HF Interference in Space from Terrestrial Sources

Radio astronomy observations are routinely undertaken in all portions of the radio spectrum except the region of the spectrum which is greatly effected by the propagation of waves through the ionosphere. The HF (high frequency) portion of the radio spectrum (0.3 to 30 MHz) is the area that is impacted by poor propagation conditions. It is especially difficult to make high resolution observations (∼one arc second) because the extremely long baseline between the antennas required for high resolution implies that the propagation conditions will be very different for the signals received by each antenna. The signal amplitude, due to propagation, can fluctuate by many dB and the phase can be irretrievably corrupted which makes it essentially impossible to do aperture synthesis in a feasible manner. These propagation difficulties can be completely alleviated by putting the radio telescope array above the ionosphere. The most widely consider methods for doing this are to put the array into Earth orbit, lunar orbit, or on the lunar surface. The most practical place to put it is in Earth orbit. Unfortunately, with this location, the ionosphere still cannot be ignored. While the ionosphere can grossly distort HF signals, it frequently allows them to pass through. Thus, numerous communication signals from the Earth can reach the low-directivity antennas in orbit and considerably raise the radio telescopes' noise level. The purpose of our project is to characterize the interference phenomena as it would be observed by an Earth orbiting radio telescope. Somewhat recent reviews of the radio frequency interference environment around the Earth in the HF portion of the spectrum are given by Erickson and by Desch. Weiler et al. have proposed an Earth orbiting low-frequency radio telescope, LFSA (Low Frequency Space Array), consisting of four to eight antennas forming a coherent interferometer. The antenna on each spacecraft is a pair of mutually orthogonal dipoles with very little directivity. This avoids the necessity of steering antennas mechanically, but the telescope has no tolerance for interference. A table of harmful interference levels for the LFSA is shown using a criterion that harmful interference is introduced when the interference level is 10% of the system noise (includes the Galactic background).