Precise Orbit Determination of a Galileo/Giove - A Satellite using Double Differences of Carrier Phase Measurements Inter Constellations of GPS/Galileo and Initial Results

Hua Su and Bernd Zimmermann

Abstract:	GIOVE-A is a first Galileo test satellite launched on December 28, 2005. The satellite payload transmits two frequency signals at L1+E5 or L1+E6. There are 13 world wide located Galileo Experimental Sensor Stations (GESS) to track and monitor the GIOVE-A signals. High accuracy of GIOVE-A ephemeris or orbit, however, is essential for navigation applications. The paper presents a new method to deal with the precise orbit determination of GIOVE-A satellite using pure carrier phase measurements and the initial results. It is well known that the highest accuracy of orbit determination can be achieved using double differences of carrier-phase measurements. But the problem for such a solution is that in order to use double differences of carrier-phase measurements, at least two Galileo satellites are needed. For GIOVE-A satellite, it is difficult to form such double differences as not enough Galileo satellites are currently in space. Therefore the GIOVE-A carrierphase measurements combined with GPS measurements would be a reasonable solution. This method is called Double Differences of Carrier Phase Measurements Inter Constellations of GPS/Galileo (DDIC). In order to use DDIC, some problems shall be solved in advance, first of all the synchronization of GIOVE-A satellite measurements with GPS time reference, secondly the different time reference bias between GIOVE-A and GPS satellites, thirdly the different system bias between GPS and GIOVE-A satellites due to different coordinate systems and different system configuration and finally the weight normalization of GIOVE-A and GPS measurements for unified processing. The time reference bias can be solved using navigation solution (or called PVT) with time synchronization parameters from GPS. All the GIOVE-A measurements are synchronized with GPS time after the navigation solution. This step implied the synchronization of Galileo time with GPS time. The system bias is also solved at the navigation step, but later after the first solution of the orbit determination. After the time synchronization, the weight normalization will be taken place. The following factors are considered for the normalization, satellite orbit characteristics (altitude and movement, etc.) and measurement accuracy. Then GIOVE-A orbit determination together with GPS satellite orbit is performed using dynamic method. After the GIOVE-A orbit is determined, the GIOVE-A satellite clock corrections are solved separately using pseudorange measurements. The initial results using one day of DD (for GPS) and DDIC (for GIOVE-A) measurements show that the GIOVE-A orbit accuracy was achieved to about 20 – 80 cm level deduced implicitly from GPS orbit results compared with IGS final orbit. The implication means in this case, GIOVE-A is considered as another GPS satellite in the orbit determination. The accuracy will be further improved using multi-day measurements. The accuracy of the GIOVE-A satellite clock solution was in ns-level, but compared with GPS satellite clock solution, the GIOVEA satellite clock bias increased quickly with time. For example, the GIOVE-A satellite clock bias was 800.522156 (µs) at 00:00:00 on Jan. 9 2008. The bias became 1258.956921 (µs) at 00:00:00 on Jan. 21, 2008. At the same time period, the determined GPS satellite clocks, for example, PRN02 Rb Clock corrections (processed with GIOVE-A satellite together) changed from 164.783651 (µs) to 167.890185 (µs).
Published in:	Proceedings of the 2009 International Technical Meeting of The Institute of Navigation January 26 - 28, 2009 Disney's Paradise Pier Hotel Anaheim, CA
Pages:	301 - 311
Cite this article:	Su, Hua, Zimmermann, Bernd, "Precise Orbit Determination of a Galileo/Giove - A Satellite using Double Differences of Carrier Phase Measurements Inter Constellations of GPS/Galileo and Initial Results," Proceedings of the 2009 International Technical Meeting of The Institute of Navigation, Anaheim, CA, January 2009, pp. 301-311.
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