RFI mitigation strategies for next generation phased array radio telescopes

The Low-Frequency Array (LOFAR) telescope is a phased array radio telescope that observes weak astronomical signals in-between strong terrestrial radio frequency interference (RFI). The RFI causes intermodulation products in the analog system due to the non-linearity of the electronic devices used. Multiple strategies are implemented in LOFAR to mitigated the RFI and their intermodulation products: RFI is suppressed in the analog domain based on their direction-of-arrival, using analog beamforming, and based on their frequency, using notch and bandpass filters. The intermodulation products are minimised by using a very linear signal chain, using several stages of low-gain, high-linearity amplifiers. However, in some LOFAR observations, the intermodulation products add in-phase during beamforming, limiting the sensitivity of the telescope. It is shown how this can be prevented by adding different time delays to each signal before digitisation.