Abstract
Abstract. The SAR (Synthetic Aperture Radar) Copernicus Sentinel-1 satellites require a high orbit accuracy of 5 cm in 3D in comparison to external processing facilities. The official orbit products delivered by the Copernicus POD (Precise Orbit Determination) Service fulfil this requirement. Nevertheless, analyses have shown discrepancies in the orbit results for the two satellites Sentinel-1A and Sentinel-1B. Since the satellites are identical in construction estimated orbit parameters like the scale factor for the radiation pressure are expected to be at the same magnitude, which is not the case. Estimation of GPS antenna offsets leads to differences between the two satellites, which might explain the discrepancies in the estimated orbit parameters. Such offset estimations are, however, very sensitive to orbit and observation modelling. It has to be assured that the results are not biased by insufficient models. First of all, stabilisation of the antenna offset estimation is achieved by improving the observation modelling by applying single receiver ambiguity resolution. The Copernicus Sentinel-1 satellites have a very complex shape with the long SAR antenna and the two large solar arrays. Antenna offset estimation based on different satellite models may give results which differ by up to 1.5 cm. The dispersion of the estimates is quite large depending also on eclipse and non-eclipse periods. Consideration of simple assumptions on satellite self-shadowing effects improves the satellite model and also the results of the antenna offset estimation. Finally, more consistent results for the two Sentinel-1 satellites are achieved by applying the antenna offset estimates.
Highlights
The European Copernicus Programme (Aschbacher and Milagro-Pérez, 2012) has been set up to establish European capacity for Earth Observations
Part of motivation for this study is that the estimated orbit parameters (e.g., solar radiation pressure (SRP) coefficient and empirical CPR parameters) of the two Sentinel-1 satellites show different magnitudes and or systematic signatures, which cannot be explained by other means than different antenna offset vectors of the Global Positioning System (GPS) antennas
The y- and z-component of the antenna offsets are estimated in all solutions and the corresponding a priori values are the differences between the main GPS antenna reference point (ARP) and COM coordinates
Summary
The European Copernicus Programme (Aschbacher and Milagro-Pérez, 2012) has been set up to establish European capacity for Earth Observations. The integer ambiguity resolution could be applied for the Copernicus Sentinel1, -2, and -3 satellites only recently, when the ground segment processing was upgraded to properly resolve half-cycle biases in the carrier phase observations from raw correlator data of the RUAG GPS receivers (Zangerl et al, 2014). This was shown for the first time for Swarm being equipped with the RUAG GPS receiver (Allende-Alba and Montenbruck, 2016).
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