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.
Figure 1. Artist’s rendition of the Glonass-K2 satellite in orbit.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).
Figure 2. The evaluations of GLONASS satellites.
For analogous updates on the other three GNSS constellations, please see:
A new investigative report by the Russian independent media group “The Project” into luxury dachas owned by high-ranking government officials revealed that most all include GNSS jammers among their amenities. Attempts by the journalists to photograph the dachas from the air using drones were routinely foiled by jamming.
Most all nations’ military and security services have equipment that can block GPS and other satellite navigation signals over areas both large and small. Russia, though, has advanced this to a fine art which it regularly demonstrates.
Russian forces always been proud of their electronic warfare capabilities. They see them as an essential counter to the effectiveness of western high-tech weapons. The news outlet “Sputnik” reported in 2015 Russian military claims that their ability in electronic warfare “makes aircraft carriers useless.”
GPS is an underlying technology for many western weapons, and for much of the west’s critical networked infrastructure. As a result, jamming and spoofing GPS and other GNSS has long been a priority for Russian forces.
In 1997 a Russian company offered a handheld four-watt GPS and GLONASS jammer that was effective at ranges of up to 150 to 200 kilometers. They also reported working with the Russian military on directional antennas for this jammer. These antennas would focus the disruption on a particular target while leaving most other users unaffected. The U.S. Army was sufficiently interested that, in 2002, they reportedly spent almost $200,000 to purchase the jammers for testing and evaluations.
In 2016 Russia announced a program to add GPS jammers to more than 250,000 cell towers as a partial defense against a U.S. cruise missile attack.
That same year a Moscow Times headline proclaimed, “Kremlin Eats GPS for Breakfast!” GPS users near the Kremlin had been regularly finding their cell phones reporting that they were 20 kilometers away at an international airport. This was playing havoc with Uber and Lyft drivers, as well as delivery services that depended upon satellite navigation. This spoofing, or sending false information to receivers, was reported to be an effort to protect the Kremlin and leaders from attack and surveillance by drones. Most drones are programmed at the factory with the locations of airports and to fly away from them. Convincing receivers near the Kremlin or elsewhere that they are really near an airport helps keep the area drone-free.
Independent technologists in Moscow also reported that this spoofing employed a classic electronic warfare technique called “herding.” GPS L2 and L5 signals and Russia’s GLONASS satellite navigation signals were jammed. This forced receivers to rely upon the L1 signal which was spoofed.
That same year this same kind of activity was also detected in the Black Sea. The RNT Foundation reported that over 600 ships had been “transported” to airport locations ashore. A subsequent report in 2019 by the non-profit group C4ADS revealed almost 10,000 instances of ships being spoofed in the Black Sea, the Baltic and in Russia’s west near Vladivostok between 2016 and 2018. It also drew a strong correlation between the movements of Russian President Vladimir Putin and the spoofing events.
Russian jamming and spoofing has not been limited to its homeland. Vehicles, ships and aircraft in other nations, as well as in international waters and airspace, have been impacted. This despite Russia’s treaty obligations under the International Telecommunications Union radio regulations which provide that “All transmissions with false or misleading identification are prohibited.”
The C4ADS report documented a massive Russian “smart jammer” operating almost continuously in Syria that had impact far beyond that nation’s borders. Smart jammers, by their definition, transmit messages that seem to be valid GPS signals, but with content that does not allow receivers to calculate a location. The operation in Syria has caused multiple warnings by the U.S. Maritime Administration of GPS disruptions in nearby international waters, and the European air traffic agency issuing warnings for international airspace in the eastern Mediterranean.
The Baltic and Scandinavia have also seen Russian GPS jamming in recent years. In 2017 the Secretary General of NATO complained about Russian naval jamming that also degraded cell phone service in Latvia, Norway and Sweden.
Early this year Norway protested Russian jamming in its far north, some of which was timed for NATO exercises. Five significant jamming events in the previous 17 months impacted, aviation, construction and other users.
Russia regularly demonstrates that GNSS jamming and spoofing can be a useful tool for internal security and an effective method of power projection. Its actions, along with the portability and proliferation of jamming and spoofing equipment, are undoubtedly meant to remind the west that Russia can take away essential GNSS services at any moment with a just the flip of a switch.
