Effects of spacecraft attitude on the NASA Scatterometer antenna calibration

Radar scatterometers are expected to become a main source of marine surface winds. Wind vector retrieval is based on the relation (geophysical model function) between the radar cross-section and wind induced surface roughness. Multiple antennas, pointed at different azimuth angles, are required to remove wind direction ambiguity inherent in a single cross-section wind vector observation. Scatterometer antennas must be well calibrated to ensure desired accuracy of the retrieved wind. Pre-launch calibration alone proved insufficient in the past so post-launch calibration and validation is planned for scatterometer missions. During cal/val activities for the NASA Scatterometer (NSCAT), a consistent difference was noted between calibration corrections calculated based on ascending vs. descending passes. This difference cannot be attributed to geophysical parameters and must be instrument related. In this paper, an attempt is made to attribute ascending/descending discrepancy to imperfect spacecraft attitude. Calibration is performed at multiple spacecraft attitude sets (roll, pitch, and yaw). The set producing the lowest difference between ascending and descending based corrections is the suggested attitude. After brief introduction to the NSCAT in the next section, calibration method using homogenous land targets is outlined.