technique of repeating the observations is not very useful in the case of sources of variable intensity, as it is not possible to distinguish between variations in the interference and in intrinsic variations in the source. The on/off technique is not useful for observations of extended astronomical objects, such as the cosmic background, since there is no region of the sky which can be considered off-source. An assumption that the intensity of the interference is constant must also be made. This is not generally true; often the interference varies in intensity with time and in a complicated way across the frequency band. 9.2.3 Data processing techniques. After-the-fact reduction of the effects of very low-level interference is of very limited usefulness in radioastronomy. One reason for this is that in order to detect power at a very low level, long integration times, which mask the identifying characteristics of the interfering signals, have to be used. The added power caused by the interference is then no longer distinguishable from the random noise one is looking for. It might be possible, in some special cases such as pulsed radar interference where the characteristics of the interfering signals are also accurately known, to process the data in a way that might reduce the effect of the interference. Continuum observations, which require a large bandwidth in order to achieve good sensitivity, can be made by a receiver covering the desired bandwidth with a number of contiguous channels (a spectral line receiver). The spectral information can then be used to identify a narrow interfering signal. However, for interference covering a bandwidth comparable to the continuum bandwidth observed, this technique is not useful. In very special and rare cases if the characteristics of the wanted signal are known, this fact can be used to separate it from the interference and noise. However, for the general case of radioastronomy observations, there seems not to be any useful data processing technique that may be used to identify and reduce interference. 9.3 Interference and long baseline interferometers High resolution observations require the use of interferometers or arrays of antennae with wide spacings between their elements. Each antenna may be subject to different interference conditions. In the extreme case of very long base-line interferometer (VLBI), the interference may affect only one antenna of the system. This may reduce the effect of interference by a small factor. On the other hand, it should be noted that VLBI experiments involve sophisticated data reduction and use of very distant antennae with consequent co-ordination problems. Interference affecting only one antenna may invalidate a whole observation. This leads to the need for world-wide protection of VLBI observations. REFERENCES AND BIBLIOGRAPHY COHEN, M. H. [1969] High resolution observations of radio sources. Annual Rev. of Astron, and Astrophys., 7, 619-664. ELLIS, G. R. A. and HAMILTON, P. A. [1966] Astrophys. Journ., 143, 227. EWEN, H. 1. and PURCELL, E. M. [1951] Nature, 168, 356. HEWISH, A., BELL, S. J., PILKINGTON, J. D. H., SCOTT, P. F. and COLLINS, R. A. [1968] Observations of a rapidly pulsating radio source. Nature, 217, 709-713. JANSKY. K.G. [1935] Proc. IRE, 23, 1158. KRAUS, J. D. [1966] Radio Astronomy, McGraw Hill, New York, USA. LILLEY, A. E. and PALMER, P. [1968] Tables of radio frequency recombination lines. Astrophys. Journ., Suppl. Series No. 144, 16, 143-174. RADHAKRISHNAN, V. [1969] Fifteen months of pulsar astronomy. Proc. Astron. Soc. of Australia, 1, 254-263. ROBERTS, J. A. [1963] Radio emission from the planets. Planet, and Space Science, 11, 221. SNYDER, L. E., BUHL, D., ZUCKERMAN, B. and PALMER, P. [1969] Microwave detection of interstellar formaldehyde. Phys. Rev. Letters, 22, 679. VERSCHUUR, G. L. [1969] Measurements of magnetic fields in interstellar clouds of neutral hydrogen. Astrophys. Journ., 156, 861-874. WEINREB, S., BARRETT, A. H„ MEEKS, M. L. and HENRY, J. C. [1963] Nature, 200, 829. WILD, J. P., SMERD, S. F. and WEISS, A. A. [1963] Solar bursts. Annual Rev. of Astron, and Astrophys., 1, 291. CC1R Document [1971]: Report of the CC1R Special Joint Meeting, Geneva, 1971, Chapter 3, Table 3.3-IV.
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