Engineering the All-Weather Antenna Tuning Unit For NDGPS Applications

Chris A. Treib, Paul K. Gingras, Michael W. Parsons and David B. Wolfe

Abstract:	The U. S. Coast Guard is part of the Department of Transportation (DOT)) team to expand the maritime Differential Global Positioning System (DGPS) service into a national transportation safety system. The U. S. Coast Guard's role is to implement a Nationwide DGPS (NDGPS) expansion effort to more than double the existing number of broadcast sites. The NDGPS system is designed to meet all surface transportation navigation requirements in the United States and will provide double terrestrial DGPS coverage across the continental United States. The USCG uses 285-325 kHz (Medium Frequency) to broadcast corrections which provide signal for up to 250 nautical miles. Unfortunately, this frequency range typically requires 340-foot towers for short monopole (tenth of a wavelength) implementation typically discussed in textbooks. Because of the unusually long wavelength, several years of consulting, modeling, and testing were required before fully understanding how to meet system requirements given the severe constraints imposed by practical implementation. The electrically short antennae places increased pressure and complexity for the Antenna Tuning Unit (ATU) to maintain a 50 + 0j Ohm load for all weather conditions. Legacy equipment lacks the sophistication to maintain a 50 Ohm, low VSWR condition, resulting in transmitter mismatches, amplifier failures, extended off-air periods, excessive technician call outs and exhausted sparing and depot maintenance. An all weather ATU presents itself as a tool to significantly improve system availability. The desire to develop, build, and field an all weather ATU started with a carefully laid out plan to incorporate the engineering process with the required contracting process to serve as a vehicle to evaluate emerging technology fairly while providing options for design improvement, quality control, and efficient production and fielding. Results are presented from C2CEN baseline testing to prototype evaluation for seven DGPS sites for 5 months. The results show the all weather ATU provides a 50 + 0 j Ohm match upon install and an ability to automatically maintain this match through periods of changing weather conditions. Availability through the prototype period increased from 97.52% to 99.26%, a marked improvement toward achieving the system availability of 99.7%. The successful fielding of the all weather ATU increases system availability and provides timely relief for maintenance and depot commands, with full fielding expected for maritime DGPS application by September 2006 and for nationwide DGPS application by December 2007. The ATU also provides new possibilities to remotely monitor far field coverage in real time via antenna current monitoring as well as a method to improve system bandwidth for rapid fielding of enhanced modulation schemes.
Published in:	Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005) September 13 - 16, 2005 Long Beach Convention Center Long Beach, CA
Pages:	758 - 769
Cite this article:	Treib, Chris A., Gingras, Paul K., Parsons, Michael W., Wolfe, David B., "Engineering the All-Weather Antenna Tuning Unit For NDGPS Applications," Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005), Long Beach, CA, September 2005, pp. 758-769.
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