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Don Casey: GPS Basics

GPS and Loran C Navigation
by Chuck Husick
  Updated March 2009

Loran C and the Global Positioning System are the two primary electronic means for marine navigation. Loran C, in use for more than 30 years continues to serve a large community of commercial and recreational users. Many fisherman rely upon their libraries of Loran C Time Difference (TD) waypoints. Although these waypoints can be converted to GPS Lat/Long format the conversions are not always precise. The system will remain in service in North America for the foreseeable future, providing a valuable back-up for the newer GPS. Loran coverage is being expanded in some other parts of the world and if a recent and very ill advised proposal from the White House Office of Management and Budget to destroy Loran is properly reversed will transition into eLoran or enhanced Loran.

While Loran C's absolute position fixing accuracy is not equal to GPS, it does provide quite precise position repeatability, usually on the order of 50 feet except at sunrise, sunset or during passage of strong weather fronts. eLoran will improve this performance and in addition provide accurate heading information when the receiver is stationary. In addition, Loran and eLoran is a more robust system than GPS, able to withstand interfering signals that would deprive a user of GPS of any useful navigation guidance. Efforts are presently underway to provide the decision makers in the top levels of Government with information that will convince them of the necessity for both maintaining and improving Loran C and eLoran.

The supremacy of GPS as the world's most popular position fixing system is the result of a number of factors. Being satellite based, it provides world-wide coverage. The operating frequency, 1.575 GHz permits use of an antenna small enough to fit within a hand-held package. The circuitry can be contained on a couple of large scale integrated circuits, allowing automatic assembly of the entire device. The massive market, in marine, aviation, automotive and other applications supports production volumes that drive costs down to levels far below previous expectations. Simply put, GPS is a truly amazing bargain.

A GPS receiver may simply show latitude and longitude and calculate and display motion data; course and speed over the ground or it may depict position on a built-in chart or map. They are available as small, hand held, battery powered units, as stand-alone navigation receivers or combined with or housed within chart plotters of varying size, capability, complexity and cost.

A characteristic of units sold for marine use is their ability to store position information in a waypoint library and provide the user with bearing, distance, elapsed time, time of arrival and deviation from the direct path to the waypoint. GPS also provides the user with precise time information (with a latency of 200-300 milliseconds), ironically improving the accuracy of the time required for celestial navigation.

Choice of a hand-held or a fixed mount GPS usually depends on the type of boat on which it will be used. For small, open boats, the ability to clip the hand-held into place when it is needed and secure it in a locker at other times can be very valuable. The built in antenna will generally have the clear view of the sky necessary for proper functioning. Although hand-held units can operate from built in batteries it is usually best to connect the GPS to the boat's 12 volt system. It's a good idea to remove the unit's batteries when they are not in use.

Most hand-held units have a small LCD screen on which data and a plot of position and waypoints can be displayed. The plot of the vessel's track can be quite valuable, for example, when retracing the route out of. a shallow cove. Some display chart information, however the primary value of the chart display will be to aid the navigator in fixing his vessel's position with reference to a proper navigation chart.

Loran C or GPS equipment powered from the boat's electrical system may shut down when the supply voltage momentarily decreases when starting an engine. While most equipment will recover in less than a minute, even a short interruption in data may be inconvenient, especially if it the position data is being used by a chart plotter. Connecting the GPS and other electronics to a battery separate from the engine starting battery will eliminate the momentary shut-down problem.

GPS data is often used as an input to a chart plotter, radar, 406 EPIRB DSC equipped VHF radio or an autopilot, however the format of the data exchange is not totally standardized. The most commonly used format is identified as National Marine Electronics Association (NMEA) 0183. If you contemplate purchasing a chart plotter or navigation software package separate from the GPS check with the manufacturers to assure that the chosen products will be fully compatible. It is also important to note that the NMEA 0183 format includes a number of versions that have evolved over the years it has been in use. A newer standard, NMEA 2000 has been developed and will gradually replace 0183 in the coming years.

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