Boat Maintenance

Making the Most of GPS


The GPS unit on your dash lets you navigate a constellation — not one of stars, but one made up of 24 satellites orbiting the Earth. The unit receives information from the Global Positioning System, operated by the U.S. Air Force, and converts the information received to a specific location, accurate to within a few yards. The GPS system lies at the heart of the Electronic Chart Display and Information System that will, one day, replace paper charts.

Twenty-four satellites, orbiting the Earth at a constant speed over a constant location — called a geo-synchronus orbit — make up the “constellation” at the heart of the Global Positioning System.


Handheld GPS Units

Regardless of the type of GPS receiver, read the instructions before you use it. Handheld, battery-powered GPS units only require that you turn the unit “On.” You don’t have to “localize” the unit. Handheld GPS receivers and GPS receivers with internal antennas are intended for use open boats or use on the flybridge of larger boats.

Mount a dash-mounted GPS receiver where you can see it from your boat’s steering station. Then, all you need to do is install the bracket – usually little more than drilling a pair of holes to mount the bracket and bolting it in place – and mount the unit into the bracket.

Once the receiver is in place, connect the red wire from the unit to a positive lead from beneath your dash and the black wire from the unit to the negative (or ground) wire. The red wire has an inline fuse holder. Install the fuse that comes with the GPS unit into the fuse holder.

If GPS has a built-in antenna, it’s ready to turn on and use.

GPS Antennas

GPS antennas aren’t like radar. If your GPS needs an antenna, it doesn’t need to be as high in the boat as possible. A good location? Mount the antenna on the stern rail or on the frame for your T-top. The antenna cable uses an “F” connector much like the one cable TV installers use to connect your cable system to your television. Plug the cable into the antenna and tighten the connector with a wrench. Route the cable to the GPS receiver, plug it in and tighten the connector and you are ready to turn the receiver on.

In the instructions for some GPS receivers, you’ll see that you are supposed to initialize the unit. When tested at the manufacturer, the receiver was turned tested and shut down. It then traveled as much as a thousand miles to the store where you purchased it. When you turn it on, it will be “confused” and you will have to help it get its bearings. This may require you enter your approximate location – it may have a list of nearby cities or it may ask you for your approximate latitude and longitude, within a degree or so.

After initializing

When you turn the GPS receiver on, it will spend at least 30 seconds acquiring data from the Global Positioning System satellite constellation. Because the satellites transmit in 30 second bursts, satellite acquisition may require up to a minute-and-a-half or longer. When it finally gets a fix on your position, it will display your latitude and longitude or – if you have a chartplotter-type GPS, it will show either show your latitude and longitude or a chart with your current position, if you have the local chart cartridge installed.

All you need do then is press the power button and wait for the receiver to display your position.

Electronic charts, like those in your chartplotter, are becoming more prevalent every year. They are not, however, the only charting system you should have on your boat. Paper charts should be kept aboard as “Get Home” charts, should your chartplotter suffer a catastrophic failure or the Global Positioning System become unavailable for national security reasons. Remember, a compass and charts were all that sailors had for centuries.

After you have your current position, you can do the rest of your navigation with a chart and a pencil – or, you can enter the latitude and longitude of a specific location into the receiver’s memory. These stored “waypoints” can then be destinations that form the basis of an entire route. If you record the location of a favorite and productive fishing location in the GPS receiver’s memory, ten years from now, the receiver will return you to within 15 feet of that same spot. A number designates each waypoint; you can add a name to each waypoint as well.

You can also enter waypoints to mark places you want to avoid, such as shoals, rocks and jetties.


Because the method of marking waypoints varies with each make and model of GPS receiver, reading the receiver’s instructions becomes essential.


Another way you can use waypoints is to build a route, using a paper chart. If, for example, you’re going from the yacht club at Galveston to a fuel dock in Sabine Pass, Texas, you can find the approximate location of the “G” buoy that marks the intersection with the Houston Ship Channel on the chart and record that location as a waypoint in your GPS.

You may then enter the location of each of the buoys in the south side of the ship channel in your GPS as a waypoint. The south side of the channel is best, because there’s good water there, should you need to leave the channel to allow a ship to pass and because the north side of the channel is a ship anchorage area. You can then create a series of waypoints all the way to Sabine Pass.

Once you enter all the waypoints you wish to pass, including your destination, you can create a connect-the-waypoints “route,” following the instructions for your make and model of GPS, and tell your GPS to save that route.

When you begin your trip, your GPS will identify your location at the yacht club. You then tell it to recall the route to Sabine Pass. As you see the “G” buoy and make your turn, the GPS will lock onto the next buoy in the Houston Ship channel. The GPS will continue to show your distance and course to the next waypoint in the route, until you reach your destination at the fuel dock in Sabine.


When GPS first became publically available, in the 1990s, its accuracy was about 600 feet – 1/10 of a nautical mile. Since that time, changes have increased its accuracy to about 15 feet with a single antenna. Greater accuracy is possible, with a system called “Differential GPS,” that requires a special GPS receiver and a boat large enough to mount two antennas, 45 feet from each other.

Turning the receiver on is simple. Learning that, if you push a button, you will know the bearing and distance to the next waypoint is straightforward. Creating waypoints and a route will make you wonder what the big deal was. If you follow the instructions in your GPS receiver or chartplotter manual, you’ll likely find that you can interface all of your instruments, from fish finder to radio, with your GPS system. Depending on the GPS and your autopilot, you can even interface those, but only for open water, although you’ll still need to stand a watch. In the close quarters of a channel or a river, you should disengage your autopilot; you do not want to end up as the hood ornament for a larger vessel.


Next Week: Buying a Used Boat

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