The ERS-1 and ERS-2 wind scatterometers, system performance and data products

On 17 July 1991 ESA launched ERS-I into a Sun-synchronous, polar orbit, followed by ERS-2 in 1995. The satellite carries three active microwave instruments: a synthetic aperture radar (SAR), a wind scatterometer, and a radar altimeter (RA) and an infrared-visible Along Track Scanning Radiometer (ATSR). The satellite has a standard orbit repeat cycle of 35 days. Two other repeat cycles of 3 days and 168 days have also been operated. The wind scatterometer is configured as a real aperture pulse radar providing three radar images of the ocean surface with a spatial resolution of 50 km and a swath width of 500 km. The three images are acquired by three different antennas: one, the mid-beam, looking to the right side of the satellite, perpendicular to the ERS ground track, one looking forward at 45/spl deg/ azimuth projection angle, with respect to the midbeam, and one looking backwards at 45/spl deg/ azimuth projection angle with respect to the mid-beam. The radar transmitter and receiver are connected to the fore-, mid-, and aft-beams, in sequence. In the ground processor, the three radar images from the different beams are co-registered onto a 25 km grid. Such co-registration is possible since the satellite uses yaw-steering, programmed such that the effects of Earth rotation is counteracted. For each grid point or 'Node', the ocean surface wind speed and direction are estimated on the basis of an empirical mathematical model, CMOD4, which relates the ocean surface wind speed and direction to the radar backscattering coefficient and the radar look angles. This model is based on the pre-launch estimate of the radar backscattering behaviour, which was determined during a multi-year programme of experiments, using airborne radars, but 'tuned' in orbit on the basis of dedicated geophysical validation campaigns and data assimilation into numerical weather prediction models.