Frequency mangement in low frequency radio telescopes

The ionosphere can be a major impediment to the operation of low frequency radio telescopes such as LOFAR (low frequency array). At frequencies in the HF band, signal paths from extraterrestrial sources can be severely distorted when traversing the ionosphere and even be reflected back into space before reaching the ground. The ionosphere is a continuously varying medium and so there can be times when there are frequencies for which ground based astronomical observations are impossible. This is exacerabated by the fact that the ionosphere can vary quite considerably over the area of the telescope array and hence cause extreme distortion to the array beam forming. It is the purpose of the following paper to investigate the effect of the ionosphere, and ionospheric variations, upon the operation of radio telecopes at low frequency. Further, to develop techniques for predicting what frequencies might be available, at a given time and season, for astronomical observations. The investigations are carried out by means of raytracing that uses a numerical solution the Haselgrove equations and an ionosphere that is based on the IRI90 ionospheric model. (Below is an example of the distortion caused to rays approaching the LOFAR telescope at a variety of elevations and from which it will be noted that the ionsophere can caused severe distortion at low frequencies.) The adaptation of frequency management techniques for HF communications to the needs of LOFAR is also investigated.