In this article, author Urs Hugentobler looks at the history of laser ranging to navigation satellites, how that ranging has improved the accuracy of the orbits of those satellites and what the future portends for this important contribution to space geodesy.
INNOVATION INSIGHTS with Richard Langley
THE LASER. It might not be in the top 10 of the most important inventions of all time, but Time magazine rated it among the most important developments of the 20th century, listing it fifth after the automobile, the radio, the television and the transistor. Lasers are now ubiquitous: they scan our purchases at the supermarket checkout; they let us read and write data on compact discs; they have replaced the scalpel in many operating theaters; and they play major roles on the battlefield with laser-guided munitions. However, one of the first practical uses of the laser was in precisely determining the orbits of satellites.
Initial experiments in ranging to satellites carrying corner-cube retroreflectors began in 1964 just a few years after the laser was invented in 1960. Satellite laser ranging (SLR) stations were built in several countries, and a number of multi-instrument satellites with retroreflectors were launched by the U.S. and other nations along with dedicated spherical satellites with no electronic instrumentation — just the retroreflectors covering the satellite’s surface. The first of these was the Laser Geodynamics Satellite, or LAGEOS. It was designed by NASA and launched in 1976. LAGEOS and the other satellites carrying retroreflectors played a significant part in NASA’s Crustal Dynamics Project (CDP). Initiated in 1979, the CDP promoted the use of SLR and very long baseline interferometry to improve our understanding of plate tectonics, the rotational dynamics of the Earth, and the structure of the Earth’s gravity field.
As a post-doctoral fellow at the Massachusetts Institute of Technology and later at the University of New Brunswick, I participated in the CDP with analyses of lunar laser ranging (LLR) data. Ranging to reflectors placed on the moon’s surface by Apollo astronauts as well as those on the Russian Lunokhod rovers was a bit more difficult than ranging to satellites given the larger distances to the reflectors and the much weaker return pulses. Among other advances, LLR was the first technique to confirm the existence of variations in the spin of the Earth with a periodicity of around 50 days.
But let’s get back to SLR. Today, thanks in large measure to the International Laser Ranging Service, ranging data is routinely collected on more than 70 satellites and lunar reflectors. Included is a growing list of GNSS satellites equipped with corner-cube retroreflectors. Laser ranging to GNSS satellites is instrumental is better modeling the orbits of these satellites. Among other benefits, better GNSS satellite orbits result in better receiver position accuracies — accuracies needed to improve monitoring of crustal strain, for example, including that associated with earthquakes.
In this month’s column, we take a look at the past, present and future of laser ranging to GNSS satellites and how laser ranging and microwave ranging are mutually beneficial. They are truly kindred spirits.
Nighttime ranging at NASA’s Next Generation SLR system at Goddard Space Flight Center, Maryland. (Credit: Felipe Hall/HTSI)
Satellite laser ranging or SLR has been an indispensable independent tool for validating the precise orbits determined for GNSS satellites using microwave pseudorange and carrier-phase observations for several decades. SLR has allowed researchers to identify several orbit-modeling issues. Adding albedo radiation pressure and antenna thrust, among other effects, into the GPS orbit model allowed them to eliminate the observed bias between microwave- and SLR-derived orbits. For the first Galileo satellites launched, SLR residuals indicated severe orbit modeling issues caused by the different shape of Galileo satellite bodies compared to those of GPS. In the future, all GNSS satellites will be equipped with laser retroreflectors, a big challenge for researchers concerning tracking scenarios and observation planning to make economic use of the ground equipment.
In this article, we will take a brief look at the history of laser ranging to navigation satellites, how that ranging has improved the accuracy of the orbits of those satellites, and what the future portends for this important contribution to space geodesy.
VALIDATION OF GNSS ORBITS
FIGURE 1. Operating principle of satellite laser ranging.
In 1964, only four years after Theodore Maiman built the first laser, the first laser echoes were obtained from NASA’s Explorer 22 satellite. SLR rapidly developed into an indispensable tool for precise orbit determination, gravity field determination, and Earth system research.
