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.
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.
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..
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.
The location of a mobile phone can be determined using client software installed on the handset. This technique determines the location of the handset by putting its location by cell identification, signal strengths of the home and neighboring cells, which is continuously sent to the carrier.
GPS devices receive satellite and microwave signals and use calculations to determine a location and to track speed and movement. This means that you can not only find out where an object with a GPS tracker is currently, you can also find out where it's been and how fast it's moving, which can be useful information.
GPS-enabled smartphones are typically accurate to within a 4.9 m (16 ft.) radius under open sky (view source at ION.org). However, their accuracy worsens near buildings, bridges, and trees. High-end users boost GPS accuracy with dual-frequency receivers and/or augmentation systems.
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.
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: TeleNavViaMoto 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
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