How to Use a GPS on the Water

Understand how they work. GPS satellites are operated by the U.S. Air Force. Twenty-four of them orbit the earth every 12 hours. They transmit their signals to equipment on the ground. GPS devices are receivers only and need an unobstructed view of the sky, so they don't always work well in forests, canyons or buildings, in contrast to the sea.

Their operation depends on the very accurate atomic clocks on board each satellite. Each transmission indicates a satellite's location and local time. Because the signals arrive in your hand at the speed of light, they arrive at slightly different times.

Your GPS receiver will lock on to the transmissions of at least four satellites. By analyzing the time differences it will place you on the earth in three dimensions. It will tell you in which direction you are heading and how fast you are traveling. It's particularly useful for ocean travel, because it looks at your absolute position on the globe adjusted for the effects of current and wind, errors that are not readily apparent when using a compass, a map, a watch and dead reckoning.

Decide on what features you need within your budget limits. Many GPS receivers allow you to work off downloadable maps or marine charts. The highest-end models today can actually tie your GPS into an autopilot and your boat's radar while communicating with a computer. It's not exactly like putting your boat under the helm of a robotic first mate, but it isn't far off.

If you can, buy a new one. Most hand held receivers today are accurate to within 10 to 20 yards. Until 2000, a timing error was introduced in all non-military GPS devices so that you could only be accurate to within the diameter of a football field, a real limitation in trying to find the entrance to a marina breakwater. Battery life and screen quality have also improved significantly. If you inherited one of these older receivers, it's time to replace it.

Many of us have a natural tendency to try to figure things out for ourselves without looking at a manual. Resist that temptation here. Modern GPS receivers are extremely sophisticated. Since you'll be depending on this for your exact position on the water, it's best to not introduce user error.

As soon as you figure out how to operate your unit's buttons, start to enter essential way points, such as the location of your slip, the entrance to the marina, your favorite fishing ground and anchorage. You can also down load popular GPS destinations at websites like TravelbyGPS.com. Start to track your progress to and from these points, comparing them with your plotting on a chart and compass to see how you fare next to your little helper.