Quiet up there

Circular diagram showing the movement of GPS satellites.

Simulation of the GPS constellation, showing the
trajectory of each satellite in azimuth & elevation
from BETA. Click the thumbnail to read memo.

Radio astronomy image comparing the results when the mitigation technique is not used.

The effect of using the PAF-based RFI mitigation
techniques described in the memo.

11 March 2016

A recent experiment by the ASKAP Commissioning and Early Science (ACES) team has shown how phased array feed (PAF) receivers can be used to mitigate the effect of radio frequency interference (RFI).

This effectively allows astronomers to ‘disappear’ RFI from their observations without losing astronomical data; something that would not be possible with traditional single-pixel feeds.

A new technical memo details the trials, in which the team used ASKAP’s BETA test array to observe the radio calibrator source PKS 1934-63, at 1225 MHz. This frequency is known to be affected by interference from GPS satellites.

According to Keith Bannister, author of the memo, the PAFs offer a unique capability of removing interference through the use of special ‘projection’ algorithms and the ability to ‘steer’ nulls – only possible thanks to the flexible beamforming capability of focal plane arrays.

“In this experiment, we observed the well-known calibrator source and trialed a number of different mitigation techniques,” said Keith, “These techniques, known as 'projection' algorithms, performed extremely well.”

“The unmitigated data would have been useless for astronomy, while the mitigated data showed the source to be exactly where it should, and surrounded by quite low levels of noise and artefacts. Thanks to such encouraging results, we'll be trying these techniques with ASKAP as it grows, and see how well we can get it to work.”

Further development of the mitigation techniques will not only benefit the operation of ASKAP in the future, but is also be transferable to any real-world astronomy application where selective RFI mitigation is needed.

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