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7. Calibration of Data

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 is the atmospheric opacity. It is possible that at certain frequencies the noise contribution from the atmosphere is larger than that from the receiver. In order to properly calibrate the data one needs to measure the total noise and determine the loss due to absorption in the earth’s atmosphere.

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, is defined as

where y = Substituting for and , and assuming that TATM is approximately equal to TAMB , one can express the system temperature as

This expression is the total noise times and this is what needs to be applied to the data to calibrate it. Once the system temperature is measured, the voltage on and off the source can be measured and the antenna temperature can be expressed as

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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