ENVISAT OCEAN ALTIMET RY PERFORMANCE ASSESSMENT

With more than four years of altimetric measurements over ocean Envisat not only ensures the continuity of the observations provided by ERS -1 and ERS-2, but significantly improves the data quality. The quality assessment of these data is routinely performed at the CLS Space OceanographyDivision in the frame of the CNES Segment Sol Altimetrie et Orbitographie (SSALTO) and ESA French Processing and Archiving Center (F-PAC) activities. This paper presents the main results in terms of Envisat data quality: verification of data availability and validity, monitoring of the most relevant altimeter(ocean1 retracking) and radiometer parameters, assessment of the Envisat altimetersystem performances. Envisat carries a new generation of dual frequency radar altimeter(RA-2) which allows correction of ionosphere effects. Very low editing ratios and low level of noise are observed, thanks to the use of the Model Free Tracker. The presence of a Dorisreceiver highly improves the accuracy of the orbit determination. All these improvements allow Envisat to reach the high level of accuracy of other precise missions such as T/P and Jason-1. This work also includes a cross -calibration analysis of Envisat data with other flying precise altimetric missions. It is essential to assess data qualityand performances, and for allowing combination of altimeterdatasets as req uired by applications and operational oceanography. Finally, Envisat altimetry data are also compared to a tide gaugedata network. Comparisons with an independent data set are indeed of great interest to detect drifts and biases. The methodology of the comparisons of altimeterdata against tide gaugemeasurements is now operational and produces results routinely for Envisat altimetry mission.