FOAM, From Ocean to inland waters Altimetry Monitoring
In collaboration with the CNES and NASA oceanographic projects (T/P and Jason-1), the OCA developed a verification site in Corsica since 1996 and LEGOS installed sites in Kerguelen in 1993 and at Vanuatu in 1999. 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 estimates of the reflector heights, whether it is sea level or inland water stage using in situ data. Now, Corsica is, like the Harvest platform (NASA side), an operating calibration site 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 altimetric height (SSH for ocean surfaces) is usually done at the vertical of a dedicated CAL/VAL site, by direct comparison of the altimetric data with in-situ data [Bonnefond et al., 2011]. 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, atmospheric corrections...) and local scales (geodetic systematic errors, land contamination for the instruments, e.g. the radiometer, land contamination for the radar echoes, e.g. tracking/retracking concerns). We intend to continue and ensure the experiments already conducted during the first part of the FOAM project (2007-2011) at various sites (Corsica, Vanuatu, Kerguelen, lakes and rivers...) where the local conditions are different from each others but where permanent instruments and infrastructures already exist and have to be reinforced.
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&2 and EnviSat 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., SARAL/AltiKa, Sentinel-3, Jason-3, Jason-CS, SWOT) we propose to integrate the calibration activities on the oceanic domain and those on different water bodies such as rivers and lakes. CAL/VAL activities on the oceanic domain have a long history and protocols are well established. CAL/VAL activities on rivers and lakes are much recent but in turn they enable to avoid the contributions of the Sea Surface Bias (SSB) and liquid tides in the range calibration and to address other problems such as the performance of the various retracking algorithms, altimeter technologies (LRM/SAR), wavelength (Ku/Ka), and more globally assess the quality of the geophysical corrections. This proposal will then pay a particular attention of the short wavelength signatures in the altimetry data since SARAL/AltiKa will be the first mission to distribute 40 Hz measurements on reduced footprints owing to the Ka band pulse.
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 drifts.
The main objectives of the FOAM project continuation is two-fold: (i) ensure the experiments already conducted during the first part of the FOAM project (2008-2012) and (ii) conduct new developments in terms of processing and technology (GNSS reflectometry (GNSS-R), GPS buoy design and processing, ...).