Determination of an Amazon Hot Reference Target for the On-Orbit Calibration of Microwave Radiometers

A physically based model is developed to determine hot calibration reference brightness temperatures (TBs) over depolarized regions in the Amazon rain forest. The model can be used to evaluate the end-to-end calibration of any satellite microwave radiometer operating at a frequency between 18 and 40 GHz and angle of incidence between nadir and 55°. The model is constrained by Special Sensor Microwave Imager (SSM/I) TBs measured at 19.35, 22.2, and 37.0 GHz at a 53° angle of incidence and extrapolates/interpolates those measurements to other frequencies and incidence angles. The rms uncertainty in the physically based model is estimated to be 0.57 K. For instances in which coincident SSM/I measurements are not available, an empirical formula has been fit to the physical model to provide hot reference brightness temperature as a function of frequency, incidence angle, time of day, and day of year. The empirical formula has a 0.1-K rms deviation from the physically based model for annual averaged measurements and at most a 0.6-K deviation from the model for any specific time of day or day of year.