An Analysis of Influences of Satellite Positioning Accuracies on Earth-Orbital Ultra-Long Wavelength Interferometry

Being free from constraints on ground-based observations such as those due to the ionosphere and artificial/natural interferences, astronomical observations in ULW (Ultra-Long Wavelength) bands are more easily carried out in space. Antennas deployed on an Earth orbit can detect signals of frequencies well below 10MHz, and space interferometry can have baselines many times longer than the Earth diameter. Space interferometry can thus reach higher angular resolutions and sensitivities at ULW bands than ground-based interferometry. During interferometric observation antennas are synchronized to receive signals simultaneously. The observation data are then transmitted to ground stations for subsequent processing, including fringe search, signal correlation, and inverse Fast Fourier Transformations. For interferometric observation accuarcies of satellite attitudes and baselines between satellites will affect image quality and data-processing speeds. Using the fringe-search method in data processing can reduce the influences of signal-delay errors on result data correlation and images. Although some instruments for high-accuracy inter-satellite ranging can be carried on-board satellites to achieve highly accurate measurements for the purpose, this would increase the complexities and costs of relevant missions. In this paper we first analyze effects of control accuracies of satellite attitudes and measurement accuracies of baselines on Earth-orbital ULW interferometry. We then simulate a particular array configuration by taking into account satellite positioning errors in the direction toward the interference phase center. The simulations reveal the relations between signal-delay errors and fringe-search ranges as appearing in data processing. We also estimate data-processing speeds needed to achieve a common level of image quality for different fringe-search ranges. The simulation results show that for the adopted parameter set an increase of 6 times in the satellite positioning accuracies leads to a reduction of 95% in the needed data-processing speed. Our analysis and simulation results can guide future design and optimization of space ULW missions.