Speed recorded by GPS differs from old-fashioned pitometer recordings.
Some technology just doesn't work well on the water. For instance, if you get far enough out on your boat, your cell phone will stop working as it loses reception from any land-based cell tower. But with GPS technology, you don't have to worry about getting too far out. As long as there's a clear line of sight between your GPS unit and the skies above -- which there always is on the open water -- then you can get a measurement of your speed.
Background
The GPS, overseen by the U.S. Air Force, is comprised of about 24 active satellites that are spread out and orbit the earth twice daily. And from the government's point of view, the use of GPS by boaters and ship captains is not only important for navigation but also for knowing their speeds. Having a constant speed reading from a GPS device helps maritime professionals, especially, to maximize fuel and plan for a more efficient journey.
Needs
The signals from the GPS satellites are so powerful that they have no trouble traveling the roughly 12,000-mile journey to Earth, where they're picked up by GPS receivers. So there's no danger of being out of range, even when out on the water. All that needs to happen is for the GPS device to receive signal from at least three GPS satellites, which will enable the device to calculate its latitude and longitude as well as the geographical speed at which it -- and the boat -- is traveling.
Devices
The companies Garmin and Magellan, two manufacturers out of the major GPS makers, both sell GPS devices designed for boating. TomTom, another manufacturer, doesn't offer GPS devices for maritime navigation, as of June 2013. These devices come installed with maps that include major bodies of water, enabling you to plot a course and navigate accurately. However, even with a GPS that doesn't have marine-specific maps, you should still be able to get a speed reading, just minus the ability to pinpoint your location on a map.
Disambiguation
It's also important to note that, depending on the type of GPS you're using, the unit of measurement for speed readings might be fixed or adjustable. For example, a typical GPS device meant primarily for use in a vehicle on the road in North America will measure in terms of miles per hour. However, a device that's geared toward boating will either be fixed or adjustable to measuring in terms of nautical miles per hour -- or knots per hour.
Tags: fixed adjustable, miles hour, speed reading
