Engineering evaluation of multi-beam satellite antenna boresight pointing using land/water crossings

The Microwave Radiometer (MWR) on the Aquarius/SAC-D mission measures microwave radiation from earth and intervening atmosphere in terms of brightness temperature (T_b). It takes measurements in a push-broom fashion at K- (23.8 GHz) and Ka-band (36.5 GHz) frequencies using two separate reflector antenna systems, each producing eight spot beams. Pre-launch measurements of the alignment of these beams with respect to the spacecraft coordinate system is used to geolocate the antenna foot-prints on ground. As a part of MWR's on-orbit engineering check-out, the verification of MWR's pointing accuracy is discussed here. The technique used to assess MWR's pointing involves comparing the radiometer image of land with high-resolution maps. When the beam's instantaneous field of view (IFOV) passes over a land/water boundary, the brightness temperature changes from a radiometrically “hot” land-scene to a “cold” ocean-scene. This “step-function” change in brightness temperature provides a very sensitive way to assess the mispointing error of the calculated MWR earth location (latitude/longitude) of the antenna footprints. This paper describes the algorithm used for the MWR geolocation validation assessment and preliminary results, presented for the MWR 23.8 GHz channel, show that the mispointing errors from the true coastline are close to meeting the specification.