Mitigation of radar interference in L-band radio astronomy

The 1215-1400 MHz band is important for spectroscopy of H I at high redshift, pulsar work, and SETI. Observations at these frequencies are complicated by pulsed interference from ground-based aviation radars. In this paper, we characterize one such radar received at the Arecibo Observatory using coherently sampled data sets obtained during a recent observation. Using these data, we demonstrate some simple methods for detection and removal of the radar pulses. One of these uses a coherent subtraction technique that has not previously been applied to the radar problem. This new technique provides an alternative to blanking, which is undesirable in pulsar and SETI work. We demonstrate that the radar studied in this paper can be suppressed by at least 16 dB in integrated spectra using the coherent subtraction technique. The maximum single-pulse power observed at the output of the canceler is similar to15 dB less. The primary limitation appears to be the detector performance; as a result, the performance using blanking is about the same. Also, we demonstrate that the matched detector for pulses from this radar is relatively insensitive to astronomical transients (e. g., giant pulses), and we quantify the risk of such transients being falsely identified as radar pulses. The techniques described in this paper can easily be adapted to radar waveforms other than the one examined here.