Now AP, the noise fluctuation in power density, in the sensitivity equation (4), is related to the total system sensitivity (noise fluctuations) expressed in temperature units throughout the Boltzmann constant, A, as shown in equation (5): and represents the sum of TA, (the antenna noise temperature contribution from the cosmic background, the Earth’s atmosphere and radiation from the Earth), and TR, the receiver noise temperature. 7.2 Sensitivity estimates It is now possible to estimate the expected sensitivity by substituting reasonable numbers for Af and t. This has been done in Table I by assuming Af to be the bandwidth of an allocated radioastronomy band and the observing (or integration) time to be 2000 seconds. In Table II Afis assumed to be the channel bandwidth of a typical spectral line observing system, and the observing (or integration) time is assumed also to be 2000 seconds. The harmful interference levels given in Tables I and II are expressed as the interference level which introduces an error of 10% in the measurement of AP(or AT), i.e.: The calculated sensitivities and harmful interference levels presented in Tables I and II are based on assumed integration times of 2000 seconds. Integration times actually used in astronomical observations cover a wide range of values. Continuum observations made by telescopes operating singly (rather than in interferometric arrays) are reasonably well represented by the integration time of 2000 seconds. It is representative of good quality observations although there are many occasions when this time is exceeded by an order of magnitude. On the other hand 2000 seconds is less representative of spectral line observations. Improvements in receiver stability and the increased use of correlation spectrometers have resulted in the more frequent use of longer integration times. Spectral line observations lasting several hours are now quite common. A more representative value would be 10 hours with a consequent improvement in sensitivity of 6 dB over that now shown in Table II.
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