You do not have to subscribe to anything. GPS is provided by the United States Department of Defense, courtesy of the the United States taxpayers. It is a complex system. Basically, the GPS receiver knows the precise time and the position of each satellite in view based on information transmitted by the GPS sat. Based on this information and intersecting spheres (whose radius is the slant range to the GPS sat and whose center is the known position), a GPS receiver can determine the position (Latitude, Longitue (and possibly altitude)). If I remember correctly, the www.howstuffworks.com articles on GPS are pretty good.

GPS or Global Positioning Systems are computer systems that consist of 24 orbiting satellites around the globe to provide exact longitudinal and latitudinal position of the user. GPS units were originally designed by the Department of Defense for military purposes and are now used on the consumer level for day to day navigation from bike trails, to GPS vehicle tracking or shopping in a new city. Some GPS units now sell for less than $100.00 and if you have one, you will never be lost again. In fact most GPS receivers can pinpoint your location on the globe within a few feet. GPS units vary in price based on their quality and capabilities, so when shopping for one it is important to know what you are planning to use it for. GPS systems are mostly used by consumers for two purposes, positioning and navigation. GPS units are placed on boats so boaters can have an idea of where they are in comparison to the shoreline. Motorcycle GPS are now becoming more popular to find great ridding locations. Even golf players use GPS to get an edge on the course. In cars they are more often equipped to provide directions to the driver from two different points. Some models come equipped with voice capabilities so that drivers don't even have to take their eyes off the road. Such as the Garmin nüvi 350 3.5-Inch Portable GPS Navigator for cars. Many handheld models are sold to outdoor enthusiasts for camping, hunting and hiking. These handheld models also range in price based on their capabilities and if the display comes in color or not. One of the top picks is the Garmin Forerunner 305 Wrist-Mounted GPS Fitness. Most GPS units will also provide you with some standard and useful information… most even comes with MP3 players, eBooks an more! ·An Odometer (Distance you have traveled) ·Time you have been traveling ·A Speedometer (How fast you are currently traveling) ·Average speed for the journey ·A map of you trail so far ·E.T.A (Estimated time of arrival) GPS units use four satellites orbiting the globe to triangulate your position on earth and can do so with extreme accuracy. If you are constantly lost behind the wheel or spend a lot of time in the outdoors purchasing a GPS unit might be the right move for you. http://www.allinonegps.com/

They receive signals from satellites which help you to determine where you are located.