GPS World, May 2017

System 2010 2011 2012 2013 2014 2015 2016 GPS 3369 3331 3339 2318 647 151 0 GLONASS 36929 74094 83715 77947 109169 106526 80362 Galileo 0 846 13337 19374 16857 24746 36651 BeiDou 8663 9453 8041 7289 8342 6663 7576 NavIC 0 0 0 474 2637 2158 2027 MAY 2017 WWW GPSWORLD COM GPS WORLD 45 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 0233 and orbit height of 19800 kilometers due to a malfunction of the launcher third stage With a sequence of maneuvers the satellite orbit heights could be increased to 22600 kilometers compared to the planned height of 23200 kilometers and the eccentricity was decreased to 0156 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 highprecision 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 fiveminute 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 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 FIGURE 4 Retroreflector array on Galileo satellites at bottom of satellite below antenna array TABLE 1 Number of normal points per year for each GNSS constellation