A downward looking advanced microwave precipitation radiometer (AMPR) for high altitude airborne measurement

The development and testing of an imaging total-power millimeter-wave radiometer operating at 10.7, 19.35, 37.1, and 85.5 GHz are described. The instrument, referred to as the advanced microwave precipitation radiometer (AMPR), is designed to be flown onboard NASA´s ER-2 high-altitude research aircraft in support of atmospheric precipitation and storm-related field measurements. A unique feature of the AMPR antenna system is the multifrequency feedhorn for operation at 19.35. 37.1. and 85.5 GHz. This antenna feedhorn offers dual polarization output at all three frequencies, although initially only one polarization is being used at each frequency. A lens is used with the feedhorn to obtain half-power beamwidths of 7.9°, 4.1°, and 1.8° at 19, 37, and 85 GHz. respectively. Three major subsystems of the AMPR are the multifrequency antennas, the four-channel downconverters, and the imaging scanner. These subsystems are combined to provide four radiometric data outputs which interface with NASA´s data acquisition system. Calibration of the AMPR allows one to perform absolute temperature measurements, which is necessary to obtain brightness temperature data from precipitation in the atmosphere