Differential and Rubidium Disciplined Test Results from an Iridium-Based Secure Timing Solution

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

Abstract:	We present results from two separate experiments intended to increase the accuracy of the real-time timing outputs available from 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 an STL 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 experiment 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 same setup demonstrated an MTIE of 656 nanoseconds over a seven-day period when the receiver’s location was only initialized to within 5 km and the rubidium clock was still warming up from a cold start (or 420 nanoseconds, starting 6 hours after the cold start). The second experiment used a nearby STL reference receiver to transmit differential corrections to the receiver under test. The reference receiver used a GPS-derived timebase that was assumed to be perfect. Using these corrections, the test receiver demonstrated a MTIE of 484 nanoseconds over 24 hours when the receiver’s location was only initialized to within 5 km.
Published in:	Proceedings of the 2017 International Technical Meeting of The Institute of Navigation January 30 - 2, 2017 Hyatt Regency Monterey Monterey, California
Pages:	1111 - 1116
Cite this article:	Cobb, Stewart, Lawrence, David, Gutt, Gregory, O’Connor, Michael, "Differential and Rubidium Disciplined Test Results from an Iridium-Based Secure Timing Solution," Proceedings of the 2017 International Technical Meeting of The Institute of Navigation, Monterey, California, January 2017, pp. 1111-1116. https://doi.org/10.33012/2017.14969
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