Co-Locating DGPS and Loran Transmitters

D. B. Wolfe, J. L. Hartline, M. W. Parsons, M. E. McKaughan, H. L. McCarter and C. A. Schue III

Abstract:	The U.S. Coast Guard is part of the U. S. Department of ransportation’s 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 in addition will provide double terrestrial DGPS coverage across the continental United States. The DGPS system operates in the Medium Frequency (MF) band where efficient antenna systems are large and costly to build. As a cost saving measure, the NDGPS project team is researching the possibility of co-locating NDGPS sites at U.S. Coast Guard maintained LOng RANge Navigation (LORAN) stations. The colocation concept offers several desirable benefits including but not limited to: significant savings in construction costs (especially in Alaska), reduced project timeline to acquire property, reduced environmental impact, and Coast Guard technicians on-site to maintain the NDGPS equipment. The U.S. Coast Guard's DGPS Radio Frequency (RF)Natural Working Group (NWG) initially proposed the concept of colocating LORAN and DGPS signals on the same tower.1 This method was used uccessfully at the NDGPS site in Savannah, GA where the DGPS signal and a NAVTEX signal are both transmitted from a single antenna tower. Special antenna matching and isolation filters are used at this site. This technique is impractical at the LORAN sites because of potential degradation of the LORAN pulse shape. Using computer simulation and some creative brainstorming, three potential diplexing techniques are presented. To reduce the potential for destructive interference of the LORAN signal, the RF NWG has investigated the possibility of using a single top-loading element (TLE) as the DGPS transmitting antenna. Additional isolation of the two signals should be possible by using the guy end that is connected to the TLE as the antenna for the DGPS signal. This portion of guy is 400 feet long and not an active element of the LORAN broadcast tower. Computer modeling and actual TLE guy impedance measurements are presented. These modeled and measured results indicate the merit of this approach. C2CEN has partnered with the U.S. Coast Guard Academy to build a matching network for the 400 feet TLE guy section and to perform proof-of-concept tests. Presently this effort will concentrate on the 625-foot and 700-foot top-loaded monopole LORAN antenna configurations. Additional schemes for using other ORAN tower configurations are presented. C2CEN, the Coast Guard Academy Electrical Engineering section, and the LORAN Support Unit (LSU) in Wildwood, NJ are working together on implementing this concept. This paper reviews C2CEN’s methodology for computer modeling of both diplexing and co-location using the LORAN tower TLE guy as the DGPS antenna. The methodology leading to the final design is presented. Modeling and actual test results are presented plus plans for future implementation. Estimates of installation and operational savings are included.
Published in:	Proceedings of the 2002 National Technical Meeting of The Institute of Navigation January 28 - 30, 2002 The Catamaran Resort Hotel San Diego, CA
Pages:	352 - 361
Cite this article:	Wolfe, D. B., Hartline, J. L., Parsons, M. W., McKaughan, M. E., McCarter, H. L., Schue, C. A., III, "Co-Locating DGPS and Loran Transmitters," Proceedings of the 2002 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 2002, pp. 352-361.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In