Geographic Information System Loran-c Coverage Modeling

Geographic information system (GIS) technologies can offer significant advantages over conventional computer programs designed to predict Loran-C coverage. While newer conventional coverage prediction programs offer sophisticated propagation and interference modeling, flexibility in the specification of positioning techniques, and improved mapping quality, geographic information systems offer the potential for a more adaptive, flexible, and interactive approach. GIS technologies permit the acquisition, management, analysis, and display of spatially related information in a single integrated system. GIS modeling also offers the ability to perform interactive spatial analysis based on requirements unique to the user not included in the design of a conventional computer program. In addition, the geographic operators provided by GIS packages facilitate the transfer of Loran-C modeling concepts to various hardware and software platforms, making coverage prediction technologies available to independent agencies worldwide. Introduction Loran-C is a ground-based, long-range, 100 kHz, radio-navigation system. The coverage of the Loran-C system is defined by the geographic areas within which a receiver can reliably acquire and track the Loran-C pulses from a set of transmitters that can provide adequate measurements to meet the needs of the user. The original Loran-C system was used primarily for marine navigation in a three transmitter, hyperbolic mode. Early receivers were designed to track a set of three transmitters consisting of a Master station and two Secondary stations (a chain) transmitting pulses at a common group repetition rate (GRI). The arrival times of the Secondary stations were measured with respect to the arrival time of the Master station to provide two time differences (TDs). The user located these hyperbolic lines of position (LOPs) on marine charts to obtain a position estimate. Current Loran-C receivers track three or more transmitters to provide a position fix usually computed by the receiver and displayed as latitude and longitude. Some receivers contain accurate clocks, enabling them to use only two signals to provide position. Time and frequency receivers may need to track only one transmitted signal to control a timing pulse or oscillator signal with respect to the Loran-C system clocks. Some receivers operate in hostile interference environments while others are used in vehicles capable of high-dynamic maneuvers. Modern avionics receivers may track as many as eight transmitters from up to four different chains. Some use individual transmitted signals as pseudo-range measurements in combination with Global Positioning System (GPS) satellite measurements to provide interoperable navigation service. Differential Loran-C service is available in some areas, where system bias errors are broadcast to users requiring high accuracy position fixes. As used today, effective coverage for the Loran-C system is a function of transmitter power, the position of the receiver with respect to the tracked transmitters, the 100 kHz propagation environment, the capabilities of the receiver, and the requirements of the user. Early coverage charts assumed a user community with single chain receivers operating in the hyperbolic mode and are not adequate descriptors of coverage for the new generation of Loran-C receivers and the variety of user requirements. GIS technologies can be used to model transmitters, propagation paths, interference sources, receiver capabilities, and unique user requirements in a flexible, interactive environment. Using the capabilities of GIS to manage, analyze, and display information in a single system, new coverage charts can be produced that can combine Loran-C modeling with other user information such as transportation routes, land use boundaries, fishing areas, or other interoperable navigation system service areas. Geographic Information System Fundamentals Geographic information systems are hardware and software systems that provide for the creation, management, analysis, and display of spatial information. Evolving from CAD packages, map display systems, and data base and spreadsheet software, GIS processes allow the user to perform complex spatial analysis difficult to achieve with any of the original systems that gave rise to GIS. GIS software is available for a variety of platforms, from mainframes to personal computers. GIS prices range from a few hundred dollars for simple thematic mapping packages to hundreds of thousands of dollars for implementation of complete vehicle tracking and optimal routing dispatch systems for cities with populations in the millions. Current users include private businesses as well as local, departmental, and national governmental agencies around the world. Applications include facilities management, natural resource inventory, land record maintenance, and site location analysis.