The radiation temperature 1 of the atmosphere is determined by using the usual ``chopper wheel'' calibration technique: one compares the emission of the atmosphere, to both an ambient load of radiation temperature (the absorbing table that is switched in to the beam), and a cold load of radiation temperature (an absorber in the receiver dewars towards which the beam is redirected by switching in a corner mirror).
The three measurements give ( is the temperature in the receiver cabin and the forward efficiency):
These measurements are combined to give and . Actually is quite stable and we can avoid too frequent cold load measurements by assuming constant.
The system noise of the atmospheric monitor is given by:
Note that this is different from the usual formula for the system temperature, since our reference plane is below the atmosphere, not above.
Thus a variation of atmospheric emission leads to a variation of the total power , given by:
where we allowed for a change in due to a variety of possible causes: variations in receiver noise, ambient temperature, forward efficiency (or ground noise).