Precise orbit determination and gravity field improvement for the ERS satellites

Abstract

Abstract. The radial orbit error has long been the major error source in ERS-1 altimetry, crippled by having only satellite laser ranging for precise tracking and relying on insufficiently accurate general-purpose gravity field models. Altimeter crossovers are used very effectively as additional tracking data to laser ranging. The ERS Tandem Mission even provides the unique possibility to simultaneously determine orbits of two similar satellites flying the same orbit.Altimeter crossovers between the two satellites then link the two orbits into a common reference frame. Tailoring of the Joint Gravity Model 3 (JGM 3) is another step to reduce orbit errors. This technique is aimed at the reduction of the geographically anticorrelated orbit error (observed in the crossover height differences) through the adjustment of selected gravity field parameters. The resulting Delft Gravity Model (DGM)-E04 has reduced this part of the orbit error by a factor of 2, performs even better with respect to the ESA-provided orbits, and also outperforms the recent Earth Geopotential Model EGM96 in this respect. ERS-1 and ERS-2 orbits for the entire Tandem Mission are computed and studied in detail, and orbit errors due to the gravity field and nonconservative forces are identified. Analyses systematically show that the orbits computed with JGM 3 have a radial root-mean-square orbit accuracy of 7 cm, with DGM-E04 5 cm. IntroductionThe Tandem The bulky satellites ERS-1 and ERS-2 were never designed for high-accuracy orbit determination, and the loss of the Precise Range and Range-Rate Equipment (PRARE) tracking system left ERS-1 even more poorly equipped for orbit determination. Yet, subdecimetric orbit accuracy is not of academic interest only. The ERS altimetric system has performed well above expectations and is unique because of its multidisciplinary character, sampling not only ocean but also land and ice surfaces, in combination with the suite of instruments on board, providing, e.g., simultaneous measurements of wet tropospheric content and surface temperature. Undoubtedly, ERS will always lag behind on the 2-cm root-meansquare orbit accuracy of the TOPEX/POSEIDON altimeter mission [Marshall et al., 1995], so only when the precise orbit determination is stretched to its very limits, ERS altimetry will be regarded a reliable source of information. Only then will ERS be able to demonstrate its additive value in ocean research and its unique capabilities in, e.g., monitoring of the ice sheet mass balance.In section 2 we will discuss the numerous advances made in ERS operational and precise orbit determination over the years up to the current state-of-the-art modeling. An important step in this is the development of the ERS-tailored Delft Gravity Model (DGM)-E04 (section 3). In section 4, DGM-E04 demonstrates that it constitutes a remarkable improvement on the ERS orbits but also has a more general applicability. Section 5 examines the effect of nonconservative forces acting on the satellite. Section 6 combines all results and attem...

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