Innovation: GPS + LORAN-C
July 1, 2006 By: James Carroll GPS WorldPerformance Analysis of an Integrated Tracking System
Before GPS, even before satellites, there was LOng RAnge Navigation, or LORAN. Using terrestrial radio transmitters, it was developed during World War II for aircraft navigation. The wartime system evolved by the mid-1950s into the present day 100 kHz LORAN-C system. LORAN's standard principle of operation is hyperbolic positioning. A receiver measures the difference in times of arrival of pulses transmitted by a chain of three to six synchronized stations separated by hundreds of kilometers. The time-difference measurement derived from the signals of two stations, when multiplied by the speed of propagation of the signals, forms a line of position (LOP); the receiver could be anywhere on this line and give the same measurement. The geometrical form of this LOP is a hyperbola. Measurements using a third station provide another hyperbola, which intersects the first at the position of the receiver. There are many LORAN chains around the globe.
 Richard Langley |
The LORAN system is being modernized to enhance its accuracy, integrity, availability, and continuity. Vacuum-tube transmitters are being replaced with solid-state designs and new primary frequency standards are being installed at transmitting stations. Manufacturers have developed compact LORAN receivers able to track multiple transmitters simultaneously and to automatically apply propagation bias corrections. Some receivers are integrated with GPS or other sensors. Receivers also feature improved antenna designs. Collectively, these improvements are known as Enhanced LORAN or eLORAN for short. Additionally, LORAN signals can be used to convey differential GPS corrections. Such a system is already operational in Europe.
Supported by the Coast Guard and the Federal Aviation Administration in the United States, a goal of eLORAN is to provide nonprecision approach for aviation users and harbor entrance and approach for marine users. Land users will benefit, too. Since LORAN has different signal characteristics from those of GPS, it can be used in locations where GPS cannot — by itself or in conjunction with GPS and other sensors. In this month's column, we look at a system that combines eLORAN with GPS and dead reckoning to overcome some of the problems in navigating in big cities.
Innovation features discussions about recent advances in GPS technology and its applications as well as the fundamentals of GPS positioning. The column is coordinated by Richard Langley of the Department of Geodesy and Geomatics Engineering at the University of New Brunswick, who welcomes your comments and topic ideas. To contact him, see the "Contributing Editors" section.
 |
The rapidly increasing use of satellite navigation encompasses a broad range of civilian users. But not all users are sufficiently aware of important operational risks in using, say, the Global Positioning System (GPS) for positioning, navigation, and timing (PNT) applications. Systems that use GPS work very well when designed properly, and when sufficient robust ranging signals from the satellites are available. When the GPS signal cannot be used for PNT, appropriate backup systems and procedures should be used. Service providers are increasingly aware of the need to provide adequate system integrity, and to provide a seamless procedure for timely switches to and from the backup procedures. As a result, there is increasing interest in integrated PNT systems.
The U.S. Department of Transportation's Volpe National Transportation Systems Center in Cambridge, Massachusetts, has developed and installed vessel surveillance and tracking systems for maritime applications and is now applying this evolving technology to surface applications in urban areas in the U.S. Positioning and timing information is provided by a system that integrates GPS with enhanced LORAN-C (eLORAN) and dead reckoning (DR).
An important issue for the integrated GPS/LORAN system is how well LORAN and DR systems can supplement GPS when the GPS signal is not usable. Performance requirements vary with application, but a basic requirement in urban areas is reliable location of the vehicle at an intersection.
This article describes the design and uses of an integrated GPS/LORAN/DR tracking system, and presents results from performance evaluations conducted in New York City, including the lower Manhattan financial district. The results obtained show that eLORAN can be an effective backup to GPS in many areas of Manhattan, including some dominated by tall buildings or open tunnels where GPS is not able to function well.
1 2 3 4 5 6 7 8
RSS
Become a Fan
Join our Network
Follow Us
Please send any technical comments or questions to our webmaster.