How GPS Works
To provide an accurate fix of your current location, the GPS device must have direct lines of sight to at least three satellites in the sky. So if you're hiking in a deep canyon of rock or concrete, your GPS device may display inaccurate location information, or fail to identify your location at all. The more satellites a GPS unit sees, the more accurate its reckoning becomes--to within 20 feet of your true position in some instances. Under those conditions, a GPS device can give you a fairly accurate reading of your current altitude, too-not terribly important information if you're in a car or on board a ship, but potentially critical if you're out in the wilds.
Like many technologies we take for granted, GPS began as advanced technology for the U.S. military. The system's precise positioning capabilities gave the armed forces a technological edge over their adversaries (and allies). When released for private and commercial use in the 1980s, GPS proved an immediate boon to ships at sea, which until then relied on a 300-year-old technology, the sextant.
Personal and portable GPS units appeared about a decade ago when designers reduced GPS electronics to one or two chips; miniaturized memory meant that the receiver and map data could be placed in a box that would fit in your hand.
The two most common types of portable GPS units are small handhelds (typically used in the field) and on-dashboard models for your car. Since most on-dash models have internal batteries, they can double as handhelds when you're walking city streets.) But there are many variations on the theme: Models for boats are the next most common, but units designed for motorcycles, bicycles, and runners, as well as GPS/PDAs, GPS sport watches, GPS/walky-talkies, and even units for tracking the movement of a hunting dog in the field, have emerged.