FIGURE 1 shows the principles of SLR operation. Essentially, an SLR station fires a series of laser pulses at passing satellites equipped with corner-cube retroreflectors, and the relatively few photons returned are collected by a telescope. The station electronics measures the round-trip travel times of the laser pulses. From these measurements, the coordinates of the SLR station or the satellite’s orbit can be determined.
Observations by a global network of SLR stations are coordinated by the International Laser Ranging Service (ILRS), which, like the International GNSS Service, is one of the space geodetic services of the International Association of Geodesy (IAG).
FIGURE 2. Retroreflector array on GPS Block IIA satellites SVNs 35 and 36.
Since the early 1990s, the ILRS has tracked GNSS satellites supporting the independent validation of the microwave-derived precise orbits. Two Block IIA GPS satellites, SVN35 and SVN36, were equipped with retroreflectors (see FIGURE 2) and they were routinely tracked from their launches in 1993 and 1994, respectively, until their decommissioning in 2013 and 2014 (actually, SVN36 was subsequently briefly reactivated in 2015 so data is available for that satellite until that year). Also in the 1990s, the ILRS started to track GLONASS satellites in support of the International GLONASS Experiment (IGEX-98). There is a retroreflector array on all GLONASS satellites (see FIGURE 3).
Range residuals of GPS and GLONASS satellites were studied in the early years by a number of different research groups. Most of their analyses showed a bias of about –5.5 centimeters for GPS satellite orbits derived from microwave tracking data by the IGS while the accuracy of the latter was estimated to about 5 centimeters. For GLONASS orbits, a negative bias of about –4 centimeters was identified, too. The accuracy of the orbits was, however, at the 10–15 centimeter level. These validation results supported several model improvements for GPS satellite orbits including, in particular, the handling of solar and Earth albedo radiation pressure and antenna thrust, reducing the observed SLR bias with respect to the IGS orbits to 1.3 centimeters with a standard deviation of about 2 centimeters.
“What are radiation pressure and antenna thrust?” you might ask. The photons making up the light coming directly from the sun or reflected from the Earth’s surface (albedo) impinge on a satellite and transfer some of their energy to it. Solar radiation pressure – the force due to the impact of the photons – is tiny, but its continuing presence has a strong perturbing effect on satellite orbits. Antenna thrust is also a small force. The transmission of GPS navigation signals results in a continuously acting reactive force in the radial direction acting on the satellite.
FIGURE 4. Retroreflector array on Galileo satellites (at bottom of satellite, below antenna array).
SLR also plays an essential role for calibrating improved radiation pressure models for the new satellite systems. All Galileo satellites have retroreflectors (see FIGURE 4), and the orbits of the first satellites to be launched, generated using the classical extended radiation pressure model of the Center for Orbit Determination in Europe (operating in the framework of the IGS Multi-GNSS Pilot Project or MGEX), had SLR residuals as large as 20 centimeters for passes with a small beta angle. (The beta angle is the angle between the sun and a satellite’s orbital plane.) The origin of this behavior is the elongated shape of the Galileo satellites compared to the more-or-less cubic shape of GPS satellites, causing much larger variations of the satellite cross-section exposed to the sun while orbiting the Earth. The observed SLR residuals triggered the development of improved radiation pressure models for Galileo satellites.
All BeiDou satellites are also believed to be equipped with retroreflectors (see FIGURE 5). As the estimated longitude of geostationary GNSS satellites such as those in the BeiDou constellation is highly susceptible to biases due to the small motion of the satellites with respect to the tracking stations, SLR may play an important role for precise orbit determination of this category of satellite.
FIGURE 5. Retroreflector array on BeiDou satellites.
The satellites of the Indian Regional Navigation Satellite System (IRNSS), also known as the Navigation with Indian Constellation system or NavIC, also carry retroreflectors (see FIGURE 6) and have been tracked by SLR stations. However, little publicly available microwave tracking data yet exists. Therefore, up to now, precise orbit determination heavily relies on SLR observations.
FIGURE 6. Retroreflector array on NavIC satellites.
