ERS-1 / ERS-2: a long story of altimeter data and improvements
Françoise Mertz-Ogor(1) , Joël Dorandeu(1) , Yannice Faugère(1) , ngan Tran(1) , Sylvie Labroue(1) , and Franck Mercier(1)
8-10 rue Hermès,
31520 Ramonville St Agne,
Since the launch of ERS-1, CLS has been much involved in calibration/validation and cross-calibration activities, including several altimetry data such as ERS-1/2, TOPEX/POSEIDON, and recently Jason-1 and Envisat, all this in the frame of CNES, ESA and IFREMER projects. The routine work consists in validating the Ocean Products data delivered to users and in monitoring the relevant parameters. This includes cross-calibration analysis of ERS-2 with ERS-1, ERS-1/2 with TOPEX/POSEIDON but also ERS-2 with Envisat.
Besides these routine activities, many studies have been performed at CLS to continuously enhance data with new algorithms and corrections. Moreover, improvements proposed by other laboratories were also assessed. From precise cross-calibration studies, a method was developed to enhance the ERS orbit by using simultaneous data from other precise orbit missions. This allowed homogenizing the ERS data with other missions for the objective of building a combined multi-mission altimeter dataset. Other studies consisted in evaluating and validating the impact of retracking on ERS-1 Ocean Products, already undertaken since phases C and G have already been produced and disseminated, and still on-going with other phases of ERS-1 (D,E,F).
With the new generation of altimeters, new instrumental and geophysical corrections have been proposed. Thus it is important to assess the improvement of these new techniques. Among several items, we can note the progress provided by the non parametric Sea State Bias estimation, the neural algorithm used to compute the wet troposphere correction, the combined high frequency atmospheric correction from MOG2D model, the GIM ionosphere correction, the FES2004 and GOT00 tidal models, the CLS01 MSS, and the new generation of Delft orbits. All these improvements contribute to precise and consistent Sea Surface Height measurements along the whole period from ERS-1/2 to Envisat.
Further work is still needed on ERS-1/2 data to provide oceanographers with a long time series as precise as possible, focusing on the homogenization and merging with other missions. Furthermore, significant improvements can also be expected in coastal areas by refining ERS altimeter data processing and enhancement in these areas. This will lead to more precise coastal altimeter mean profiles that are essential for applications and studies using past and present altimeter data.