Nonlinearity Analysis For JPSS-1 ATMS Instrument

For microwave radiometer, nonlinearity is the inherent nature of square-law detector and need to be accurately characterized through TVAC test. However, instrument noise, gain fluctuation, as well as radiation contamination from various sources make it difficult to directly determine nonlinearity from TVAC data with required accuracy. Previous study on deep space scan observations shows that for ATMS instrument, there is a non-negligible scan angle dependent near-field radiation term intrudes in the radiation path of warm load, cold space and scene, and needs to be corrected in calibration. Further study on TVAC data shows the similar impacts of the near-field radiation term on observations, if not being corrected, will lead to large bias on characterizing the nonlinearity parameters.

In this study, based on TVAC test data sets derived at different environment temperature, near-field radiation term was modeled as scan angle scene temperature dependent function of reflector emissivity and physical temperature. The correction model was applied to correct the blackbody brightness temperature of cold, warm and scene targets in calibration process, results show that there is distinct improvement in nonlinearity parameter accuracy. ARTS is a full radiance transformation system for JPSS ATMS TDR processing, and can be used as a test bed for microwave instrument calibration. The antenna emission corrected nonlinearity parameters derived from TVAC are applied in on-orbit calibration in ARTS, and was evaluated at selected channels at which the radiative transfer model simulation can be used as truth.