Navigating Above the GPS Constellation – Preliminary Results from the SGR-GEO on GIOVE-A

M. Unwin, R. De Vos Van Steenwijk, P. Blunt, Y. Hashida, S. Kowaltschek, L. Nowak

Abstract:	Europe’s first Galileo demonstrator satellite, GIOVE-A played a major role in the run up to the Galileo satellite navigation system through the transmission of prototype navigation signals to the world. It was designed and built by SSTL for ESA for an operational life of 2 years, but was retired after 6 years of operations on 6 June, 2012 to make way for the Galileo IOV satellites. Less well known is the fact that GIOVE-A carried a secondary payload consisting of a novel GPS receiver experiment called the SGR-GEO (Space GNSS Receiver for Geostationary Earth Orbit). The SGR-GEO’s primary goal was to demonstrate the application of GPS receivers in altitudes above that of the GPS constellation (20,000km). GIOVE-A is now located at its retirement altitude of around 23,300km - some 100km above the operational altitude for the Galileo navigation constellation. During GIOVE-A’s mission, it was not possible to operate the SGR-GEO due to the payload transmissions in the same frequency band. Following the completion of payload transmissions, the SGR-GEO experimentation was able to commence. On 13 November 2012, the SGR-GEO achieved its first navigation fix in Medium Earth Orbit above the GPS constellation. Designed and built by SSTL’s GNSS Receivers team with the support of the European Space Agency and the BNSC (now UKSA), the SGR-GEO is a 12 channel L1 C/A code GPS receiver with several adaptations enabling its application in high altitude orbit. It features an OCXO to provide stable clock reference, weak signal tracking algorithms and a high gain antenna to improve acquisition and tracking of signals, and an on-board orbit filter to enable continuous navigation output through periods when the navigation solution is under-determined. Since operations started in November 2012, the SGR-GEO was operated for several months continously. The orbit of GIOVE-A is no longer being determined as accurately as during its mission phase, but some assessment of the SGR-GEO position fix accuracy has been possible through the fitting of orbit model to the GPS data. The tracking of GPS signals by the SGR-GEO allows the investigation of the antenna pattern of the transmitting satellites, including the sidelobes not normally visible to ground-based users. Perhaps the main obstacle to the adoption of GPS navigation at high altitudes is the limited visibility of GPS signals. The GPS antenna sidelobes, if available, can give high altitude receivers a far greater availability of signals, but only limited information about the magnitude of these sidelobes is present in the public domain. Since its initial turn on, SGR-GEO has routinely been tracking signals from GPS sidelobes. Standard tracking loops with a 1 ms integration time were initially used to assess of transmit antenna patterns. Subsequently, longer integration times of 10 ms have allowed a higher level of detail in mapping the antenna patterns of all blocks of the current GPS constellation. This paper will present preliminary results from the SGR-GEO, including positioning and mapping of the GPS constellation transmit antennas using weak signals. We further look into the performance of the SGR-GEO as a spacecraft navigation and timing subsystem applied to satellite platforms in high altitude orbits, extrapolating to expected performance at geostationary altitude. We discuss the benefits such a subsystem might offer operators in cost reduction and satellite autonomy and the challenges that the sparse signal environment presents to receiver and spacecraft designers. Finally we look at the future of GNSS navigation at high altitudes, considering the advantages and disadvantages of the new and modernised signals from the various GNSS constellations that are currently being deployed by space agencies around the world.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	3305 - 3315
Cite this article:	Unwin, M., Van Steenwijk, R. De Vos, Blunt, P., Hashida, Y., Kowaltschek, S., Nowak, L., "Navigating Above the GPS Constellation – Preliminary Results from the SGR-GEO on GIOVE-A," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 3305-3315.
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