A STUDY OF THE MICROWAVE SOUNDING UNIT ON THE NOAA-12 SATELLITE

The microwave sounding unit (MSU) on the NOAA series of polar-orbiting environmental satellites is a four-channel Dicke radiometer making passive measurements with the channels centered at 50.30, 53.74, 54.96, and 57.95 GHz, respectively. Onboard blackbody and cold space calibrations are performed once every 25.6 seconds for each scan line. The MSU data are extensively used for atmospheric temperature retrieval and climatological studies. After the launch of the NOAA-12 satellite on May 14, 1991, it was observed that the system gain in channel 2 decreased by approximately a factor of 2 from its pre-launch value and that it required a special correction to the pre-launch calibration coefficients for channel 2. Recently, it was discovered that the pre-launch calibration coefficients may be inaccurate because of an error found in the MSU manufacturer's computer software that was used to process the thermal-vacuum chamber test data for the pre-launch calibration. In this study, we reprocessed the pre-launch calibration data and obtained a new set of nonlinearity coefficients, which can be used to generate more accurately the atmospheric brightness temperatures. We also examined some in-orbit MSU data, particularly, the blackbody and cold space counts used for onboard calibrations. In addition, the in-orbit variation of the temperatures of the Dicke loads and the blackbody targets monitored by the platinum resistance thermometers (PRT's) was also investigated. This provides us a better understanding of the MSU in-orbit data. Test cases show that antenna temperatures generated from the new nonlinearity coefficients can be 0.5 K lower than the current operationally retrieved atmospheric temperatures. Alternative calibration algorithms for modeling the MSU nonlinear contributions were also explored. Calculated results show that the antenna temperatures will be further reduced if the alternative calibration algorithms are employed. The results from this study may be used to improve the accuracy of temperature retrievals from the NOAA-12 MSU data.