The design, development, application, and capabilities of a variable frequency microwave radiometer are described. This radiometer has demonstrated the versatility, accuracy, and stability required to provide contributions to the geophysical understanding of ocean and ice processes. The design technique utilized a closed-loop feedback method, whereby noise pulses were added to the received electromagnetic radiation to achieve a null balance in a Dicke switched radiometer. Stability was achieved through the use of a constant temperature enclosure around the low loss microwave front end. The Dicke reference temperature was maintained to an absolute accuracy of 0.1 K using a closed-loop proportional temperature controller. Versatility was achieved by developing a microprocessor based digital controller which operates the radiometer and records the data on computer compatible tapes. Accuracy analysis has shown that this radiometer exhibits an absolute accuracy of better than 0.5 K when the sensitivity is 0.1 K. The sensitivity varies between 0.0125 K and 1.25 K depending upon the bandwidth and integration time selected by the digital controller.
Computational techniques were developd to (1) predict the radiometric brightness temperature at the input to the radiometer antenna as a function of the geophysical parameters, (2) compute the required input radiometric brightness temperature as a function of the radiometer output using a mathematical model of the radiometer, (3) achieve computational efficiency through a simplified algorithm to determine the expected radiometric brightness temperature, and (4) calculate the emissivity of a layered dielectric media such as ice over water. The effects of atmospheric absorption due to oxygen, water vapor, nonprecipitating clouds have been included. Correction factors for the finite antenna beamwidth, surface roughness, and wind induced foam were employed in these computations.
Remote sensing experiments were conducted from an aircraft platform using this radiometer. The purpose of these experiments was to demonstrate that the accuracy and versatility of this instrument had been achieved in actual field experiments. Four significant scientific observations were accomplished during these experiments. These observations consisted of the first radiometric mapping of an ocean polar front, exploratory experiments to measure the thickness of lake ice, first discrimination between first year and multiyear ice below 10 GHz, and the first known measurements of frequency sensitive characteristics of sea ice
