The GEM-T2 gravity model, previously used for the ERS-1 POD, to have fully-consistent ERS-1 and Geosat altimeter data sets, was found to be of inadequate precision, with radial orbit errors of the order of 1.5 metre.
Since GEM-T2, gravity models improved for ERS-1 mainly due to the inclusion of DORIS tracking data to the French SPOT-2 satellite, which has an orbital altitude and inclination similar to ERS-1. Because of this advantage over the GEM-T2 model, the more up-to-date PGS-4591 model, complete to degree and order 50, was used to compute off-line precise ERS-1 orbits for internal purposes on a semi-operational basis, reducing the radial orbit error to some 30 cm. More recently, newer models became available, the first of a series of high-quality models to be applied for the TOPEX/Poseidon POD. One of these models, JGM-2, is complete to degree and order 70, and was the basis for all ERS-1 operational orbits provided since June 1992, having a radial precision of the order of 15 cm or less.