: Driving fish, North Korean tests

    “Seen & Heard” is a monthly feature of GPS World magazine, traveling the world to capture interesting and unusual news stories involving the GNSS/PNT industry.

    GO, FISH!

    Scientists at Ben-Gurion University in Israel discovered goldfish are good drivers. An aquarium on wheels uses lidar, an onboard camera, and motion-detection software to track a fish in the tank. When it swims toward a side of the tank, the vehicle rolls in the same direction. Fish learned to navigate a small area to hit a target for a reward at least 15 times per half-hour, showing their sense of direction isn’t limited to a watery environment.

    Photo: narvikk/iStock/Getty Images Plus/Getty Images
    Photo: narvikk/iStock/Getty Images Plus/Getty Images

    NORTH KOREA GUIDED BY GLONASS

    North Korea’s recent missile tests are being guided by GLONASS rather than BeiDou, claims a source close to the Chinese military. In January, Pyongyang fired at least four short-range ballistic missiles into the sea off the coast of the Korean peninsula. North Korea also tested hypersonic missiles on Jan. 5 and 11. As reported by the South China Morning Post, the source said North Korea has been using GLONASS and has benefited from Soviet technology.

    Photo: Prakhar Gupta/iStock Editorial/Getty Images Plus/Getty Images
    Photo: Prakhar Gupta/iStock Editorial/Getty Images Plus/Getty Images

    AUTOMATED AUTOWAYS IN INDIA

    The National Highways Authority of India (NHAI) will start using GNSS/3D automated machine guidance to construct highways. First up: the new Lucknow-Kanpur Expressway. Three machine-control systems will be integrated, guided by a computerized 3D model of the project. NHAI expects the technology to almost double the speed of highway construction, as well as provide stakeholders with mobile updates.

    Photo: Tonga Meteorological Services, Government of Tonga
    Photo: Tonga Meteorological Services, Government of Tonga

    SATELLITES REGISTER OCEAN ERUPTION

    While imagery satellites captured the explosive Jan. 15 underwater volcanic eruption in Tonga, GNSS satellites also registered its effects. The energy released reached the ionosphere and was picked up by GNSS signals passing through it (see JPL’s findings). The eruption blanketed Tonga’s main island in ash and devastated its western coastline, affecting up to 80,000 people, according to news reports. It also caused damage to an underwater cable, resulting in a major communication outage. The tsunami that followed reached as far as Japan and the U.S. West Coast, and caused a massive oil spill off of Peru.

  • GNSS constellations create four strong winds

    GNSS constellations create four strong winds

    Matteo Luccio
    Matteo Luccio

    First, there was one. In July 1995, the U.S. Air Force declared the Global Positioning System had met all the requirements for full operational capability (FOC). Soon thereafter, there were two. In December of that same year, Russia’s Globalnaya Navigazionnaya Sputnikovaya Sistema (Global Navigation Satellite System, or GLONASS), also achieved FOC. For a quarter century, that was it.

    Then, last year, the number doubled, as both the European Union’s Galileo and China’s BeiDou Navigation Satellite System (BDS, named after the Big Dipper asterism, which is known in Chinese as Beidou) achieved FOC.

    The Indian Regional Navigation Satellite System (IRNSS, aka Navigation Indian Constellation, or NavIC, which means “sailor” or “navigator” in Hindi) and Japan’s Quasi-Zenith Satellite System (QZSS, also known as Michibiki) are not global yet, but plan to become so. Currently, NavIC is an autonomous regional satellite navigation system, and NavIC-based trackers are compulsory on commercial vehicles in India. QZSS currently complements GPS to improve coverage in East Asia and Oceania, but Japan plans to have an operational constellation of seven satellites for autonomous capability by 2023. The Korea Positioning System (KPS) plans to join the party by 2035.

    Who’s next? Will it be another country or a private company? Given that the state-sponsored systems are free to end users, I don’t see what the business model would be for a private GNSS constellation, unless it were to piggyback on one built mainly for another purpose.

    Surveyors who have begun to routinely use three or more constellations are over the moon. One, quoted in this month’s cover story, recalls that “the use of GPS for construction staking was an extremely risky proposition” because its residuals exceeded most construction tolerances. Using multiple GNSS constellations, however, has increased confidence in the accuracy of results to the point that some construction companies are relying on GNSS receivers for staking. Additionally, multi-constellation receivers can now increasingly be used under tree canopies and against structures, whether natural or built.

    Whatever their mix of military, political and commercial motivations for building, deploying and operating their own GNSS constellations in addition to the original two, the European Union, China, India, Japan, Korea and whichever entity may follow are greatly improving satellite-based positioning, navigation and timing (PNT) for all users everywhere — by increasing accuracy, shortening the time to first fix, and making GNSS more impervious to jamming and spoofing.