MORE APPLICATIONS OF SLR FOR GNSS
Because GNSS is a one-way measurement technique, only pseudoranges and carrier phases can be measured, and clock synchronization is indispensable for positioning and orbit determination. Radial orbit errors can therefore be absorbed to a large degree by satellite clock corrections. For the very stable clocks on board Galileo satellites, the SLR residuals show the same behavior as the microwave-derived clock corrections indicating that the clock corrections are, in fact, caused by radial orbit errors. SLR therefore provides a way to break this correlation and to separate radial orbit errors and satellite clock corrections. This makes it possible to study and to characterize the physical behavior of onboard clocks including temperature-induced clock variations.
Separation of orbit errors and satellite clock variations is crucial when using the first two Full Operational Capability Galileo satellites, which were released into wrong orbits, for relativistic experiments. In a dual launch on Aug. 22, 2014, the two satellites were put into orbits with an initial eccentricity of 0.233 and orbit height of 19,800 kilometers due to a malfunction of the launcher third stage. With a sequence of maneuvers, the satellite orbit heights could be increased to 22,600 kilometers (compared to the planned height of 23,200 kilometers) and the eccentricity was decreased to 0.156. The satellites are, nevertheless, fully functional, and the very stable hydrogen masers on board should allow scientists to improve the uncertainty of the relativistic redshift parameter α beyond the current value determined in 1976 using the Gravity Probe A satellite. Regular SLR tracking of the two satellites plays an essential role in this experiment to separate clock variations due to orbit errors from those caused by the gravitational redshift.
Eventually, SLR may also be used as a tool for high-precision time synchronization of stable GNSS clocks combining one-way laser transmissions with two-way active laser operation, similar to the concept of the European Laser Timing experiment foreseen using the Atomic Clock Ensemble in Space (ACES) on the International Space Station and already tested for BeiDou satellites.
SLR TRACKING OF THE GNSS CONSTELLATIONS
In the near future, more than 100 GNSS satellites carrying retroreflectors will be operational. This includes GPS Block III satellites, which will carry retroreflectors starting with SV-9. Tracking the full GNSS constellation will pose a big challenge for the ILRS concerning economic use of its ground equipment. Optimized tracking scenarios and session planning strategies will be indispensable.
Already today, the ILRS regularly tracks a large number of GNSS satellites. TABLE 1 shows the number of SLR normal points from ranging to the various GNSS constellations available at the ILRS data centers since 2010. Normal points are compressed full-rate data obtained by averaging individual range measurements typically over five-minute intervals. As part of the Laser Ranging to GNSS Spacecraft Experiment or LARGE project of the ILRS, the tracking of GLONASS satellites was extended to the entire satellite constellation as shown in FIGURE 7.
FIGURE 7. Number of SLR normal points per month for GLONASS satellites.
To assess the capability of SLR for GNSS precise orbit determination based on the number of tracking stations and the distribution of observations, we performed a simple simulation. The covariance analysis included observations of a single SLR station compared to networks of 6 and 17 globally distributed stations. For each station, three normal points were simulated per satellite pass for a full 24-satellite Galileo constellation: two observed at 30° rising and setting elevation angles and one at maximum elevation angle. No unfavorable weather conditions were considered and observations of different stations were assumed to be uncoordinated.
Formal errors of the determined orbits are shown in FIGURE 8 for the radial, along-track, and cross-track components. As expected, orbits determined with observations from one day’s observations by a single station reach formal errors in the few 10s of kilometers range (plot on the left in the first row). If observations from three days are used for orbit determination, the errors on the middle day reduce to about 100 meters (right, first row). The situation significantly improves if a global network of six stations is considered. Even for a single day of observations, an orbit precision of a few decimeters is reached (left, second row) while the orbit uncertainty further decreases to a few centimeters if observations from three days are used (right, second row). If, however, in an effort to reduce the number of observations per pass, only measurements at satellite culmination are acquired, the orbit precision is in the kilometer range for a six-station network and observations from one day (left, third row). If observations from three days are used, the orbit precision is at the meter level (right, third row). Using three normal points per pass for a 17-station network, the orbit precision reaches a few centimeters even within one day (left, last row) and about 1 centimeter for observations from three days (right, last row). It should be noted that the covariance analysis does not consider any systematic observation or orbit modeling error.
