A quantitative assessment of RFI in the near-earth environment

Abstract

Central to the issue of making observations from space near the earth is the assessment of the environmental noise levels at very low frequencies. It is well known from previous space-borne observations, particularly from the Radio Astronomy Explorer satellites, that the radio frequency interference (RFI) levels below about 10 MHz are very high, sometimes exceeding the galactic background noise by at least 40 dB. In this paper I will review the known sources of rf noise, their spectral and intensity characteristics, and how serious a threat each represents to monitoring in the earth's neighborhood It is clear that the quietest frequency/time regime for monitoring galactic and extragalactic sources is over the dayside of the earth in the 0.8 – 10 MHz band Observations made in 1969 from RAE-1 achieved an interference free duty cycle of nearly 5096 in the band below 4 MHz on the nightside of the planet. It is felt that by going to an altitude of ~ 10R_e, observations during the present epoch could match or exceed the 1969 RAE observing conditions, particularly on the dayside of the planet.