How GPS Works
The Global Positioning System--the technological underpinning of all GPS devices--is composed of orbiting satellites (which transmit time and position signals) and GPS receivers (land-, sea-, or air-based devices such as the handheld or car-mounted GPS units that consumers use) that mathematically calculate their current location based on data from the satellites. The system works nonstop, anywhere on the globe.
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 driving through a deep tunnel of 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. Under those conditions, a GPS device can give you a fairly accurate reading of your current altitude, which can be 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 had relied on a 300-year-old technology, the sextant.
Personal and portable GPS units appeared in the early 1990s, when designers reduced GPS electronics to one or two chips; miniaturized memory meant that the receiver and map data could reside 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 models for your car. Many in-car models have a pedestrian mode that lets them double as handhelds when you're walking city streets. But there are many variations on the theme: A lot of today's smart phones include some sort of GPS capabilities. Companies also make models for boats, plus units for motorcycles, bicycles, and runners, as well as GPS PDAs, GPS sport watches, GPS walkie-talkies, and even units for tracking the movement of a hunting dog in the field.