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To outline the relative importance of all parameters, I will now derive an
approximate formula for the calibration factor in the limiting (but
frequent) case where the opacities are weak and equal in the image and
signal bands.

(25) 
where is the physical temperature of the absorbing layers. Thus

(26) 

(27) 

(28) 
Eliminating gives

(29) 
Although is a weighted average of the physical temperature of the
cabin and the outside temperature, it is not very different from
. Hence we can simplify :

(30) 
It is important to note that in this formula, is dependent mostly
on the outside temperature and pressure (and site altitude of course), and
weakly on because is the (physical) temperature of the
absorbing layers. is the effective temperature seen by the
antenna, and hence small compared to . The parameters are
, and . Setting reasonable numbers :
yield

(31) 
Next: SKYDIP effects
Up: Critical parameters, biases and
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20181213