GPS, do you really know it? Here are some useful tips


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We usually blindly trust the electronic instrumentation on board, and we do wrong, because instruments can be wrong too, GPS included: here’s what you need to know to avoid nasty surprises and optimize their operation.

A GPS receiver (short for Global Positioning System) is an instrument that can instantly calculate the latitude, longitude and elevation of where your boat is located. Originating in the 1970s for military purposes, it spread widely after 2000, when Bill Clinton ordered the removal of the limitation on the accuracy of the signal used by civilian GPSs (before that date the residual error was about 100 meters) making them as accurate as military ones: to this day the error is calculated to be in very few meters. In order to work, GPS needs the simultaneous view of 4 satellites to solve complex equations that provide longitude, latitude, local time and elevation data. All other information provided by the instrument, such as speed and course, is deduced by difference between successive points. Most GPSs on the market work on at least 12 channels and the higher-end ones up to 20: this number represents the number of satellites the unit can track simultaneously to lock onto signals.


board, the installation of the GPS antenna will not create any particular problems: since it is an omnidirectional and very sensitive device, you can house it wherever you are most comfortable. If you have a fiberglass boat, on you will need to have an external antenna but will be sufficient to mount an apparatus with a built-in antenna below deck. Multifunction displays on the market and even some handheld VHF models now incorporate GPS antenna. Returning to the classic outdoor antenna, as long as it has a good view of the sky the accuracy of the receiver is not affected: but in certain situations this accuracy may fail. For example, many marine GPSs when they can lock on to only 3 satellites fix the elevation at zero, to infer latitude, longitude, and time: other algorithms exist to handle weak and intermittent signals, which can further deteriorate accuracy. On the display of your GPS among the various screens you will find the parameter related to position accuracy, (in English, accuracy): check that the number reported is around 10 meters at most (or 30 feet), a higher number could be a symptom of a suboptimal installation or an antenna with bad reception.



When you purchase a new GPS you should check that the coordinates displayed are compatible with the coordinates of the nautical chart you are using. In fact, there are various geodetic systems that define the virtual representation of the globe such as WGS84, NAD83, ED50 and many others. Almost all GPS on the market is set to the World Geodetic System 1984, or WGS84, convention. Compatibility is expressed on each nautical chart: most nautical charts are plotted on this geodetic system but it is good to always check. A GPS setting error or a nautical chart that is based on a different geodetic system can lead to differences of even several hundred meters, enough to cause you to run into rocks. Obviously this is a problem you will face if your GPS, fixed or portable, is not cartographic, and therefore does not provide for real-time overlay between your location and the reference map.



Learn not to blindly trust the GPS, always operating plausibility checks against the position provided by the instrument: some devices stop updating coordinates if they lose signal, and by just taking a quick look at the map you may not notice the problem. During a long navigation, especially if you lose sight of the coast, it is a good idea to regularly mark the GPS position on a nautical chart, in case of problems you will have a recent reference point. Also keep in mind that to err is human, so pay special attention: the most common use of a GPS receiver is to set waypoints that you want to reach in navigation, and it happens to everyone at one time or another to head for a point with the wrong coordinates, where to get to which you are on a collision course with obstacles and rocks.


If you are navigating to a waypoint created with the cursor on a chart plotter (or on a navigation program) check that there are no hazards such as shoals or rocks between you and the selected point: then zoom in on the route and analyze its path to the waypoint, because as the area represented increases, the detail will decrease, so you may not spot buoys, small shoals, outcropping rocks, and the like. When navigating, you will also need to keep an eye on lateral separation from the originally plotted course. This separation is called Cross Track Error, or XTE. If you are not on your original course you may be faced with obstacles that you have not considered. When navigating along a series of waypoints (a fairly common situation, for example, when organizing a trolling session), chances are you will not pass exactly on the waypoint you had set. At some point you will have to decide to move on to the next waypoint-usually the chartmaker is set to do this automatically when it realizes that you are no longer approaching that waypoint but have gone beyond it. The GPS may then decide to route to the next waypoint, and if you do not check the course on the chart you may have nasty surprises along the new route recalculated from where you are. Don’t forget to practice with the traditional nautical chart, because in case of an electrical problem (dead batteries, faulty alternator) you may find yourself in the middle of the sea without the instrumentation. To avoid finding yourself with a square and protractor in the storm, our advice is to keep an extra handheld GPS on board.



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