Envisat Precise Orbit Determination

The Navigation Office at ESOC has been closely involved in all aspects of the Precise Orbit Determination (POD) of Envisat throughout the mission lifetime. These activities started shortly after launch through membership of the Orbit Validation Team (OVT). During this time ESOC was involved in both calibration and validation of the DORIS doppler data, the SLR data and the different POD solutions provided by the OVT members. These activities were concluded in May 2003 with an orbit comparison campaign led by ESOC. After this phase, routine monitoring of the overall performance of the Envisat DORIS doppler data, SLR data and the MOE and POE orbit solution from CNES has continued. The results of these activities are reported bi-weekly on our website. This paper will give an overview of all these activities performed during the last two years. 1. ORBIT COMPARISON CAMPAIGN ESOC conducted in May 2003 as part of the OVT activities an orbit comparison campaign. The goal of the comparison was to better quantify the absolute orbit accuracy of ENVISAT. For this comparison cycle 12 of ENVISAT was selected which corresponds to a period of 35 days starting at 10.12.2002 and ending at 13.01.2003. Two days, 18 and 19 December, were excluded within this period from the comparison. These days were selected because of the large inclination manoeuvre on 18 December and the resulting loss of DORIS and SLR tracking data for nearly a day. Six different centers contributed their POD solution to the comparison. The CNES POD solution used in this comparison is the same solution as the one that is found on the ENVISAT altimeter GDR. The JPL-DORIS solution only uses the DORIS tracking dataset and is based on the EGM-96 gravity field where all the other centers use DORIS and SLR data and the GRIM5-C1 field. Three complementary analysis methods where applied to all contributions, namely pair-wise orbit comparison, SLR tracking data analysis and altimeter crossover analysis. 1.1 Pair-wise orbit comparison Table 1 and Table 2 provide the pair-wise radial and total orbit difference for all solutions. The top triangle shows the RMS difference and the bottom triangle provides the order within the total set of comparison pairs. The RMS of differences between the orbit solution and the true ENVISAT orbit can be seen as a measure for the orbit error, denoted as RMSA and RMSB for two solutions A and B respectively. If the two solutions are independent, their two orbit error signals are fully uncorrelated. The pair-wise error value RMSAB follows from: