Some GPS phones use wireless-assisted GPS to determine the user's location. In wireless-assisted systems, the phone uses the orbiting GPS satellites in conjunction with information about the cell phone's signal. Sometimes called enhanced GPS, wireless-assisted GPS can often get a fix on the user's location faster than a GPS-only receiver. Some wireless-assisted systems can work inside buildings, under dense foliage and in city areas where traditional receivers cannot receive signals. You can learn more about wireless-assisted GPS through Lucent Technologies.
GPS Receiver Basics
Like a cell phone, a GPS receiver relies on radio waves. But instead of using towers on the ground, it communicates with satellites that orbit the Earth. There are currently 27 GPS satellites in orbit -- 24 are in active use and 3 act as a backup in case another satellite fails.
In order to determine your location, a GPS receiver has to determine:
- The locations of at least three satellites above you
- Where you are in relation to those satellites
The receiver then uses trilateration to determine your exact location. Basically, it draws a sphere around each of three satellites it can locate. These three spheres intersect in two points -- one is in space, and one is on the ground. The point on the ground at which the three spheres intersect is your location.
A GPS receiver has to have a clear line of sight to the satellite to operate, so dense tree cover and buildings can keep it from getting a fix on your location.
GPS receivers and cell phones have a lot in common, and both are very popular. In the next section, we'll look at some of the features of GPS-enabled cell phones.