TABLE OF CONTENTS The purpose of calibration is to establish the connection between the input brightness temperature and the output quantity (volts, watts, digital counts) of the radiometer.
In principle, a full knowledge of all component specifications, waveguide losses, reflection coefficients, and physical temperatures would render calibration superfluous, as the behavior of the radiometer then could be perfectly predicted by modeling. In real life, however, such accurate predictions are very difficult, and modeling is best suited for prediction of relative dependencies such as: how the radiometer output will vary with the temperature of a certain amplifier. Thus basic absolute calibration becomes a vital and often time-consuming part of a radiometer development and maintenance.
For Dicke radiometers we have from (4.3): V OUT = constant ( T A - T R), where T R is known. For noise-injection radiometers: T A = T R - T I, where T R is known and T I is proportional to some output quantity, usually digital counts. In both cases only one calibration point is needed, and the input-output relationship can be adequately described. It is then assumed that the radiometer in question is perfectly linear an assumption that it would be nice to verify through calibration at several points.
For total power radiometers we have from (4.1): V OUT = constant ( T A + T N), where T N cannot be regarded as well known (i.e., to better...