    In 1978, the year that the U.S. Department of Defense launched the first NAVSTAR GPS satellite (“NAVSTAR” was later dropped from the system’s name), Neil Young sang “Four Strong Winds” (originally written by Ian Tyson and performed by him with his wife Sylvia as the Canadian folk-duo Ian and Sylvia).

    Now, GNSS has “four strong winds,” two lighter ones and several more breezes to follow. As a sailor and a navigator, I welcome them heartily. As this magazine’s editor-in-chief, I don’t mind that, like Jeep, Kleenex, Popsicle and Xerox, GPS probably will stick in popular culture as a generic term for global satellite navigation systems way past its accurate description of what is in the box.

    Matteo Luccio | Editor-in-Chief
    [email protected]

  • Launch of first Glonass-K2 satellite postponed until 2022

    Russia has postponed the launch of its first next-generation Glonass-K2 satellite until early in 2022, according to a report from TASS. The launch was originally scheduled for the fourth quarter of this year. The delay is due to a portion of onboard equipment requiring further work following a ground-based test run.

    Glonass-K2 is the next-generation navigation satellite of Russia’s GLONASS positioning system. The K2 satellites are expected to provide navigation precision of less than 30 cm.

  • Report: GNSS simulators market to be worth $165M by 2025

    Report: GNSS simulators market to be worth $165M by 2025

    Photo: Philipp Berezhnoy/iStock / Getty Images Plus/Getty Images
    Photo: Philipp Berezhnoy/iStock / Getty Images Plus/Getty Images

    According to a new research report, “GNSS Simulators Market by Component Type (Single Channel and Multichannel), GNSS Receiver (GPS, Galileo, GLONASS and BeiDou), Application (Vehicle Assistance Systems, Location-based Services and Mapping), Vertical, and Region — Global Forecast to 2025,” the market is estimated to grow from $106 million in 2020 to $165 million by 2025, at a compound annual growth rate (CAGR) of 9.3% during the forecast period.

    Factors such as rapid penetration of consumer internet of things, the contribution of 5G in enabling ubiquitous connectivity and increasing use of wearable devices utilizing location information are expected to drive the adoption of the GNSS simulators hardware, software and services, the MarketsandMarkets report said.

    Market segments

    The report stressed several key points in regard to the hardware segment, automotive vertical segment and GPS segment.

    The GNSS simulators market, by component, is segmented into hardware, software and services. Of these three, the hardware segment is expected to account for a larger market size in the coming years in the GNSS simulators market as it enables signal simulations and plays a key role in the simulation process.

    The GNSS simulators market, by vertical, is segmented into military and defense, automotive, consumer electronics, aerospace, marine and other verticals (agriculture, construction, and rail and road). According to the report, the automotive vertical segment is expected to hold a higher CAGR during the forecast period in the GNSS simulators market.

    “GNSS offers a cost-effective and accurate way of determining position in real-time,” the report said. “Ride-sharing apps, such as Uber and Lyft, car-sharing, usage-based insurance apps, dynamic toll charging and parking apps all depend on the real-time location of the car. GNSS offers sufficient accuracy for all these apps by providing location coordinates leading to its adoption in the automotive vertical.”

    Next, GNSS receivers are segmented into GPS, GLONASS, Galileo, BeiDou and others, with the others comprised of NavIC and QZSS receivers. The satellite tracking service provided by GPS is useful in various commercial and personal applications. The increasing commercial application of GPS vehicle tracking system for taxi companies and cab services is contributing to the rising adoption of GPS location services. It makes the navigation easier by providing relevant details, such as distance and shortest route to the desired location, the report said.

    Market size and location

    North America is expected to have the largest market size during the forecast period, with the key factor favoring the growth of the GNSS simulators market in North America being the increase in technological advancements coupled with the rise in government support for the adoption of UAVs. The increasing number of players across different regions is also further expected to drive the GNSS simulators market globally, the report said.

    In addition, Asia-Pacific is projected to grow at the highest CAGR during the forecast period, as it has witnessed considerable developments and adoption of solutions across the region. According to the report, Asia-Pacific has a large potential for GNSS and space-enabled applications, and it has a wide consumer base due to its booming economies accompanied by an increasing share of local R&D and technology innovations.

    Major players

    Major vendors in the global GNSS simulators market include Spirent Communications, Rohde & Schwarz, VIAVI Solutions, Hexagon, Keysight Technologies, u-blox, Orolia, Cast Navigation, Accord Software and System, IFEN, Racelogic, Syntony GNSS, Teleorbit, iP-Solutions, Pendulum Instruments, Saluki Technology, Shanghai Huace Navigation Technology, Averna, GMV NSL, Brandywine Communications, Jackson Labs Technologies, Hunan Shuangln Electronic Technology, Work Microwave, Qascom and M3 Systems.