This is the most complete calibration procedure, with the minimum number of assumptions. The first step is to measure the power detected by the receiver on the Sky, then on a warm load (the Chopper), then on a cold (liquid nitrogen) load, averaged over the channels of a specific backend. For a perfectly linear detector, the output power is proportional to the total input noise (Receiver + Load), hence the receiver temperature is simply given by
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The description above refers to what happens when OBS when you use the CAL COLD command (in either TREC or AUTO mode).