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The radiometer equation relates bandwidth, length of observation, and amount of random noise, to a resulting signal-to-noise ratio, i.e., your ability to tell whether a radio signal is coming from the direction you are aiming at. It is of interest in any situation where the sensing ability of a radio receiver is of interest, including radar and communications as well as radio astronomy.
There is random radio-frequency noise from the sky, from the antenna, and from the circuits and transmission lines themselves. An assurance of the presence of a signal-of-interest can be established if the observation-time is sufficiently long. The gist of the equation is to characterize to what degree random noise might masquerade or cancel a signal according to the above factors.
Radio observations also pick up competing (non-random) signals since no antenna is perfectly directional. These are generally accounted for by noting the characteristics of the antenna's directionality, learning some of the specifics by additionally carrying out some observations not exactly at, but near the direction of interest. Doing this is somewhat analogous to apodization for optical telescopes.