FIGURE 8. Formal errors of Galileo orbits in radial (red), along-track (green) and cross-track (blue) directions. First row: one SLR station, 1-day arc (left), middle of 3-day arc (right); second row: six stations, 1-day arc (left), 3-day arc (right); third row: six stations with tracking only at culmination, 1-day arc (left), 3-day arc (right); fourth row: 17 stations, 1-day arc (left), 3-day arc (right). Note the different scaling for the various plots.
This simulation is very simple and not very realistic, but nevertheless indicates the capability of precise orbit determination for GNSS satellites using a limited number of observations per station. The simulations demonstrate two facts. Firstly, even with just two or three normal points per satellite of a GNSS constellation, a significant fraction of the observation time of a station is required. Typically, a mid-latitude station can acquire about 60 normal points per day for a 24-satellite constellation, amounting to several hours of observation time per day. Secondly, the improvement in formal orbit accuracy only increases with the square root of the number of stations. More important than the number of normal points is their distribution along the orbit requiring SLR observations from several stations distributed over the globe.
These two findings make it obvious that coordination among SLR stations is indispensable for making economic use of the observing time of SLR stations while providing good coverage of normal points along all satellite orbits. To cope with weather conditions, this coordinated scheduling of GNSS SLR tracking may have to be optimized in real time.
CONCLUSIONS
SLR has played an important role in validating GNSS-derived satellite orbits for the past several decades. For new GNSS constellations and new orbit types, SLR proves to be essential for calibrating radiation pressure models and allows us to separate orbit- and temperature-induced variations of onboard clocks. Eventually, the role of SLR will become even more important by contributing to the precise orbit determination of GNSS satellites. Given the large number of GNSS satellites from several constellations equipped with retroreflectors, coordination of observation scheduling among SLR stations will be crucial for optimizing the benefit-to-cost ratio.
Concerning the distribution of SLR observations over the constellations, the following conclusions may be drawn:
For the validation and calibration of radiation pressure models, it is sufficient to acquire well-distributed observations along the orbit of one satellite for each constellation block type for a range of solar beta angles, that is, of one satellite block type per orbital plane.
For contributing to precise orbit products, optimally combined with microwave GNSS observations, the tracking of all satellites of a constellation is needed. This requires a coordinated scheduling of observations among SLR stations.
For determination of the gravitational redshift parameter using the two Galileo satellites in eccentric orbits, good coverage of the orbits of both satellites is required (as long as the satellites run on one of the onboard hydrogen maser clocks).
For BeiDou and NavIC geostationary satellites, SLR coverage is needed for all satellites to resolve biases in the microwave tracking technique.
In the long term, SLR observations could contribute, together with microwave observations, in providing operational high-precision orbit products for all GNSS constellations jointly by the ILRS and the IGS in the framework of the IAG’s Global Geodetic Observing System.
ACKNOWLEDGMENTS
This article is based on the invited paper “Ranging the GNSS Constellation” presented at the 20th International Workshop on Laser Ranging held in Potsdam, Germany, Oct. 10–14, 2016. Figure 1 was adapted from an image in “Expert Advice: Laser Reflectors to Ride on Board GPS III” published by GPS World. GPS, Galileo, BeiDou and NavIC retroreflector images obtained from the ILRS. The GLONASS retroreflector image was obtained from ISS Reshetnev. Opening photo: Nighttime ranging at NASA’s Next Generation SLR system at Goddard Space Flight Center, Maryland (Credit: Felipe Hall/HTSI).
URS HUGENTOBLER is a professor of satellite geodesy at the Technische Universität München, Germany, and head of the Satellite Geodesy Research Facility in the Institute for Astronomical and Physical Geodesy. He is also a former chair of the IGS Governing Body. His research activities include precise positioning using GNSS, precise orbit determination and modeling, reference-frame realization, clock modeling and time transfer, using both the legacy and new satellite systems. Hugentobler obtained his Ph.D. from the University of Bern, Switzerland, in 1997.
FURTHER READING
Author’s Conference Paper
“Ranging the GNSS Constellation” by U. Hugentobler, presented at the 20th International Workshop on Laser Ranging held in Potsdam, Germany, Oct. 10–14, 2016.
