Engineering the Ideal Medium Frequency Datalink for NDGPS Application

C.A. Treib, M.W. Parsons, D.E. Shofner, D.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 combined several different radiators from other agencies to establish the NDGPS network, saving approximately $45 million dollars and 10 years of installation time. One disadvantage of this approach is the complexity of maintenance issues due to multiple non-standard radiator configurations. The USCG studied several configurations and has found the ideal radiator by weighing the advantages of each design criteria and optimizing each given several constraints. The ideal radiator must overcome extreme environmental changes, provide adequate signal strength in all weather and minimize the use of hazardous materials. 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. Since Nationwide and Maritime DGPS sites each have their own constraints, two separate designs will be provided to address system differences. Two fundamental design philosophies are presented. The classical approach assumes efficiency is the driving factor in antenna design and strives to maximize the real antenna input impedance and radiation resistance. The contemporary approach weighs bandwidth as an important factor in antenna design and seeks a balance between bandwidth and efficiency. Spectacular results were achieved using this method as efficiency was inadvertently maximized simultaneously with bandwidth. The contemporary approach was tested and described as a 150e (150-foot enhanced tower) radiator, providing increased efficiency and bandwidth and is the clear choice for Maritime DGPS sites. The 150e Maritime ideal radiator is a 150 foot tower with 6 Top Loading Elements (TLE) with lengths of 150 feet each, 72 ground radials with lengths of 330 feet each, and a #8 copper 20 foot by 20 foot 4 inch mesh ground mat at the base of the tower. The proposed ideal Nationwide DGPS radiator pends structural approval, and is predicted to be a 190 foot tower with 6 TLE with lengths of 190 feet each, 100 ground radials with lengths of 330 feet each, and a #8 copper 20 by 20 foot, 4 inch mesh ground mat at the base of the tower. By studying the effects of changing one design parameter at a time, each parameter may be optimized for NDGPS application. Integrating all design parameters provides the configuration for the 150e and 190e and forms the basis for upgrading legacy antennas systems as well as constructing new NDGPS sites. The improved antenna design ensures NDGPS construction goals are met on time and that next generation satellite correction signals may be broadcast from upgraded sites. The increased datalink performance attracts new users and spurs existing users to greater productivity.
Published in:	Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003) September 9 - 12, 2003 Oregon Convention Center Portland, OR
Pages:	1 - 16
Cite this article:	Treib, C.A., Parsons, M.W., Shofner, D.E., Wolfe, D.B., "Engineering the Ideal Medium Frequency Datalink for NDGPS Application," Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003), Portland, OR, September 2003, pp. 1-16.
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