Written by Henry Berglund, Frederick Blume, and Celia Schiffman
15 December 2014
The Russian GLONASS Global Navigation Satellite System experienced a ten-hour outage beginning at 21:00 UTC on April 1st, 2014 that affected a significant number of GNSS receivers operated by UNAVCO and our collaborators. Many GNSS stations use both GPS and GLONASS to increase accuracy and reliability in real-time applications. However, an error in the GLONASS broadcast navigation message caused some receivers to completely fail to track GLONASS signals, and in some cases GPS signals, for several hours.
The GLONASS outage coincided with the Mw 8.2 Pisagua earthquake, and while UNAVCO's Development and Testing staff were aware of the GLONASS outage while it was happening, the extent to which GPS tracking was also affected did not become clear until we began processing data recorded during the earthquake. UNAVCO and other global analysis centers were able to accurately estimate daily positions at the affected sites despite some missing data. However, kinematic solutions during the event may have been interrupted at a number of sites with affected receivers.
The majority of UNAVCO's receivers only track GPS satellites and thus were unaffected by the outage. Newer multi-constellation GNSS receivers, which have the capability to track GPS, GLONASS, and newer GNSS constellations such as Galileo, experienced some tracking problems. Receiver manufactures use proprietary Receiver Autonomous Integrity Monitoring (RAIM) software to recognize and exclude erroneous data, which should include when satellites are broadcasting incorrect orbital information. Some receivers were able to recognize the error in the GLONASS ephemeris and excluded GLONASS observations throughout the outage, while some receivers failed to exclude the faulty data and computed inaccurate position and receiver clock bias estimates. We infer that the erroneous clock bias estimates are what prevented a subset of receivers from tracking during the outage.
UNAVCO's Development and Testing staff and collaborators from the International GNSS Service (IGS) and GNS New Zealand analyzed data collected from 316 GLONASS enabled receivers in our archive as well as 79 sites from GeoNet NZ. The results show that both GLONASS and GPS tracking were affected at a number of the sites, depending on the type of receiver and the ability of the RAIM software to detect the error. The New Zealand network exclusively uses GNSS-enabled receivers, and therefore a significant portion of their network was impacted during the outage.
The cause and duration of the outage has not been publically explained by the GLONASS control center. It is speculated that a programming error was made and that it took several hours before the correction could be made, both due to the complexity of the messages and the procedure used to upload the corrections to each of the 24 satellites.
Prior to this outage, the GPS community has not experienced the failure of an entire constellation. The use of multiple constellations was intended to provide redundancy in such an event; the fact that the failure of one constellation could cause a complete system failure exposed a critical design flaw in GNSS receivers. Manufacturers are currently working hard on developing new firmware with improved RAIM algorithms to ensure that similar failures cannot occur in the future. UNAVCO's analysis of the behavior of the global population of GNSS receivers is an important contribution to this effort.
Impact analysis of the April 1st, 2014 GLONASS outage on the GNSS enabled sites within the UNAVCO archive. Yellow and red circles indicate stations where receiver tracking was interrupted during the time period when GLONASS transmitted illegal navigation messages. Click here for an interactive global version of this map, created by Henry Berglund, UNAVCO.
Last modified: Friday, 17-Jul-2015 17:45:55 UTC