IMPROVEMENT OF ERS-1 ORBITS USING ALONG-TRACK ACCELERATIONS FROM DORIS DATA ON SPOT2

In long-arc precise orbit determinations of altimetric satellites such as ERS-1, large errors may occur from mismodelling of aerodynamic dra g and solar radiation pressure. Such surface forces for nonspherical s atellites require accurate modelling of the effective area and particl e-surface interactions, but the dominant source of errors is neutral a ir density as derived from thermospheric models for aerodynamic drag. Several techniques can be employed to alleviate air-drag mismodelling but all require the solution of additional parameters from the trackin g data. However, for ERS-1 the sparsity of laser range data limits the application of such empirical techniques. To overcome this, use can b e made of the dense DORIS Doppler tracking for SPOT2 which is in a sim ilar orbit to ERS-1. A recent investigation by CNES examined the use o f drag scale factors from SPOT2 to constrain the ERS-1 orbit. An impro vement to that methodology is to consider along-track mismodelling as observed by timing errors in the Doppler data for each pass of SPOT2. The along-track mismodelling as observed by timing errors in the Doppl er data for each pass of SPOT2. The along-track correction to the acce leration as derived from SPOT2 can then be applied to ERS-1 orbits, so lving for a scale factor to absorb systematic errors - particularly th at arising from the 50 km altitude difference. Results are presented o f the associated improvement in ERS-1 orbits as derived from concurren t SPOT2 arcs. It will be seen that the procedure not only improves the laser range fit, but more importantly, leads to more precise radial p ositioning as evident in the altimeter and crossover residuals.