System calibration of the 1.4 GHz and 5 GHz radiometers for soil moisture remote sensing
Two microwave radiometers at the frequencies of 1.4 GHz and 5 GHz were mounted on a mobile tower and used for a remote sensing of soil moisture experiment at a Beltsville Agriculture Research Center test site. The experiment was performed in October 1979 over both bare field and fields covered with grass, soybean, and corn. The calibration procedure for the radiometer systems which forms the basis of obtaining the final radiometric data product is described. It is estimated from the calibration results that the accuracy of the 1.4 GHz radiometric measurements is about + or - 3 K. The measured 5 GHz brightness temperatures over bare fields with moisture content greater than 10 percent by dry weight are about 8 K lower than those taken simultaneously at 1.4 GHz. This could be due to either (1) a 5 GHz antenna side lobe seeing the cold brightness of the sky, or (2) the thermal microwave emission from a soil being less sensitive to surface roughness at 5 GHz than at 1.4 GHz.