On-ORBIT antenna reflector loss measurements for Advanced Technology Microwave Sounder (ATMs) calibration

The Advanced Technology Microwave Sounder (ATMS) onboard the Suomi National Polar-orbiting Partnership (NPP) satellite is a total power radiometer and scans across the track within a range of ±52.77° from nadir. It has 22 channels and measures the microwave radiation at either quasi-vertical or quasi-horizontal polarization from the Earth´s atmosphere. From the ATMS pitch-over deep space scan observations, it is found that the antenna reflector losses play an important role in calibration and dominates the scan angle dependent features in the ATMS antenna temperatures. Since the losses are small, they are difficult to measure by traditional means. However, they can be assessed directly from pitch over observations by using deep space radiation measured at different scan angle. This paper describes a physical model developed for the correction of reflector emissivity, which incorporates the angular dependent terms derived from the pitch-over maneuver data. Based on the pitch-maneuver data, the expected V/H polarization emissivity value is in the range of 0.002 to 0.0065. Considering the facts that ATMS is heritage of historical NOAA series microwave sounding instruments like MSU and AMSU, on which the reflector emissivity correction model can also be used to improve TDR/SDR data quality.