Radar initial orbit determination method from angular track and Doppler shift measurements

The growth in the overpopulation of resident space objects calls for space surveillance initiatives. In particular, the threat posed by in-orbit collisions and fragmentations, as well as by satellites re-entry requires an efficient space objects cataloguing capability. Ground-based sensors are the main contributors to build up and maintain a catalogue of space objects. In this context, survey radars can provide angular track, slant range, and Doppler shift measurements without the need for transit prediction, allowing either the refinement or the initial determination of the target orbital state. In the latter case, a proper Initial Orbit Determination (IOD) technique is required to reconstruct the orbital state of the observed object. This work presents the IODAD algorithm (Initial Orbit Determination from Angular and Doppler shift measurements), a novel radar IOD method when slant range is not available, and thus relying only on the angular and Doppler shift measurements. The proposed IOD algorithm combines the optical admissible region, computed from the angular track measurement, with the measured Doppler shift to compute a first estimate of the orbital state. This combination forks depending on whether the radar is monostatic or bistatic. At the end, the first estimate is refined through a batch filter and the IOD result is returned in terms of mean state and covariance. Unlike existing methods, the new algorithm offers greater flexibility and ease of operational application, as it does not need long measurements tracks as input, nor a specific advanced computational technique. Numerical simulations show the potential of the IODAD algorithm, both through nominal and sensitivity analysis, proving its validity to any survey radar. In addition, a comparison with an existing method demonstrates the significantly better performance of the proposed method. Finally, the results are confirmed by analysing a real dataset of transits concerning calibrator satellites.