| 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 (~ 70dB) 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 were chosen from real clocks and transmitters at LORAN-C stations operating in the western U.S. The errors arising from the system components were quantified using a metric based on a time variance measure TVAR – developed at NIST. 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 1.5 meters (~5 ns). 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 and 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.
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| Steve Smith is a Senior Development Staff Member in the Engineering Science & Technology Division at the Oak Ridge National Laboratory in Oak Ridge, Tennessee. He has been involved in the design and development of numerous analog and digital electronic systems and circuitry to support numerous programs within the Department of Energy (DOE) operations, particularly those in the uranium enrichment, waste handling, and fuel reprocessing areas. Specific development tasks have included the design and testing of analytical instrumentation, radiation-hardened electronics, specialized communications and control systems, and equipment monitoring and diagnostic hardware and software. His present areas of interest include wireless communications devices, navigation systems, analog and digital signal processing, MEMS devices, and advanced intelligent and autonomous sensor systems. He holds B.S., M.S., and Ph.D. degrees in Electrical Engineering from the University of Tennessee and is also presently serving as an adjunct faculty member there. He has over 25 years' experience in AM/FM/TV broadcast engineering and holds 19 U. S. patents (plus 12 others pending) in the areas of communications and signal processing. He has also received two NASA Technology Utilization Awards for developments in the satellite communications field. He is a member of IEEE, ISA, ION, and SBE and has served on the IEEE 1451.3 (Vice-Chairman), 1451.5, and 1588 Standards Committees. |