ERS‐1/2 orbit improvement using TOPEX/POSEIDON: The 2 cm challenge

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Abstract
The ERS orbit error reduction method using TOPEX/POSEIDON (T/P) data as a reference [Le Traon et al., 1995a] was applied to ERS‐1 cycles from phases C, E, F, and G and to the first 16 cycles of the ERS‐2 mission (phase A). T/P M‐GDR (geophysical data record) (version C) and ERS‐1/2 ocean product (OPR) data were used. ERS‐1/2 orbits are the D‐PAF (processing and archiving facility) orbits and, when necessary, ERS‐1/2 altimetric corrections were updated to make the T/P and ERS‐1/2 corrections homogeneous. The adjustment method has been refined, and formal error on the estimation is now calculated. The ERS‐1/2 orbit error estimation is thus estimated to be precise to within about 2 cm root‐mean‐square (rms). E‐E crossover differences are reduced from 12 to 17 cm to only 6.5 cm rms for all processed cycles. Similarly, the T/P‐E crossover differences are reduced from 11 to 14 cm to only 7 cm rms. The adjusted D‐PAF orbit error varies between 6 and 12 cm rms. The adjustment has also been performed for the Joint Gravity Model 3(JGM 3) orbits of ERS‐I phases C, E, and F. The rms difference between the corrected orbits for the D‐PAF and JGM 3 orbits is only about 1 cm rms, while it is about 11 cm before T/P orbit error correction. This shows that the adjustment is almost insensitive to the initial ERS‐I orbit used. It also confirms the 2 cm precision of the method. We also do repeat‐track analysis on the 35 day repeat cycles of ERS‐I phase C. The mean difference in sea level variance before and after orbit error correction is 34 cm2 (D‐PAF orbit) and 17 cm2 (JGM 3 orbit). The corrected ERS‐1 and T/P sea level variabilities, however, are in excellent agreement. The study thus shows that ERS‐1/2 orbit error must be corrected before analyzing large‐scale oceanic signals and combining ERS‐1/2 with T/P data. The proposed method provides a very effective correction and thus significantly enhances the quality of ERS‐1/2 data. Corresponding data sets will be distributed to the scientific community by Archiving, Validation, and Interpretation of Satellite Oceanographic Data (AVISO).

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