Chapter 16 Radio Frequency Interference

Abstract

A basic requirement of radio astronomy is access to a spectrum in which observations can be made without detrimental interference from transmissions by other services. In the early years of radio astronomy, when most of the radio astronomy bands below a few GHz were allocated, bandwidths of radio astronomy systems were generally no greater than a few MHz, and the comparable allocated bandwidths largely sufficed. Some allocations were made for radio lines, most importantly the hydrogen (H1) line, for which 1420–1427 MHz was reserved. In the following decades, as radio astronomy at frequencies in the range of tens of GHz developed, bandwidths of order 1 GHz were allocated, and later, a substantial fraction of the spectrum above 100 GHz was allocated to radio astronomy. However, spillover of radiation from transmitting services into radio astronomy bands occurs, and generally it has been necessary to choose observatory sites in radio-quiet areas of low population density and to take advantage of terrain shielding where possible. These considerations have led to the choice of sites in South Africa and Western Australia for international development of several of the largest arrays. Also, with the increase in computing capacity at observatories, detection and removal of interfering signals in astronomical observations have become important parts of data analysis. In particular, digital analysis allows the received bandwidths to be divided into as many as 106 spectral channels, which allows those containing interference to be identified and removed. A general discussion of interference in radio astronomy is given by Baan (2010).