The first four Galileo satellites used for in-orbit validation were launched in October 2011 and October 2012.They are now transmitting their signals on an operational basis. Thanks to the simultaneous use of these four satellites, the European Space Agency was able to compute the first autonomous Galileo-only fix using broadcast ephemerides in March 2013.
Using data from the real-time service of the International GNSS Service (as supported by the Multi-GNSS Experiment), real-time protocols and new high-precision multiple signal messages and a new generation multi-constellation network of GNSS stations, the Centre National d’Etudes Spatiales (CNES) has been able for the first time to compute decimeter-accuracy Galileo orbits in real time.
The networks used in this work include the CNES/Institut Géographique National REGINA (REseau Gnss pour l’Igs et la NAvigation) network and the Deutsches Zentrum für Luft- und Raumfahrt (DLR) and associated organizations CONGO (COoperative Network for GNSS Observation) network (real-time access courtesy of Oliver Montenbruck). The filter used for the multi-constellation real-time orbit determination is a CNES proprietary tool based on a Kalman filter.
The following figure shows the network of stations used in this experiment:
The CNES orbits have been compared to an accurate reference orbit computed by Technical University München (TUM) as part of the MGEX project. The following figure shows the 3D orbit differences for the two solutions (for the ProtoFlight Model (PFM) and Flight Model 2 (FM2) satellites), over the 10 days of the experiment. Excluding the first day during which the filter converges, the 3D root-mean-square orbit difference is about 15 centimeters. This demonstrates the feasibility of accurate real-time Galileo solutions using currently available networks and software tools.