Test jigs and measurements for the scanning antenna system of the multifrequency imaging microwave radiometer MIMR

The multifrequency imaging microwave radiometer (MIMR) is an instrument which has as its objective the remote sensing analysis of the Earth surface. Due to the polar orbit of the platform on which it is embarked, 99.8% of the Earth surface will be covered after three days of revolution around the planet. To complete the coverage in such a short time, with the exception of two small caps (0.2%) on the poles, the polar orbit is not enough and an antenna beam scanning is necessary across the satellite track. The MIMR antenna system is composed of a parabolic offset reflector, the main reflector, illuminated by a feed cluster of nine feeds at six frequencies: 6.8, 10.65, 18.7, 23.8, 36.5, 89 GHz. Each feed provides a double linear polarisation and two feeds at 36.5 GHz and four feeds at 89 GHz guarantee the necessary overlap between the footprints in these two frequency bands. The main reflector, pointed toward the Earth surface, rotates with the feed cluster around a scan axis to provide a beam scan of /spl plusmn/60/spl deg/ in the cross track direction. The radiometric remote sensing is interrupted outside this angular range when the feed cluster, because of its rotation, enters into the field of view of a cold calibration reflector (CCR) placed in a fixed position and pointed toward the cold space. The CCR obscures the main reflector for a limited period of time giving rise to a new antenna geometry adequate for the instrument calibration. Alenia Spazio has performed measurements on the MIMR antenna. From a compact range, the radiometric parameters of interest have been extracted with adequate data processing for different relative mutual positions of the antenna elements.