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).