Digital Filters in Radio Detectors of the Pierre Auger Observatory.

Ultra-high energy cosmic rays (UHECR) are the most energetic observable particles in Universe. The main challenge in detecting such energetic particles is very small flux. Most experiments focus on detecting Extensive Air Showers (EAS), initiated by primary UHECR in interaction with molecules of the atmosphere.

One of the observation method is detecting the radio emission from the EAS. This emission was theoretically suggested in 1960's, but technological development allow successful detection only in the last several years.
This detection technique is used by Auger Engineering Radio Array (AERA). Most of the emission can be observed in frequency band 30 - 80 MHz, however this range is contaminated by radio frequency interferences (RFI). These contaminations must be reduced to decrease false trigger rate.

Currently, there are two kind of digital filters used in AERA. One of them is median filter, based on Fast Fourier Transform. Second one is the notch filter, which is a composition of four infinite impulse response filters. Those filters have properly work in AERA radio detectors for many years.

Dynamic progress in electronics allows to use more sophisticated algorithms of RFI reduction. Planned modernization of the AERA radio detectors' electronic allows to use finite impulse response (FIR) filters, which can fast adapt to environment conditions. These filters are: Least Mean Squares (LMS) filter and filter based on linear prediction (LP).
Tests of new kind of filters are promising and show that FIR filters can be used in next generation radio detectors in AERA experiment.

Author: Dariusz Głas
Conference: Title