USING TOPEX POSEIDON DATA TO ENHANCE ERS-1 DATA

This paper presents a relatively straightforward method for efficientl y reducing the ERS-1 orbit error using Topex/Poseidon data. The method is based on a global minimization of Topex/Poseidon-ERS-1 (TP-E) dual crossover differences. The TP-E crossover differences give an estimat e of the ERS-1 radial orbit error almost directly, leading to a geomet ric estimation of orbit error. Smoothing cubic-spline functions are th en used to obtain a continuous estimation of the orbit error over time . The splines can also be adjusted to minimize the ERS-1-ERS-1 (E-E) c rossover differences. This allows a better estimation of the orbit err or, especially poleward of 66-degrees where no TP-E crossovers are ava ilable. The method was successfully applied to the final TP and ERS-1 datasets [i.e., the TP GDRs (geophysical data records) and the ERS-1 O PRs (ocean products)]. The authors used one full 35-day ERS-1 cycle an d five TP cycles concurrent with ERS-1 data. Only crossovers with time differences less than 5 days are used in the adjustment so that most of the large-scale oceanic signal is preserved. Just by using dual TP- E crossovers, E-E crossover differences are reduced from 18 to 10 cm. Also using the single E-E crossovers in the adjustment significantly i mproves the solution poleward of 66-degrees. The E-E crossover differe nces are thus globally reduced to only 8 cm. The method was also shown to be almost insensitive to the initial ERS-1 orbit error. The result s demonstrate that the orbit of ERS-1 can be determined with an accura cy similar to TP. The method also provides a precise, homogeneous ERS- 1-TP dataset. This dataset can be used to map sea level variation or m ean sea surface with high accuracy and excellent resolution. More gene rally, this study shows that when two satellites are flying simultaneo usly, the more precise one can be used as a reference. This is of grea t importance for future altimetric missions.