TABLE III - Comparison of observational results with harmful interference limits given in Tables I and II Table 111 shows that very sensitive observations are being made at various radioastronomy observatories. The system temperatures, bandwidths and integration times chosen for the calculations leading to harmful interference limits given in Tables I and II and in Fig. 1, represent practical values currently being used by the radioastronomy service throughout the world. Were interference to be encountered with intensities increasing above these limits, radioastronomy observations would become increasingly untrustworthy. Changes in receiving systems can be expected to give improved performance in the future. It is safe to assume that within ten years observations will be made routinely at sensitivity levels better than those shown in Tables I and II. Such improvements might result from changes in any one of the factors entering into Equations 6 and 7. It appears unlikely however, that major improvements could result from changes in receiver noise temperature. At frequencies of 150 MHz and less, the receiver temperature is not a large contributor to the total system temperature. At the high frequency end of the spectrum now being used by radio astronomers, improvements in receiver technology are likely to have their largest effect. If receiver temperatures of 10 K can be achieved at frequencies in excess of 30 GHz then improvements in sensitivity of 6 dB will result in this millimetric region of the spectrum. 8. The radioastronomy antenna The typical radioastronomy antenna has high directivity in order to obtain the best possible angular resolution of the observed sources, and a large collecting area (high gain) for good sensitivity. In modern systems beamwidths of the order of minutes of arc to seconds of arc are used (100 millidegrees-10 millidegrees), corresponding to antenna gains of more than 70 dB. The high gain combined with the good sensitivity of a radioastronomy receiving system makes it possible for the radio astronomer to observe very faint power fluxes indeed. For example at 1420 MHz, with a receiver sensitivity of 10-27 W/Hz (-270 dB(W/Hz)) and with an effective collecting area of the antenna of 4000 m2 (61 dB gain), the detectable power flux density is: Obviously a different antenna would yield a different sensitivity level. To obtain a general feel for the interference problems which may be applied to all radio telescopes, large and small, the conditions where the telescope is pointed away from the interfering source should be considered. The harmful power flux and power flux-density shown in Tables I and II are based on the isotropic case and should be regarded as the general interference criteria for high sensitivity radioastronomy observations 9. Interference 9.1 Types of interference It is convenient to divide harmful interference into three main categories: Category 1: Strong interference that causes non-linear operation of the receiver, sometimes to the point of harming the sensitive input amplifier.
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