Poseidon-3 Radar Altimeter: New Modes and In-Flight Performances

On June 20, 2008, 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 1s). 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.