SAC-C orbit reconstruction using the experimental GPS/GLONASS receiver LAGRANGE

The Argentinian satellite SAC-C was launched in November 2000. It carries onboard an experimental prototype of the first Italian-built, combined GPS/GLONASS space receiver LAGRANGE, developed for the Italian Space Agency (ASI) by LABEN S.p.a. It also carries a second GPS receiver provided by JPL known as Blackjack. The SAC-C prototype of the LAGRANGE receiver is based on the first-version of the Advanced GPS/GLONASS ASIC (AGGA) developed by the European Space Agency (ESA). The dual-frequency LAGRANGE receiver is provided with 12 channels that can be dynamically allocated to GPS and GLONASS frequencies. The receiver was operating during a period of 56 days until 23 January 2001, when it suffered a radiation-induced failure and ceased operations before the end of the commissioning phase. However, data were collected during the entire period and were made available for analysis. This paper will address the results from the LAGRANGE data processing. It is of interest to evaluate the performance of this receiver based on the availability of the well-known, long-heritage BlackJack receiver onboard the same spacecraft. The two receiver antennas were placed very to each other, thus allowing a very similar satellite tracking scenario. In this experimental setup the LAGRANGE receiver suffered from important limitations such as channel-dependent pseudorange biases, very low availability of measurements at the L2 frequency and a clock with a very high drift. A new strategy to cope with the biased pseudorange measurements (“realignment”) was implemented, which allowed to obtain a navigation solution otherwise impossible. This strategy, based on the availability of another GPS receiver (BlackJack), did not allow the recovery of the absolute LAGRANGE clock error. This important quantity was inferred through reference to a precise orbit of the SAC-C satellite obtained using BlackJack carrier phase data. An along-track orbit difference was the key to calibrate the LAGRANGE clock. The two receivers were later compared in their performance in terms of an L1-only, doubly differenced carrier phase-based Precise Orbit Determination analysis. When the orbits obtained from single frequency data from each of the two receivers are compared in an RTN reference frame with the same precise reference orbit— obtained from dual-frequency BlackJack data—the residuals show very similar behavior in terms of both mean and standard deviation. This can be interpreted as an indication that the two receivers performances are quite similar. Studies are ongoing to assess wether use of combined iono-free pseudoranges and carrier phase LAGRANGE measurements provide indications of a better performance of the receiver when the comparisons are made with the precise SAC-C orbit.