This chapter describes the trades between navigation services from low Earth orbit (LEO) compared to medium Earth orbit (MEO) where the GNSS core-constellations of GPS, GLONASS, Galileo, and BeiDou reside today. Closer to Earth, LEO offers less path loss than navigation systems in MEO, improving signal strength by 1000 fold (30 dB). This strengthens us to interference and aids substantially in urban and indoor environments. Their drawback is coverage: it takes nine LEO satellites to match the footprint of one MEO.
This chapter opens with a brief history of early US and Russian satellite navigation systems in LEO, progressing to satellites in LEO today and LEO constellations on the horizon. We then give the needed mathematical background to derive and contrast LEO and MEO signal-to-noise ratio, satellite footprint, mean motion, and Earth coverage, highlighting the advantages and drawbacks of LEO and MEO from a navigation perspective. We then discuss the role of LEO in satellite navigation today, showcasing the performance of the Satelles Iridium-based GPS augmentation system and demonstrating the benefit of stronger signals. Though extremely valuable as a complement to GPS, Iridium lacks the numbers to fully replace GPS as a standalone navigation system in all capacities as only one satellite is typically in view at mid latitudes. We close by looking to the future of LEO in navigation and demonstrate that the scale of the proposed Broadband LEO constellations of OneWeb, SpaceX, Boeing and others, coupled with the more benign LEO radiation environment, can deliver the positioning performance of GPS with commercial-off-the-shelf (COTS) components.
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Fig. 1 was produced using data from the Union of Concerned Scientists Satellite Database which is constantly updated. Orbital parameters we taken from NORAD, specifically the master list of Two Line Elements. The Matlab code to produce the plot has been shared publicly on github.
Figs. 7, 9, and 10 were made using this Matlab code publicly shared on github.
A brief summary of this work can be found in the GPS World article by the authors.