Abstract
Absolute calibration of radiometers is usually implemented onboard using one hot and one cold external calibration targets. However, two-point calibration methods are unable to differentiate calibration drifts and associated errors from fluctuations in receiver gain and offset. Furthermore, they are inadequate to characterize temporal calibration stability of radiometers. In this paper, a preliminary study with linear radiometer systems has been presented to show that onboard external three-point calibration offers the means to quantify calibration drifts in the radiometer systems, and characterize associated errors as well as temporal stability in Earth and space measurements. Radiometers with three external calibration reference targets operating two data processing paths: i.e., (1) measurement path and (2) calibration validation path have been introduced. In the calibration validation data processing path, measurements of one known calibration target is calibrated using the other two calibration references, and temporal calibration stability and possible calibration temperature drifts are analyzed. In the measurement data processing path, the impact of the calibration drifts on Earth and space measurements is quantified and bounded by an upper limit. This two-path analysis is performed through calibration error analysis (CEA) diagrams introduced in this paper.
Highlights
For more than six decades radiometers have been used to measure important geophysical parameters over extended time periods to analyze the Earth and other astronomical systems, and track changes in them
For instance, the blue lines corresponding to the calibration validation data processing path in the calibration error analysis (CEA) diagram shown in Figure 12 are independent of the atmospheric brightness temperature
The maximum error in the atmospheric brightness temperature measurements due to calibration drifts undetected in the calibration validation data processing path follows this trend as well
Summary
For more than six decades radiometers have been used to measure important geophysical parameters over extended time periods to analyze the Earth and other astronomical systems, and track changes in them. Internal noise diodes, owing to their low mass and power consumption, have been used in some radiometer systems to introduce additional calibration references to overcome these problems and assess long-term calibration stability and accuracy [8] Such internal calibration sources cannot be used for absolute calibration of a radiometer system as they are not sensitive to changes beyond the point they are introduced to the system, such as antenna losses [9]. This paper, on the other hand, aims to address this issue by presenting a novel three-point onboard calibration method that utilizes an additional external calibration reference target, and offers the means to quantify calibration drifts, temporal stability, and associated measurement errors. On the other hand, are ignored for simplicity
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