Radio Frequency Interference

We describe the nature of the interference challenges facing radio astronomy in the next decade. These challenges will not be solved by regulation only, negotiation and mitigation will become vital. There is no silver bullet for mitigating against interference. A successful mitigation approach is most likely to be a hierarchical or progressive approach throughout the telescope and signal conditioning and processing systems. We summarise some of the approaches, including adaptive systems. 1. The RFI Challenge and Spectrum Management If future telescopes like the SKA are developed with sensitivities up to 100 times greater than present sensitivities, it is quite likely that current regulations will not provide the necessary protection against interference. There is a range of experiments (eg redshifted hydrogen or molecular lines) which require use of arbitrary parts of the spectrum, but only at a few locations, and at particular times, suggesting that a very flexible approach may be beneficial. Other experiments require very large bandwidths, in order to achieve enough sensitivity. As shown in Figure 1, presently only 1-2% of the spectrum in the metre and centimetre bands is reserved for passive uses, such as radio astronomy (Morimoto 1993). In the millimetre band, much larger pieces of the spectrum are available for passive use, but the existing allocations are not necessarily at the most useful frequencies. Current regulations alone will be inadequate, we need technology as well as regulation. We cannot (and do not want to) impede the telecommunications revolution, but we can try to minimise its impact on passive users of the radio spectrum and maximise the benefits of technological advances. Further information on many of the topics discussed below is available on http://www.atnf.csiro.au/SKA/intmit/. 2. Classes of Interference It is important to be clear of what we mean when we talk about interference. Radio astronomers make passive use of many parts of the spectrum legally allocated to communication and other services. As a result, many of the unwanted signals are entirely legal and legitimate. We will adopt the working definition that interference is any unwanted signal entering the receiving system. Interfering signals vary a great deal in their source and nature. This naturally leads to different mitigation approaches. Local sources of interference