Recent PNT Improvements and Test Results Based on Low Earth Orbit Satellites

Gregory Gutt, David Lawrence, Stewart Cobb, and Michael O’Connor

Abstract: This paper describes the latest innovations and experimental results for assured time and location based on Low Earth Orbit (LEO) satellites. Interest is growing worldwide to identify additional sources of PNT to augment existing GNSS sources. Industry and government groups are actively seeking independent timing solutions to ensure uninterrupted operation of the critical infrastructure, including data centers, financial markets, cellular networks, and power distribution. Military users are also interested in robust and reliable geolocation capabilities, even in environments where GPS or GNSS is not available. Commercial and academic entities serving these industries are actively exploring a wide range of technologies to support a comprehensive solution for assured PNT. Results from three separate user equipment configurations are presented – each of which is intended to increase the real-time performance of the Satellite Time and Location (STL) system, a non-GNSS solution for assured time and location that is highly resilient and physically secure. STL uses the Iridium constellation of 66 communications satellites in Low Earth Orbit (LEO) to transmit specially structured time and location broadcasts. Due to their high RF power and signal coding gain, the STL broadcasts are able to penetrate into difficult attenuation environments, including deep indoors. Like GNSS signals, these broadcasts are specifically designed to allow a receiver to obtain precise time and frequency measurements to derive its position, navigation, and time (PNT). STL is able to augment or serve as a back-up to existing GNSS PNT solutions by providing secure measurements in the presence of high attenuation (deep indoors), active jamming, and/or malicious spoofing. The first configuration substituted a rubidium clock (a very stable frequency source based on atomic physics rather than quartz crystals) for the Temperature Compensated Crystal Oscillator (TCXO) used as the STL receiver’s timebase during earlier experiments. Using the rubidium clock, the STL receiver demonstrated a Maximum Time Interval Error (MTIE) of 170 nanoseconds over a two-week period at a static location which was precisely known a priori. The second configuration utilized the same hardware setup, with the receiver configured to a static location that was unknown, and with a rubidium clock that was still warming up 6 hours after a cold start. The demonstrated MTIE for this configuration was 420 nanoseconds over a seven-day period. The third configuration utilized a nearby STL reference station to transmit localized error measurements to the STL ground segment. The reference receiver used a GPS-derived timebase that was assumed to be perfect. Timing results from 3 receivers, with different internal OCXO oscillators, are presented.
Published in: Proceedings of the 49th Annual Precise Time and Time Interval Systems and Applications Meeting
January 29 - 1, 2018
Hyatt Regency Reston
Reston, Virginia
Pages: 72 - 79
Cite this article: Gutt, Gregory, Lawrence, David, Cobb, Stewart, O’Connor, Michael, "Recent PNT Improvements and Test Results Based on Low Earth Orbit Satellites," Proceedings of the 49th Annual Precise Time and Time Interval Systems and Applications Meeting, Reston, Virginia, January 2018, pp. 72-79.
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