For ground-based radio astronomical observations the signal entering the receiver has been attenuated by the earth's atmosphere. In addition to this there is broadband emission, and the signal is refracted, that is, the signal path is bent and there are changes in the path length. Usually these effects change slowly with time, but there can also be rapid changes such as scintillation and anomalous refraction. Thus the propagation properties of the atmosphere and detailed features of its radiation must be taken into account, if the measured radiation is to be interpreted properly. In Sect. 1.2 we stated that the earth’s atmosphere is fairly transparent to radio waves if their frequency is above the cut-off given by the critical frequency of the free electrons in the ionosphere. This cut-off frequency varies depending on the electron density but usually it is in the region below 25 MHz. Most radio astronomical measurements are made at frequencies well above this limit. At the low-frequency limit ionospheric effects can be important; these are of great intrinsic interest for geophysics, but usually are of little consequence with regard to radio astronomy.
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