The precise orbit results from a data reduction process in which all available tracking data (Single-Lens Reflex, radar altimeter crossovers, PRARE range and Doppler data) and most accurate correction, transformation and dynamical models are taken into account and in which high level numerical procedures are applied.
These orbits are "optimal" achievable representations of the real orbital motion under the circumstances of tracking situation and the "state of the art" model situation. The precise orbit product for the ERS satellites are the satellite ephemeris (position and velocity vector) including time tag, given in a well-defined reference frame, together with the nominal satellite attitude information and a radial orbit correction.
Three different orbit solutions together with the combined solution are available to be used for processing of the radar altimeter measurements and the determination of geodetic/geophysical products. The new precise orbits for the ERS-1 and ERS-2 have been computed at DEOS (Delft Institute of Earth Observation and Space Systems), GFZ (Deutschen GeoForschungsZentrums) and ESOC (European Space Operations Centre) using different software (GEODYN, EPOS-OC and NAPEOS respectively) and different altimeter databases (RADS - Radar Altimeter Database System-, ADS -Altimeter Database and Processing System). Combined solutions have been created using three individual solutions for each satellite.
All orbits were derived using consistent models in the same LPOD2005 terrestrial reference frame.These new orbit solutions show notable improvement with respect to DGME04 orbits (Scharroo and Visser, 1998). Thus, RMS crossover differences of new orbits improved by 4-9 mm. Careful evaluation of the various solutions has shown that the combined solution for both ERS-1 and ERS-2 has the best performance.
All orbit files are in the SP3c format. The combined solution is also in PDS format.