Mitigation of Pulsed Interference to Redshifted H i and OH Observations between 960 and 1215 Megahertz

The neutral hydrogen 21 cm spectral line (1420.4 MHz) and the four 18 cm lines of the hydroxyl molecule (1612–1720 MHz) are observable at redshifts that put their measured line frequencies well below their protected frequency bands. Part of the redshift ranges ( z = 0.171–0.477 for H I and z = 0.37–0.73 for OH) fall in the 960 to 1215 MHz band that is allocated to aircraft navigation. Most of the signals in this band are pulsed emissions of low duty cycle so much of the time between pulses is interference-free. This paper outlines the structure and measured properties of signals in this band and demonstrates a signal processing strategy that is effective at removing the pulsed signals from spectra at sensitivities produced by several hours of integration.