| Abstract: |
Microwave radiometers have been used to measure important geophysical parameters over extended time periods to examine variations in the Earth System. Detecting small, long term trends in the Earth System accurately via microwave radiometers requires precise absolute calibration and increased stability. Thus, it is critical to track calibration drifts, and correct radiometer measurements when calibration errors occur.
Absolute calibration of microwave radiometers is usually performed onboard by observing one hot and one cold target to compute the receiver gain and offset. However, such two-point algorithms are inadequate to detect calibration drifts as they cannot distinguish calibration errors from receiver gain fluctuations. This study aims to address this issue by presenting a three-point onboard calibration method which offers the means to quantify calibration drifts, associated measurement errors, and to achieve on-board SI traceability.
Assuming a linear relationship between the input brightness temperature and radiometer output (counts, power or voltage), measurement errors due to calibration drifts will be discussed in radiometer systems implementing two-point onboard calibration. Detectability of these errors will be examined. Then, a new radiometer system which operates in two separate modes and utilizes three external calibration targets (cold space and two warm blackbodies) will be introduced. The first mode is called “the measurement mode”, and in this mode the radiometer measurements are calibrated using all three external calibration targets. In the second mode, which is called “the calibration validation mode”, however, two external calibration targets are used to calibrate the third calibration target measurements. Since the calibration temperatures are known, the calibration validation mode can provide information about calibration drifts that may occur in the lifetime of the radiometer. Subsequently, Calibration Error Analysis (CEA) diagrams which integrate information from the two modes to analyze the impacts of calibration drifts on radiometer measurements will be presented.
Finally, the cost of having an additional calibration target in the radiometer systems, desired radiometer parameters (integration time, calibration temperatures etc.) to maximize the detectability of calibration errors and minimize their impacts on radiometer measurements, possible ways to correct errors due to calibration drifts, and the effects of system nonlinearities will be discussed. |
