GPS World, November 2017

Parameters of the Earth 1990 PE 90 geodetic system PZ 90 replaced the Soviet Geodetic System 1985 SGS 85 used by GLONASS until 1993 PZ 90 is a terrestrial reference system with its coordinate frame defined in the same way as that of the International Terrestrial Reference Frame ITRF The initial realization of PZ 90 had an accuracy of one or two meters However in an effort to bring the system closer to the ITRF and GPSs WGS 84 geodetic reference system two updates to PZ 90 were carried out The first update resulting in PZ 9002 referring to 2002 was adopted for GLONASS operations on Sept 20 2007 and brought the frame of the broadcast orbits and hence derived receiver coordinates closer to ITRF and WGS 84 Another realization PZ 9011 adopted on Dec 31 2013 reportedly reduced the differences to the sub centimeter level TABLE 1 lists the defining constants and parameters of PZ 90 The new GLONASS K satellites transmit additional signals GLONASS K1 transmit a CDMA signal on a new L3 frequency 1202025 MHz and GLONASS K2 in addition will feature CDMA signals on the L1 and L2 frequencies Control Segment Similar to GPS and other GNSSs GLONASS requires a network of ground stations for monitoring and maintaining the satellite constellation and for determining the orbits of the satellites and behavior of their operating AFSs The tracking network uses stations only within the territory of the former Soviet Union supplemented with satellite laser ranging stations to help with orbit determination since all GLONASS satellites contain laser reflectors see FIGURE 3 Having a non global network of tracking stations for determining the satellite orbits and AFS behavior results in slightly degraded GLONASS signal in space range error SISRE Recently a number of tracking stations overseas have been established in conjunction with the development of the Russian satellitebased augmentation system SBAS the System for Differential Correction and Monitoring SDCM SDCM will function in a similar fashion to the Wide Area Augmentation System or WAAS the U S SBAS and the other SBASs in operation The addition to the tracking network of the overseas SDCM stations which already includes stations in Antarctica and South America with more stations coming could help improve SISRE Roscosmos also uses a global network of IGS and other tracking stations to monitor the health of the GLONASS constellation see FIGURE 4 Performance SISRE has improved over the years and is currently at the level of about 1 to 2 meters In part this is due to the better performance of the on board AFSs carried by the latest GLONASS M satellites compared to the first GLONASS M satellites Their relative one day stability has improved from 10 13 to 24 10 14 FIGURE 5 shows a time series of recent values of SISRE determined by the Information and Analysis Center for Positioning Navigation and Timing These error levels can result in pseudorange based positioning errors using GLONASS broadcast orbits and clocks about a factor of two worse than those provided by GPS although at any given instant positioning accuracy will also be impacted by atmospheric effects and multipath and these could dominate the signal in space errors Much higher positioning accuracies can be obtained using GLONASS orbits and clocks provided by the IGS and its participating analysis centers This is particularly true if carrier phase measurements are used instead of or as a supplement to pseudorange measurements A combination of appropriately weighted GPS and GLONASS measurements has shown to be beneficial in terms of availability accuracy and efficiency especially for high accuracy positioning carried out using the real time kinematic or RTK approach november 2017 www gpsworld com GPS World 47 FIGURE 3 Circular reflector array on a GLONASS K1 satellite surrounding navigation signal inner antenna elements Photo from Reshetnev Information Satellite Systems Parameter Value Earth rotation rate 72921 15 10 6 radians s 1 Gravitational constant including the atmosphere 398 6004418 109 m3s 2 Speed of light 299 792 458 ms 1 Second zonal harmonic of the geopotential 1082625 75 10 6 Ellipsoid semimajor axis 6 378 136 m Ellipsoid flattening 1 298257 84 Equatorial acceleration of gravity 978 03284 mGal Correction to acceleration of gravity at sea level due to the atmosphere 087 mGal TABLE 1 Fundamental geodetic constants and some of the parameters of the PZ 90 geodetic system as used by GLONASS