Early Work on Satellite Laser Ranging
“Satellite Laser Ranging: Current Status and Future Prospects” by J.J. Degnan in IEEE Transactions on Geoscience and Remote Sensing, Vol. GE-23, No. 4, July 1985, pp. 398–413, doi: 10.1109/TGRS.1985.289430.
“Reflection of Ruby Laser Radiation from Explorer XXII” by H.H. Plotkin, T.S. Johnson, P. Spandin and J. Moye in Proceedings of the IEEE, Vol. 53, No. 3, March 1965, pp. 301–302, doi: 10.1109/PROC.1965.3694.
Early Work on GPS Orbit Modeling
“Extended Orbit Modeling Techniques at the CODE Processing Center of the International GPS Service for Geodynamics (IGS): Theory and Initial Results” by G. Beutler, E. Brockmann, W. Gurtner, U. Hugentobler, L. Mervart, M. Rothacher and A. Verdun in Manuscripta Geodaetica, Vol. 19, 1994, pp. 367–386.
The International Laser Ranging Service
“The International Laser Ranging Service” by M.R. Pearlman, J.J. Degnan and J.M. Bosworth in Advances in Space Research, Vol. 30, No. 2, July 2002, pp. 135–143, doi: 10.1016/S0273-1177(02)00277-6.
SLR Tracking of GNSS Constellations
“Satellite Laser Ranging to GPS and GLONASS” by K. Sósnica, D. Thaller, R. Dach, P. Steigenberger, G. Beutler and D. Arnold in Journal of Geodesy, Vol. 89, No. 7, July 2015, pp. 725–743, doi: 10.1007/s00190-015-0810-8.
“IRNSS Orbit Determination and Broadcast Ephemeris Assessment” by O. Montenbruck, P. Steigenberger and S. Riley in Proceedings of ION ITM 2015, the 2015 International Technical Meeting of The Institute of Navigation, Dana Point, California, Jan. 26–28, 2015, pp. 185–193.
“Initial Results of Precise Orbit and Clock Determination for COMPASS Navigation Satellite System” by Q. Zhao, J. Guo, M. Li, L. Qu, Z. Hu, C. Shi and J. Liu in Journal of Geodesy, Vol. 87, No. 5. May 2013, pp. 475–486, doi: 10.1007/s00190-013-0622-7.
“Contribution of SLR Tracking Data to GNSS Orbit Determination” by C. Urschl, G. Beutler, W. Gurtner, U. Hugentobler and S. Schaer in Advances in Space Research, Vol. 39, No. 10, 2007, pp. 1515–1523, doi: 10.1016/j.asr.2007.01.038.
Effect of Radiation Pressure on GNSS Satellite Orbits
“CODE’s New Solar Radiation Pressure Model for GNSS Orbit Determination” by D. Arnold, M. Meindl, G. Beutler, R. Dach, S. Schaer, S. Lutz, L. Prange, K. Sósnica, L. Mervart and A. Jäggi in Journal of Geodesy, Vol. 89, No. 8, Aug. 2015, pp. 775–791, doi: 10.1007/s00190-015-0814-4.
“Enhanced Solar Radiation Pressure Modeling for Galileo Satellites” by O. Montenbruck, P. Steigenberger and U. Hugentobler in Journal of Geodesy, Vol. 89, No. 3, March 2015, pp. 283–297, doi: 10.1007/s00190-014-0774-0.
“Impact of Earth Radiation Pressure on GPS Position Estimates” by C.J. Rodriguez-Solano, U. Hugentobler, P. Steigenberger and S. Lutz in Journal of Geodesy, Vol. 86, No. 5, May 2012, pp. 309–317, doi: 10.1007/s00190-011-0517-4.
“Test of the Gravitational Redshift with Stable Clocks in Eccentric Orbits: Application to Galileo Satellites 5 and 6” by P. Delva, A. Hees, S. Bertone, E. Richard and P. Wolf in Classical and Quantum Gravity, Vol. 32, No. 23, 2015, doi: 10.1088/0264-9381/32/23/232003.
