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International audienceThe India-France SARAL/AltiKa mission is the first Ka-band altimetric mission dedi-cated to oceanography. The mission objectives are primarily the observation of the oceanic mesoscales but also include coastal oceanography, global and regional sea level monitoring, data assimilation, and operational oceanography. Secondary objectives include ice sheet and inland waters monitoring. One year after launch, the results widely confirm the nominal expectations in terms of accuracy, data quality and data availability in general.Today's performances are compliant with specifications with an overall observed performance for the Sea Surface Height RMS of 3.4 cm to be compared to a 4 cm requirement. Some scientific examples are provided that illustrate some salient features of today's SARAL/AltiKa data with regard to standard altimetry: data availability, data accuracy at the mesoscales, data usefulness in costal area, over ice sheet, and for inland waters

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the altimetry satellite Jason-2 was launched from the Vandenberg site in California. Dedicated to the measure of ocean surface topography, one of the main instruments on-board is a radar altimeter, Poseidon-3, which essentially measures the altimetric range between the spacecraft and the surface.Poseidon-3 is a dual frequency radar altimeter operating in Ku and C bands, very similar to its predecessor Poseidon-2 on-board Jason-1. However some significant improvements have been implemented to improve its tracking capabilities over coastal and inland waters, that is, its capacity to maintain data acquisition over land or mixed land-sea terrain.The performance assessment is excellent: the range measurement accuracy is close to 1.5 cm for 1s averaging and the significant wave height (SWH) noise is less than 12 cm (for a 2m SWH at 1σ ).In terms of range, the short-term drift (along an orbit) is around 1 mm, and the long-term drift is negligible so far. The tracking success is close to 100% over oceans and 80% over land surfaces, the new acquisition and tracking modes inducing significantly higher data availability in comparison with Poseidon-2.We assess Poseidon-3 main improvements, with the presentation of the new modes of echo acquisition and tracking: the median tracking algorithm, DIODE/DORIS acquisition, and the coupling between DIODE and digital elevation model (DEM) information. The median tracking algorithm is shown to reinforce the robustness of the altimetry echoes outside the standard Brown conditions. DIODE acquisition mode increases data availability in land-to-water transitions, providing up to 5 km of extra measurements along track, which constitutes an asset for coastal and small water areas (lakes, rivers) observations. Both are now implemented as the default mode on Jason-2. DIODE/DEM mode remains experimental and requires further adjustments but shows promising features such as acquisition of water surfaces in rough terrain.

This paper describes the Ka-band altimetry payload and system that has been studied for several years by CNES, ALCATEL SPACE and some science laboratories. Altimetry is one of the major elements of the ocean observing system to be made sustainable through the GEOSS (Global Earth Observation System of Systems) and GMES (Global Monitoring of the Environment and Security) programs. A short review of some mission objectives to be fulfilled in terms of mesoscale oceanography in the frame of the GEOSS and GMES programs is performed. To answer the corresponding requirements, the approach consisting in a constellation of nadir altimeter is discussed. A coupled Ka-band altimeter-radiometer payload is then described; technical items are detailed to explain how this payload shall meet the science and operational requirements, and expected performances are displayed. The current status of the payload development and flight perspectives are given.