GPS Navigation

Finding your position to within a few feet anywhere on the earth's surface is simple, thanks to GPS.

By Mark Corke

It's hard to believe GPS only became fully operational in 1995. GPS, the acronym for Global Positioning System, uses satellites to send signals down to earth, which, when received by suitable equipment, can display very accurate positioning data. Currently there are 34 active and healthy GPS satellites orbiting the earth; four of these must be "visible" to the GPS receiver at any one time for a satisfactory fix.

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 worldwide coverage. The operating frequency, 1.575 GHz permits use of an antenna small enough to fit within a handheld package. The circuitry can be contained on a couple of large-scale integrated circuits, allowing automatic assembly of the entire device. The huge 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, handheld, battery-powered units, as stand-alone navigation receivers, or combined with or housed within, chartplotters of varying size, capability, complexity, and cost. Most, if not all, smartphones incorporate a GPS receiver and with a suitable app it's possible to use these as a small handheld navigation tool, offering much of the functionality of the smaller stand-alone GPS units. 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.

GPS

Choice of a handheld or a fixed-mount GPS usually depends on the type of boat it will be used on. For small, open boats the ability to clip the handheld into place when it is needed, and secure it in a locker at other times can be very valuable. The built in antenna must have a clear view of the sky for proper functioning, so these units do not always function well below decks. Although handheld units can operate from built-in batteries, it is usually best to connect the GPS to the boat's 12-volt system whenever possible. It's also a good idea to remove the unit's internal alkaline batteries when they are not in use. Most handheld units have a small LCD screen on which data and a plotted 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.

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, which in some cases can lead to the loss of stored waypoints and tracking information. Connecting the GPS and other electronics to a battery separate from the engine starting battery will eliminate the momentary shutdown problem.

GPS data is often used as an input to a chartplotter, 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 or the newer NMEA 2000 which is gradually replacing the earlier 0183 protocols. If you contemplate purchasing a chartplotter or navigation software package separate from the GPS, check with the manufacturers to be sure 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.

— Published: December 2015


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