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Passive microwave sensors known as radiometers are calibrated receivers that make absolute measurements of weak natural blackbody noise power emissions to infer geophysical properties of the Earth's atmosphere and surface. Because of the high accuracy needed in measuring these geophysical parameters, frequent on-orbit radiometric calibrations over natural surfaces with stable radiometric emissions are highly desirable. This paper discusses the suitability of the Amazon rain forest as such a radiometric calibration target. Results of analysis of on-orbit microwave radiometer measurements from the Amazon are presented to demonstrate repeatable temporal and spatial emission signatures that can be used for radiometric calibration.

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A physically based model is developed to determine hot calibration reference brightness temperatures (T(B)s) 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 degrees. The model is constrained by Special Sensor Microwave Imager (SSM/1) T(B)s measured at 19.35, 22.2, and 37.0 GHz at a 53 degrees 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.
This is a brief review paper on microwave radiometry as a remote sensing tool in geoscientific investigations. Topics covered include basic radiometric principles, radiometer receivers, and applications. This paper is one in a series of mini-reviews sponsored by the Wave Propagation Standards Committee of IEEE and is intended primarily for those persons who have not had occasion to study extensively in the subject.
Microwave Remote Sensing