Document probably older than you think
Calibration compensates for imperfections and unknowns in the instrument use, including antenna defects (surface quality, focus), pointing errors, atmospheric transmission and fluctuations, receiver and backend gain and instabilities, etc...
This paper describes the way OBS (30-m or 15-m version) calibrates spectra, tries to assess the accuracy of the calibration factor, , under practical observing conditions, and then list precautions to take when observing.
Note: this is the on-line version of the "Amplitude Calibration". A PDF version is also available.
Related information is available in