TABLE OF CONTENTS The basic calibration is carried out to find the calibration curve of the radiometer receivers, that is, the relationship between known input levels and corresponding digital output counts. Primarily, this is done with the receivers subjected to their nominal environment. During such exercises, the linearity of the receivers must be verified and the sensitivity (defined as the standard deviation of the output signal for a constant input level) checked. Having calibrated the receivers, these, together with proper calibration sources, can be used to measure the losses in antenna reflectors, feeds, and waveguides.
However, the receivers cannot be expected to operate under a nominal, constant temperature when in orbit. First, a thermal analysis might show that the temperature within the radiometer instrument will oscillate by typically up to several degrees during an orbit cycle. Second, unforeseen thermal gradients may be experienced in orbit. So it is necessary to investigate radiometer performance under nonideal thermal conditions.
The results from the experiments in Chapter 10 show that it is possible to find the true brightness temperature at the antenna when the radiometer temperature is different from its nominal value. To do this, each receiver must be calibrated subject to different temperature levels and temperature oscillations. These oscillations shall be planned to resemble as much as possible the oscillations that may occur in orbit. Many temperature sensors must be embedded in each receiver and their output recorded during calibration exercises, and included in the housekeeping data...
