Worldwide Real-Time GNSS Performance Monitoring

Abstract

The Global Navigation Satellite Systems (GNSS) are extensively used today with a wide variety of applications including Safety of Life ones. The number of GNSS devices is expected to grow to 8 Billion by 2020 [1]. GNSS has become ubiquitous. There is a growing demand for precise, robust and reliable location information, in combination with the ongoing evolution of GNSS technology with multi-constellation and multi-frequency receivers in the near-future for mass market. GPS and GLONASS are operational systems with respectively about 30 and 24 satellites, and Galileo satellites are also available for users since Initial Services declaration at the end of 2016. The BEIDOU constellation deployment is going on with several recent launches. With regional augmentation systems like SBAS, BEIDOU GEO/IGSO, QZSS, and NAVIC, the users will benefit from nearly half a hundred GNSS satellites at the end of the decade. A growing demand for use means a growing demand for monitoring. Each GNSS constellation and augmentation is generally checked individually by their respective operational ground segments, but since the standard is now a multi-constellation one, there is a need for a full GNSS awareness or surveillance. The International GNSS Monitoring and Assessment (IGMA) pilot project is an interesting IGC and IGS initiative. It should permit to define and develop a full GNSS monitoring concept, with a good sharing of Key Performance Indicators (KPIs). CNES (French Space Agency) is already monitoring the European Galileo and SBAS EGNOS systems on a continuous basis with off-line analysis. In order to go further, some software have been developed recently and used over the last months for a real-time and worldwide check of GNSS constellations. The article will give a description of the CNES GNSS real-time monitoring concept, implementation, and early results for KPIs. The paper will include the methodology used to assess the GNSS Signal in Space performances with a description of KPI evaluation algorithms. IGS real-time streams in RTCM MSM format are used for the observation. CNES uses some of the receivers of its worldwide CNES/IGN REGINA 30 station network, and some other IGS stations which are taken into account as well for a very robust coverage. A typical number of stations between 20 and 30 gives good results with worldwide coverage. The CNES redunded and operational RTCM caster is generally used to retrieve the streams from all over the World. Such monitoring can be operated on a continuous basis. The “usual” IGMA like KPIs have been implemented with a real-time output, i.e. for the availability of each satellite in a healthy mode, the User Range Error (URE) RMS Global Average for each satellite, the Position Dilution of Precision (PDOP) maps for geometry. UTC Signal In Space errors for the time services can be checked locally for some UTC time laboratory GNSS calibrated receivers. Horizontal error maps due to URE errors are also computed in real-time. The navigation messages can be analyzed more deeply in details if necessary. The monitoring is for stand-alone solutions with those KPIs. Pseudo-range residuals are also evaluated for each reference station, and a median value for each satellite gives a good estimate of quality of pseudo-range. The pseudo-range residuals are evaluated with station positions fixed to precise ITRF ones. This KPI is very interesting since only observables and ephemerides from stations are necessary. Thus using a receiver network with a high Depth of Coverage permits a very robust monitoring as for Code Differential operational systems for Maritime Users etc… Fundamental URE KPIs are evaluated thanks to the IGS Real-Time CNES PPP correction stream which cover all GPS, GLONASS, Galileo, and BEIDOU constellations. The CNES Precise Point Positioning (PPP) correction has a “top level” accuracy of a few centimeters, which gives a good reference for URE calculation. Inter-signal biases from PPP stream are used to have a consistent URE determination with respect to the considered signals and ephemerides. Satellite Broadcast Ephemerides are consolidated directly from stations or can also be retrieved from the consolidated streams from German BKG caster. Early results fully demonstrate the real-time monitoring concept, and good results are obtained in particular for Galileo URE which has the best range accuracy today. Real-time PDOP and Positioning Error maps can also be generated. In addition, satellite real-time availabilities are easily checked in particular for Galileo and GPS. [1] GNSS Market Report, Issue 5, May 2017, GSA