The sensitivity of a radioastronomy receiving system to wideband radiation improves when the bandwidth is increased (equations (4) and (6)). The reason for this is the following: the noise power increases with bandwidth, but, since the signal also is broadband noise, so does the signal. Actually the signal-to-noise power ratio remains constant, independent of the bandwidth. However, as the bandwidth increases, the precision of the determination of the power levels improves (by a factor of p/A/), and thus the sensitivity is correspondingly improved. Equations (4) or (6) suggest that one may achieve any desired sensitivity by making the bandwidth and/or the observing time, large enough. In reality, however, factors other than the statistical ones described above, sooner or later put a practical limit on the sensitivity of a radioastronomy observation. Examples of such other effects are the stability of the receiver, fluctuations in the Earth’s atmosphere and the patience and endurance of the observer. The sensitivity levels given in Tables I and II use values for the bandwidth and integration time for which these other factors usually are insignificant. However, one should bear in mind that these sensitivity levels are not fundamental limits and that they actually have been exceeded in cases where the utmost sensitivity was required for a successful experiment. It should be recognized that astronomical sources of radiation exist which may interfere with highly sensitive observations; their power flux-densities can exceed those given in Table I. The Sun is a powerful source of emission. Because of solar interference, certain investigations can only be conducted at night. Other experiments are possible during daytime except during periods of solar activity, especially for frequencies below about 200 MHz. The quiet Sun is of large angular diameter and constant in flux; it usually presents no difficulties. Below 38 MHz, radiation from Jupiter may also exceed the limits given in Table I. At such frequencies Jupiter is a sporadic radio source which emits strongly only a few per cent of the time at highly predictable periods. These periods of emission can be avoided. Below 1 GHz, many other cosmic radio sources exceed the power flux-densities given in Table I. These sources however, are generally at known positions and of known constant flux and vary only slowly in frequency. In principle and in practice the radioastronomer can make corrections for their effects. This is necessary when performing observations at the highest possible sensitivity. On the other hand, low level terrestrial interference normally has an unknown position, flux and spectrum, and can be highly time variable, so corrections cannot be made for its effects.
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