ION GNSS 2012
Session F6B: Alternatives to GNSS: Wide Area

Title: Ranging/Timing Using the NDGPS Signal
Author(s): P.F. Swaszek, University of Rhode Island; G.W. Johnson, Alion Science & Technology; R.J. Hartnett, US Coast Guard Academy; A.B. Cleveland, USCG C3CEN
Room: 206 (NCC)

Position, Navigation, and Timing (PNT) have become a mainstay of everyday life in the United States; many useful applications have become possible through the Global Positioning Service (GPS). Unfortunately, it has long been known that the GPS is vulnerable and so the development of alternative "backup" PNT systems are recommended.
A wide variety of technological solutions to PNT are possible; in the radio frequency (RF) domain we have the so-called "signals of opportunity" systems. This phrase refers to the availability of RF signals whose primary purpose is not PNT, but that could still be used in a PNT receiver. As an RF communications signal with detailed time/frequency domain characteristics, the Nationwide Differential GPS (NDGPS) has the potential to serve as a signal of opportunity ranging signal.

Prior published work has examined the performance of the NDGPS system; however, these efforts have focused on the data communications capability (often through coverage diagrams assessing availability) and the resulting navigation accuracy of the corrected GPS signals (often through evaluation of the spatial decorrelation of the corrections), not the use of the signal as a ranging source. At ION GNSS 2007 these authors considered combining pseudoranges from both Loran and the NDGPS system. In unpublished work, we considered the possibility of a stand-alone NDGPS positioning system. The results of this last study, unfortunately, indicated that the NDGPS system is too sparse to provide sufficiently accurate pseudoranges for stand-alone precise 2-D positioning. Our view, however, is that NDGPS has great potential as a signal of opportunity: the NDGPS transmitters are located at government controlled sites widely dispersed over CONUS and the signal itself can easily be synchronized to UTC. As such, the NDGPS signal could be useful in providing precise time to a timing user or providing an additional range measurement in a ranging solution or as an integrity check to a GPS positioning user; these applications require only one accurate pseudorange.

At ION NTM 2012, these authors described a theoretical assessment of the ranging performance of a single NDGPS signal. Specifically, we explored the timing characteristics of the signals, known estimators for those characteristics (within its context as a communications signal), and the limiting effects of noise, terrestrial propagation delays, and multipath. We also proposed two improvements: decision-directed estimators of the timing parameters and the transmission of a second, correlated NDGPS signal from each site. This paper details our further work towards establishing the potential of ranging from the NDGPS signals; specifically, experimental results of ranging using a beacon signal synchronized to UTC and theoretical analysis of both decision-directed estimation and the two signal concept.

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