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Ocean Surface Topography from Space
FOAM: From Ocean to inland waters Altimetry Monitoring
Configuration of the Corsica experiment. The colored surfaces correspond to local geoids determined by using the Catamaran-GPS method.
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Pascal Bonnefond

P. Exertier, O. Laurain, F. Pierron
Y. Ménard
F. Lyard, S. Calmant, J.-F. Crétaux, L. Testut
G. Jan
V. Ballu


In collaboration with the CNES and NASA oceanographic projects (T/P and JASON-1), the OCA developed a verification site in Corsica since 1996 (see figure above). CALibration/VALidation embraces a wide variety of activities, ranging from the interpretation of information from internal-calibration modes of the sensors to validation of the fully corrected sea-level estimates using in situ data. Now, Corsica is, like the Harvest platform (NASA side), an operating calibration sites able to support a continuous monitoring with a high level of accuracy: a 'point calibration' which yields instantaneous bias estimates with a 10-day repeatability of around 30 mm (standard deviation) and mean errors of 3-4 mm (standard error).

In-situ calibration of altimeter sea surface height (SSH) is usually done at the vertical of a dedicated CAL/VAL site, by directly comparing altimetric data with in-situ sea level data. This configuration leads to handle the differences compare to the altimetric measurement system at the global scale: the Geographically Correlated Errors at regional (orbit, sea state bias, ...) and local scales (geodetic systematic errors, land contamination for the instruments, e.g. the radiometer). We thus intend to include other existing sites (Vanuatu, Kerguelen, ...) where the conditions are somewhat different from the Mediterranean sea and where instruments and infrastructures already exist.

In order to increase statistically the sea surface bias estimation but also to cover larger areas, we suggest to extend the calibration opportunities by using, not only over-flying passes, but also satellite passes located far away from the CAL/VAL site (few hundreds kilometers). This CAL/VAL method has been developed since 2003 and validated on the Corsica tide gauges network dedicated to Jason-1 CAL/VAL activities. In such a case, two main effects interfere in the SSH bias determination, the geoid slope and the ocean dynamics. In order to correct from the geoid slope, distant SSH altimetric data are propagated along a succession of known mean sea level profiles up to the in-situ reference site. The ocean dynamics differential effect can increase with the distance from the CAL/VAL site. It is corrected by using an ocean numerical model (T-UGOm).

Due to the increasing need of altimetry to monitor inland waters and in preparation to future missions (e.g., AltiKa, SWOT) we propose to integrate the calibration activities on the oceanic domain with those on different water bodies such as rivers and lakes. CAL/VAL activities on rivers and lakes enable to avoid the contributions of the SSB and liquid tides in the range calibration and to address other problems such as the performance of the various tracking/retracking algorithms and more globally assess the quality of the geophysical corrections. The proposed CAL/VAL activities are thus focused not only on the important continuity between past, present and future missions but also on the reliability between offshore, coastal and inland altimetric measurement. Purpose is to aggregate the past effort of several groups, in order to notably establish a homogeneous network of calibration site geographically distributed for more robust characterization of the existing and future radar altimeter system instrument biases and their drift.

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