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Radio astronomy observations in the HF (1-30 MHz) portion of the electromagnetic spectrum could result in new insights into astrophysical processes. However, this particular part of the spectrum is mostly inaccessible from the ground due to the effects of the Earth's ionosphere. One solution is to observe from Earth orbit, thereby avoiding most of the absorption and phase distortions from the ionosphere. However, in the 1-30 MHz band of interest, the ionosphere is neither a perfect reflector nor is it a perfect transmission medium. Terrestrial signals leak through and increase the background radio noise or introduce spurious signals into the measurements, making the detection of faint sources difficult. All terrestrial HF communications signals, especially over-the-horizon radar, are potential interferers to low frequency radio astronomy. Ideally, radio telescopes on the moon's far side would provide a perfectly shielded environment, but at much greater cost and difficulty than a similar system in Earth orbit.
We are investigating methods of predicting signal strengths at the top of the ionosphere with respect to time, frequency, and solar behavior. Existing ionospheric models provide a description of the general, global state of the ionosphere. This information is used as an input to our ray-tracing software to predict the likelihood of leakage through the ionosphere. Sources are distributed in frequency, ray launch angles, and geographic location. Because of the ionosphere, rays can be focussed (increasing the interference intensity) or defocussed (decreasing the interference intensity). Plots of the ray paths from potential interferers, showing the focussing or defocussing effects will be presented. The collective effect of a number of widely separated interferers is to potentially increase the noise at the satellite location. The ultimate goal of this research is to determine if there exist temporal and frequency windows, where the radio leakage is on the order of the cosmic background noise, that permit high-resolution low-sensitivity radio astronomical measurements from Earth orbit. The final model should to be able to predict those times using solar and geophysical parameters.
