Reference Antenna Techniques for Cancelling Interference from Moving Sources
Daniel Mitchell (U. Sydney) mitch@physics.usyd.edu.au

Interference mitigation techniques which modify signals from auxiliary reference antennae to model and cancel interference from an astronomical observation can be applied in the time domain, where the RFI voltage is modelled and subtracted from the astronomy signal path (adaptive noise cancelling), or they can be applied to the auto and cross-correlated voltage spectra in the frequency domain (post-correlation cancelling). For ideal receivers and a single, statistically stationary interfering signal, both pre and post-correlation filters can result in complete cancellation of the interference from the observation. The post-correlation method has the advantage of being applied on 10s or 100s of millisecond time scales rather than 10s or 100s of nanosecond time scales. However since the cancellers use cross-correlated power measurements to gauge the interference power in the astronomy signal path from that in the reference signal paths, if the transmitter location changes during the time integration the varying phase differences cause decorrelation of the cross-correlations which can limit the effectiveness of the algorithms. If the decorrelation is not too severe it can be corrected for, at the expense of a noise increase. The time domain adaptive cancellers are allowed to slightly vary their internal coefficients and adapt to the changing phases, which means that they avoid the decorrelation problem, however the freedom to move also results in a noise increase. In this paper the performance of both types of cancellers in the presence of interference from a moving source is compared.