"How GPS Phones Work"

Imagine driving to a job interview and realizing that you're lost. Your first impulse would probably be to call the business that's interviewing you and ask for directions. But if you're not sure where you are, getting directions can be tricky.

But suppose you use your phone for another purpose -- to figure out exactly where you are and to get turn-by-turn directions to where you're going. New phones that include global positioning system (GPS) receivers can do exactly that. With the right software or service package, they can pinpoint your location, give directions to your destination and provide information about nearby businesses.

Cell Phone Basics

A cell phone is basically a sophisticated two-way radio. Towers and base stations, arranged into a network of cells, send and receive radio signals. Cell phones contain low-power transmitters that let them communicate with the nearest tower.As you travel, you move from one cell to another, and the base stations monitor the strength of your phone's signal. As you move toward the edge of one cell, your signal strength diminishes. At the same time, the base station in the cell you are approaching notices the strength of your signal increasing. As you move from cell to cell, the towers transfer your signal from one to the next.As you travel, the signal is passed from cell to cell.

In remote locations, towers may be so far apart that they can't provide a consistent signal. Even when towers are plentiful, mountains and tall buildings can interrupt their signals. Sometimes people have a hard time getting clear signals inside buildings, especially in elevators.Even without a GPS receiver, your cell phone can provide information about your location. A computer can determine your location based on measurements of your signal, such as:

* Its angle of approach to the cell towers
* How long it takes the signal to travel to multiple towers
* The strength of your signal when it reaches the towers

Since obstacles like trees and buildings can affect how long it takes your signal to travel to a tower, this method is often less accurate than a GPS measurement.

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.

Wireless-Assisted GPS

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 Phones

Nearly all new cell phones sold in America have some GPS receiving capability built in. Those that don't can connect to a server that uses techniques discussed in the last section to analyze their signals and determine their location. This allows the phones to transmit a person's location to a Public Safety Answering Point (PSAP) after dialing 911. But that's all a lot of phones can do with their GPS.However, some phones have a complete GPS receiver located in the phone or can connect to one with wires or through a Bluetooth connection. These GPS-enabled phones can understand programming languages like Java and can provide turn-by-turn directions or information about nearby businesses and attractions. Others can work like a tracking device. To use any of these features, you must have:

* A GPS-enabled phone or a compatible GPS receiver
* A calling plan that supports transmission of maps and GPS data
* A service plan or software that provides the actual maps and directions or provides information about the phone's location

Source: HowStuffWorks.Com