Beam Spoiling Correction for Spaceborne Microwave Radiometers Using the Two-Point Vicarious Calibration Method

The vicarious warm and cold calibration techniques are combined to provide an end-to-end two point calibration method for spaceborne microwave radiometers. The method uses stable external calibration sources to permit an end-to-end calibration of the complete radiometer, including its primary antenna. Both gain and offset corrections to the radiometer calibration can be computed since vicarious reference points at both the cold and warm ends of the measurement range are available. The method is demonstrated using the WindSat radiometer. Calibration errors are found which vary with azimuthal scan position in a manner that suggests that the cause is beam spoiling from on-board spacecraft obstructions. The impact on gain and offset calibration errors of the on-board obstructions can be determined from the vicarious calibration. This information is used to characterize the beam spoiling-specifically to determine the decrease in the antenna's beam efficiency and the mean brightness temperature entering the far sidelobes of the antenna, both as functions of azimuthal scan position. With this characterization available, a calibration correction algorithm can be constructed that is based on the root cause of the problem.