Errors due to the reflectivity of calibration targets

For a microwave total-power radiometer, we consider the error introduced by neglecting the difference in the antenna reflection coefficient between when it views a distant scene and when it views a nearby calibration target. An approximate expression is presented for the error, and measurement results are presented that enable one to estimate the resulting uncertainty in the measured brightness temperature. This uncertainty ranges from about 0.1 K to several kelvins for the representative cases considered