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.

In this article, we'll review the basics of how cell phones and GPS receivers work. Then, we'll explore how phones combine these technologies.

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.

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

Common uses for GPS phones include:

Location Tracking: Some employers use GPS-enabled phones to track their employees' locations, and some business offer location tracking services for GPS-enabled phones. The Wherifone locator phone provides GPS coordinates and can dial emergency phone numbers. Parents and caregivers can track the phone's location by phone or online and can receive notification if it leaves a designated "safe area." Wearable Environmental Information Networks of Japan has also introduced the Dog @ Watch, a GPS watch phone for children.
Turn-by-Turn Directions: GPS-enabled phones with view screens can often display turn-by-turn directions as well as announce them through the phone's speaker. In general, companies that offer these services charge a monthly fee and use a database of maps to provide the directions. The services are only as good as their database -- outdated maps can provide inaccurate directions. Some turn-by-turn direction services include: TeleNav ViaMoto MapQuest Find Me smart2Go, which requires a separate Bluetooth GPS receiver and a memory card Destinator SP, which is a software package for smartphones
Outdoor Location Services: Trimble Outdoors offers maps and location-based services for hiking, mountain biking, geocaching and other outdoor activities.
Other Location-Based Services: Some companies hope to deliver news, coupons, advertisements and other information to cell phone users based on their location.

Some other GPS-enabled phones include:

Mio A701 Smart Phone
SeveralMotorola phones offered through Sprint/Nextel

Follow the links on the next page for lots more information about cell phones, GPS systems and related technology.

Caught in the Act and Enhanced 911

Not everyone who uses a GPS phone does so with the best intentions. Police easily pinpointed the location of a teenager in Tennessee who made multiple prank calls to 911 from a GPS-enabled phone [ref]. Police also arrested a California man who used a GPS phone to stalk his former girlfriend by concealing it under the hood of her car [ref].

The United States Federal Communications Commission's (FCC) Enhanced 911 (E911) program requires that all cell phones transmit their phone number and location when dialing 911. The FCC gave phone manufacturers, service providers and PSAPs until the end of 2005 to comply with this ruling. This is one of the reasons many new telephones have GPS receivers built in, even if they can't provide turn-by-turn directions.

Although the E911 program can help make sure people get help in an emergency, some people have raised privacy concerns about the program. In addition, many PSAPs do not yet have the capability to receive location information from cellular phones.

You can learn more about E911 through the FCC or from Consumer Reports.

How GPS Phones Work

Cell Phone Basics

GPS Receiver Basics

GPS Phones

Frequently Answered Questions

Do cell phones have real GPS?

Lots More Information

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