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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. Without simultaneous measurements at both polarizations at the same frequency, the conversion from ATMS antenna temperature to sensor brightness temperature becomes nonunique if the antenna subsystem has a significant spillover from cross-polarization. In addition, the antenna temperature could be contributed from both the near- and far-field radiation through the sidelobes of the ATMS antenna subsystem. An analysis of the ATMS antenna gain measurements reveals that the efficiencies of both ATMS antenna sidelobes and cross-polarization are frequency dependent. From the ATMS pitchover maneuver data, it is found that the contributions of spacecraft radiation through the near-field sidelobes are significant and dominates the scan-angle-dependent features in the ATMS antenna temperatures. A theoretical model is developed for the conversion from antenna to sensor brightness temperatures, which incorporates the angular dependent terms derived from the pitchover maneuver data.