Prospects for Low Earth Orbit Individual and Swarm Smallsat Position, Navigation and Timing Without GNSS

James Schatzman

Abstract:	We consider the ability to determine position and motion precisely of a single LEO satellite carrying an RF receiver, without GNSS. Observables are RF (frequency) measurements of narrow band or unmodulated terrestrial beacons. We consider requirements such as LO frequency accuracy of 10-12 (relative deviation), position accuracy of 0.1 m, and velocity accuracy of 1 mm/sec. If GNSS were available, typical timing accuracy in space is around 10 ns and position accuracy is several meters at best – and therefore GNSS alone does not meet the time or space accuracy requirements. We also consider the case where GNSS is denied or otherwise unavailable. Critical issues are the stability of the local oscillators on board the spacecraft, the stability of the beacon frequencies, the accuracy of as-received frequency measurements, the spatial distributions of the beacons around the globe, and accuracy of the orbital propagation. As-emitted transmitter frequencies are not required to be known, except approximately so that the Doppler-shifted signals can be tuned. With accurate orbital modeling, and with an orbital average of 1 or 2 beacons in view, the single vehicle case supports position measurement accuracy better than what is obtainable with raw GNSS measurements. In the swarm case, when vehicle attitudes are measured accurately with star trackers and a sufficient quantity and quality of inter-vehicle measurements are available, the relative timing, positions and motions of the vehicles can be determined to greater precision than what is possible for a single spacecraft, due to the redundancy of measurements. Adding realism to the spacecraft clock model and the orbital propagator accuracy results in the single-vehicle position and velocity potentially better to what is achievable with high quality GNSS-based systems.
Published in:	Proceedings of the 35th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2022) September 19 - 23, 2022 Hyatt Regency Denver Denver, Colorado
Pages:	562 - 574
Cite this article:	Schatzman, James, "Prospects for Low Earth Orbit Individual and Swarm Smallsat Position, Navigation and Timing Without GNSS," Proceedings of the 35th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2022), Denver, Colorado, September 2022, pp. 562-574. https://doi.org/10.33012/2022.18367
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