A Calibration Scheme for Microwave Radiometers Using Tipping Curves and Kalman Filtering

Calibration of microwave radiometers is a critical task and remains a key issue for the accuracy of brightness-temperature measurements. The tipping-curve calibration method is a well-established technique for ground-based microwave radiometers measuring at frequencies where the opacity of the atmosphere is low. This method relies on the known relationship between the tipping angles of the radiometer and the atmospheric opacity at those angles. Atmospheric inhomogeneities slightly disturb this relationship and therefore lead to calibration errors. The calibration scheme presented in this paper uses the determined tipping-calibration accuracy and incorporates the statistical behavior of radiometer gain and system-temperature variations in a Kalman filter framework. In this paper, a calibration simulation is set up to test the capability of the proposed scheme by reconstructing simulated brightness temperatures first. Moreover, the technique is applied to experimental data. The calibration quality is evaluated with the detrended-fluctuation-analysis method. Model and experimental results show that the calibration accuracy can be increased by a factor of two or even higher. Finally, we apply the calibration technique to a microwave radiometer with internal calibration, resulting in a reduction of the calibration noise.