CLS experience and expertise for altimeter data processing is world-wide recognised. CLS is regularly working as an altimetry expert for CNES, European Space Agency and from time to time for NASA. In the frame of the in-orbit verification of the French-American mission Jason-1, CLS has led the comparison of Ku band data for three altimeter radars, Topex, Poseidon-1 and Poseidon-2 for range noise and sea state bias. This study was part of developments for new retracking algorithms led at the Space Oceanographic Direction.
CLS tasks:
For this study, CLS was in charge of:
- comparing the features and products of Topex, Poseidon-1 and Poseidon-2
- developping homogeneous data processing and data analysis over several cycles.
- investigating new areas for further studies
The major results of the study are presented below. Complete detail can be found in the Marine Geodesy volume, dedicated to Jason-1Calibration/Validation, in the article: "Comparison of the Ku-Band Range Noise Level and the Relative Sea State Bias of the Jason-1, Topex and Poseidon-1 Radar Altimeters" July -December 2003.
Major results of the study:
- The satellite Topex/Poseidon embarked two radar altimeters, the operational altimeter, TOPEX, and the experimental one, Poseidon-1. They have and are still providing data of unprecedented quality. However, there are two major differences between these altimeters. The first is related to the estimated range noise; the second is linked to the sea state bias (SSB) models. To better understand possible sources of these differences, these altimeters have been cross-comparing to the radar altimeter Poseidon-2, on board Jason-1. Poseidon-1 and Poseidon-2 belong to the same instrument family (same design and manufacturer), behave in the same way and thus show the same observed differences when compared with Topex.
- A comparative analysis of the features of these three radar altimeters has been performed, starting from the on-board acquisition of the ocean return and ending with the ground generation of the high level accuracy oceanographic product. This analysis has pointed out that the sources for these differences lie in both, the waveform tracking processing and the presence or absence of a retracking procedure whether on-board or on ground.
- To analyse the impact of the retracking, the same ground retracking algorithm has been applied to the waveforms of the three radar altimeters. These consistent data sets have been then analysed using the same tools (spectral analysis, "collinear analysis", …). The analysis of the outputs has shown that:
- the noise level for the three radar altimeters is definitively the same, at the level of 1.6 cm for 1 Hz data.
- the source of the relative SSB between Jason-1 and TOPEX lies in the different behavior of the on-board tracking software, and is estimated to be equal to 2% of SWH (Significant Wave Height).
Users:
These studies have been carried out in close partnership with CNES teams in charge of instruments and teams in charge of production and data distribution.
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