GPS World, July 2014

THE SYSTEM GlONaSS Seen by IGS continued from page 15 coordinate system were estimated Table 1 shows that the positions derived from the normal good GLONASS BM compare very well to the IGS precise orbits Except for a minor rotation about the z axis one obtains zero rotations about the orthogonal axes in the inertial coordinate system Table 1 also shows that the bad positions were obtained from the reference positions by a rotation of about 05 degrees around the inertial x axis The RMS of 71 meters should be compared to the entire effect of up to 200 kilometers per coordinate Comparing this RMS of 71 meters with the RMS of the orbit determination of about 05 meters per coordinate also says however that the true transformation is more complicated than one represented by just a series of three rotations We did not further investigate how this more or less consistent rotation could enter into the GLONASS BM It seems to be clear however that a systematic error slipped into the realization of the GLONASS BM which were activated at a common reference Solution Rot x arcsec Rot y arcsec Rot z arcsec RMS m good BM 00 00 04 2 bad BM 17632 53 21 71 Table 1 Rotation of the entire system of good orbit positions April 1 0 00 20 45 UTC with respect to precise IGS reference orbits good BM and rotation of the entire system of bad orbit positions April 1 21 00 April 2 07 00 UTC with respect to precise IGS reference orbits bad BM epoch for all satellites but uploaded to individual satellites at different times Figure 1 suggests that the problem was almost immediately recognized by GLONASS operators already an hour later the first two satellites started to transmit BM with the usual accuracy level Figure 1 also supports the idea that the problem was remedied satelliteby satellite A back of the envelope calculation revealed that the satellites were above the horizon of at least one of the Russian uplink sites at the times of switching back to the correct BM Summary and Conclusions The GLONASS event was one that we might have described by the phrase such a thing can never happen For the user community the situation was aggravated by the fact that the event was not reported through the official Russian channel by issuing a Notice Advisory to GLONASS Users NAGU This definitely should have happened in the interest of transparency The above analysis was based on information available through the IGS It was performed weeks after the event It is worth noting however that the information needed for the analysis was available in real time The reference orbit used in the analyses could have been replaced by the IGS predicted orbits generated in the ultra rapid series In view of the importance of BM for all users and in particular for the users of IGS real time products the IGS might consider monitoring the quality of BM for all GNSS Fixing the GlONaSS bug Report from Moscow In a May 23 conversation with journalists Javad Ashjaee president of JAVAD GNSS decried the recent controversy about monitoring stations on both U S and Russian soil saying it was based in misinformation and misinterpretations inflated by a political crisis He also supplied a different perspective on the GLONASS signal outage than has been reported in other media outlets There was speculation in early April that it took GLONASS 11 hours to correct a software bug because it took that long for all the satellites to pass over a control station on Russian soil This was not the case I have learned from conversations with their engineers and with the head person responsible for all of this One engineer made a mistake and uploaded the wrong software Until they could find it and debug it and it took them 11 hours to do so they could not upload correct software to the satellites The 11 hour outage was not due to a wait for all satellites to pass over ground control stations on Russian soil to receive a fresh upload of data continued Ashjaee GLONASS has the capability like GPS to make updates via inter satellite communication The delay was caused by the time it took to find the bug in the erroneous software that had been uploaded and correct it Ashjaee addressed the monitoring station controversy saying that Russia had sought GLONASS monitoring stations in the United States not for uploading any data but for monitoring GLONASS satellites to provide more accurate orbit and clock information for the free benefit of all users For Ashjaees full discussion of the U S Russian monitoring station controversy see www gpsworld com glodispute For news updates on the situation see www gpsworld com monitornews GPS World July 2014 www gpsworld com 16