Do Earth-based transmitters affect our cosmological observation?
24 January 2014
In a recent paper by CAASTRO researchers André Offringa (ANU) and Martin Bell (University of Sydney), together with international colleagues, the Low-Frequency Array (LOFAR) radio telescope in The Netherlands was used to analyse what kind of effect terrestrial transmitters have on an observation. For astronomy, these transmitters are sources of interference. The authors assumed that sources of interference are uniformly distributed over the surface of the Earth. If this model is correct, a telescope would see many distant sources that have a faint effect and a few strong sources close to the telescope. Having mathematically derived what effect the number of sources and the curvature of the Earth have on the power level of the interference, the researchers show that the actually observed interference follows such a model very well.
While modern interference-detection algorithms can remove most of the interference caused by terrestrial (and orbiting) transmitters, it is hard to identify such interfering sources in an observation because distant transmitters will have a very low effect. There are so many of such transmitters that they might actually disturb some of the deepest observations. One particular science case that requires deep observations is the Epoch of Reionisation. Various telescopes, including LOFAR and the Murchison Widefield Array (MWA), are collecting hundreds of nights of data to allow the detection of this cosmological event.
To estimate the contribution of distant sources on such observations, the model that was found for the close-by transmitters was further extrapolated to the situation of distant transmitters. As their present publication reveals, it appears that, in theory, these distant transmitters could severely limit deep observations. In fact, their extrapolation shows that some of the observations that have already been successful should have been limited by interference, which is obviously incorrect. This can be explained by assuming transmitter signals are not received coherently by the telescope. In other words, their combined power does not add up linearly, because each transmitter has different transmitting properties. This suggests that, after all, it should be possible to make very deep observations of the sky without being limited by interference.
Image caption: Observed power levels (blue lines) and derived models (red lines) with constraints on interference power (crosses)
by A. Offringa