Extracting Precise (11/2-M) Tactical Positioning Data from LF Radio Transmissions

This paper describes a series of low-frequency (LF) time-transfer measurements performed during 2005 to support the prototyping of a frequency-agile, programmable-bandwidth radionavigation system, the theater positioning system (TPS), which is being implemented for the U.S. Army to support soldier training and combat systems testing in GPS-denied environments such as dense forests and in urban terrain. The fundamental basis for the system is a spread-spectrum signal which is launched from multiple widely-spaced, generally terrestrial transmitters. The radiolocating receiver acquires these continuous, overlapping CDMA signals, decodes them, and extracts the transmitter locations and times of transmission from data streams embedded in the respective TPS signals. The radionavigation solutions are then obtained, but with downstream corrections for spherical-earth geometry and RF propagation corrections for the groundwave signals. The TPS signal structure is also specifically designed to provide an effective back-up navigation source to GPS in difficult reception situations and afford maximal rejection of power-line noise to improve reception in urban areas. A final feature of the TPS signals permits wide-area broadcasting of low-rate data for commands, DGPS corrections, status information, and the like. The prototype timing signals described in this paper were chosen from real clocks and transmitters at LORAN-C stations operating in the western U.S. With proper clock selection and configuration, and with a new optimization technique to reduce the long-term flicker noise in the LORAN propagation paths, the equivalent system errors reduced to about 1frac12 meters. This represents the timing-error component; positioning errors can be similarly reduced with appropriate calibrations. These results demonstrate performance nearly comparable to current GPS figures and confirm that TPS can be implemented with high accuracy and stability while maintaining independence from GPS an- - d its vulnerability to jamming. This paper presents our prototyping method and discusses the application of these results to future TPS developments and other military and civilian navigation systems