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Out at sea, and without a good idea of your bearings? Need to double-check your speed and track en route to the islands? A GPS device can tell you everything you need to know about where you are, and where you’re going.
Affordability, compactness, and ease of use make a GPS unit one of the most valuable assets any navigator can have with them on the water, and it’s just as good off the water, too.
A global network ensures service across the globe day or night, and the newest generation of devices comes with features that make navigation quite user-friendly.
No More Excuses for Getting Lost at Sea
2nd generation GPS technology puts as much as 10-foot accuracy in the palm of your hand, or smaller. Ranging from $100 to well over $1000, today’s GPS receivers can do more than just give numerical coordinates. Some models come with built-in mapping software for land travel that can give you directions to a destination, providing information about attractions along the way. 
Boaters can benefit immensely from the ability to figure out their bearing and course instantly. Many products come with nautical software, allowing users access to fully detailed charts and the ability to plot courses.
The information can be downloaded from a computer or stored in cards that plug directly into the receiver. 
Many manufacturers, such as Magellan, Garmin, Raymarine, and Eagle Electronics offer combo units that will tell you where you are, how to chart a course, and where the fish are beneath you.
The GPS’ ease of use makes it a worthwhile accessory for any cruising enthusiast, and at the very least is a good choice as a back-up navigational aid to your sextant.
Like a Man-made Orion Up in the Sky
Orbiting Earth at an altitude of 12,000 miles is a constellation of 27 GPS satellites originally launched by the U.S. military. Today those satellites handle all the governmental and commercial global positioning needs of the planet, with European and Russian systems in development. The network is such that at any given time anywhere on Earth there are at least four satellites visible to a GPS receiver.
The receiver functions by using a formula of Three-Dimensional Trilateration to pinpoint your exact location on Earth. First, it involves the unit detecting a minimum of three satellites orbiting overhead. It knows, based upon information stored within its memory, where every satellite will be at a given point in time. Then, using high-frequency radio waves, the receiver determines its precise distance from each one of the satellites.
Be Glad it Handles the Geometry for You
Because it knows the relative location of each satellite above Earth and the exact distances from itself to the individual satellites, the receiver can figure out a precise point where it must be, using trilateration.
It figures its distance from the first satellite is a point somewhere on the surface of an artificial sphere with the satellite at its center. The distance to satellite two is a point on another artificial sphere that intersects the first sphere. And the same goes for the third satellite until essentially the receiver has three virtual spheres out in space that overlap each other. 
There are two points at which all three spheres intersect. From here the receiver realizes one of those points is somewhere out in space and determines the other point is its own exact location on Earth.
The mathematics are a bit complex, and the explanation difficult to visualize, but just remember that the receiver does all of this for you within a matter of seconds.
Once your position has been determined, you can figure out your course, speed, distance to destination, and sunrise/sunset information.
If and when the GPS receiver is communicating with four satellites simultaneously, it is able to also determine its altitude above sea level. The more satellites it can see, the better the receiver can ascertain its exact position. Atmospheric conditions and interference, however, can affect the transmission of radio signals and minutely decrease GPS accuracy.
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