Effect of Center of Mass Position Error on ICESat Precision Orbit Determination

Abstract

he Ice, Cloud and land Elevation Satellite (ICESat) was launched in January 2003 carrying a laser altimeter designed to detect changes in the polar ice sheets. Like radar altimeters, POD is an essential component to create surface elevations from altimeter measurements. POD team at Center for Space Research (CSR) has generated POD solutions over the periods when the Geoscience Laser Altimeter System (GLAS) was operated. Table 1 show the eighteen GLAS campaigns (L1a and L1b are one campaign with different attitude mode) that were completed by the October 11 th , 2009, totaling about 600 days. ICESat POD solutions are generated by ingesting GPS data into the CSR’s POD system, Multi-Satellite Orbit Determination Program (MSODP) using 30 hour estimation arcs from which the central 24 hours are reported as the POD ephemeris product. This ephemeris product has been used to generate SLR residuals to estimate the accuracy of the ephemeris product. The statistics of the SLR residuals and a simulation study show that the POD product has a radial orbit accuracy of less than 1.5 cm 1 , which exceeds the POD requirement of 5 cm in radial component. Ball Aerospace and Technologies Corporation (BATC) measured the position of COM of the combined ICESat bus and GLAS instrument in the Satellite Coordinate System (SCS) prior to launch with an accuracy of ±5 mm in all three axis in SCS 2 , where +xsc is in the zenith direction, +ysc axis is along the solar panel axis, and +zsc axis completes the right hand system (see Fig. 1). After launch, the COM moves slowly, mainly along the +zsc axis, as the orbit maintenance maneuvers use propellant. The Laboratory for Atmospheric and Space Physics at the University of Colorado at Boulder (LASP) updates the COM position in the SCS using models developed by BATC following each maneuver. Since the POD solutions are generated by using GPS double-differenced carrier phase measurements, MSODP models the position of the GPS receiver phase center with respect to COM using the updated COM position. To reduce the impact of the COM position error and the GPS receiver phase center location error on the radial component of ICESat orbit, a COM offset parameter in +xsc axis (radial component) is estimated for the POD solutions (see Fig. 2). In this paper, COM offset correction parameter in the cross-track component (along the +ysc axis for “airplanemode’, and along the +zsc axis for “sailboat-mode”) is introduced, and the impact of this new parameter on the POD solutions is investigated. COM offset correction parameter in the along-track component cannot be estimated due to