Comparsion of different microwave radiometric calibration techniques

In this paper we compare three techniques typically used for calibrating a microwave radiometer and understanding its design stability. The first calibration technique utilizes cold and hot load measurements to construct calibration curves. For our case, we used sky as the cold load and microwave absorber at ambient temperature as hot load. The second calibration technique utilizes measurements of the brightness temperature of the sky at various zenith angles to determine the atmospheric opacity needed for the calibration curves. The third calibration technique utilizes measurements of internal reference load and micro-controller inside the radiometer at both polarizations to estimate the system gain fluctuations and the receiver noise temperatures. We calibrated our University of Florida C-band Microwave Radiometer (UFCMR) every two weeks during the first Microwave Water and Energy Balance Experiment (MicroWEX-1). We found that, the first and third techniques produced similar calibration results with 1 Kelvin/volt standard deviation. However, the third technique was the most stable during the entire experiment with the smallest standard deviations which were 2.41 Kelvin/volt for the slope of the calibration curves at H-pol and 7.46 Kelvin/volt for the slope of the calibration curves at V-pol during MicroWEX-1