The SeaWinds scatterometer instrument

The SeaWinds scatterometer instrument is currently being developed by NASA/JPL, as a part of the NASA EOS Program, for flight on the Japanese ADEOS II mission in 1999. This Ku-band radar scatterometer will infer surface wind speed and direction by measuring the radar normalized backscatter cross-section over several different azimuth angles. This paper presents the design characteristics of and operational approach to the instrument itself. The SeaWinds pencil-beam-antenna conical-scan design is a change from the fixed fan-beam antennas of SASS and NSCAT. The purpose of this change is to develop a more compact design consistent with the accommodation constraints of the ADEOS II spacecraft. The SeaWinds conical-scan arrangement has a 1-m reflector dish antenna that provides a time shared dual-antenna beam at 40 and 46 degree look angles. The dual-beam operation provides up to four azimuth look directions for each wind measurement cell. At an orbit height of 803 km, the conical scan provides a broad and contiguous wind measurement swath of about 1800 km for each orbit pass. Radiometric measurement performance from a conical scan is inherently stable because of a common antenna apparatus, a measurement cell well defined by the narrow antenna beamwidth, and only two fixed-beam incidence angles for the multiple azimuth looks. A tracking filter is required to accommodate variations in the Doppler shift of the echo during the scan period. Key specifications of the SeaWinds instrument and associated tradeoffs and performance are described