A signal from an astronomical source can be reduced by absorption in the earth’s atmosphere. Depending on the wavelength, the atmosphere acts as a very extended source and can diminish the signal from the astronomical source as well as add noise. The noise contribution is then the sum of the receiver noise and the atmospheric noise and is given by

Here

In order to do this one first measures an ambient temperature load. The voltage measured is given by

where K is a constant. Then one measures the sky at the same elevation as the source. The voltage from this measurement is given by

The system temperature,

where y =

This expression is the total noise times

The system temperature depends on the elevation and has to be measured for each source, when the source changes in elevation and when the atmospheric conditions change.

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