Buzz
The iPhone integrates GPS, WiFi, and cellular towers to precisely locate your position and offer navigational guidance.
AP Photo/Kin CheungPicture yourself on your way to a job interview, only to realize you're lost. Your immediate instinct might be to contact the business interviewing you for help with directions. However, without knowing your exact location, obtaining directions becomes difficult.
But imagine using your phone for a different purpose — to accurately determine your whereabouts and receive step-by-step directions to your destination. Modern phones equipped with global positioning system (GPS) receivers can do just that. With appropriate software or service packages, they can locate you, guide you to your destination, and even offer details about nearby businesses.
In this article, we will go over the fundamentals of how mobile phones and GPS receivers function. Additionally, we’ll examine how phones merge these two technologies to enhance user experience.
Understanding Cell Phones
A cell phone functions as an advanced two-way radio. A network of towers and base stations, organized into cells, transmit and receive radio waves. Cell phones are equipped with low-power transmitters that allow them to communicate with the nearest tower.
As you move, you shift from one cell to another, with base stations continuously monitoring the strength of your phone's signal. When nearing the edge of a cell, your signal strength weakens, while the base station of the approaching cell detects a rise in signal strength. This signal transfer happens seamlessly as you transition from one cell to the next.
In remote areas, towers may be spaced too far apart to offer a reliable signal. Even in areas with a high density of towers, obstacles like mountains and tall buildings can disrupt signal transmission. Inside buildings, especially in elevators, it’s common for signals to weaken or become unclear.
Even without a GPS receiver, your cell phone can estimate your location. A computer can calculate your position based on measurements of your signal, such as the strength of the received signal from surrounding towers.
- The direction from which the signal approaches the cell towers
- The time it takes for the signal to reach multiple towers
- The signal strength when it arrives at the towers
Because obstacles like trees and buildings can influence the time it takes for your signal to reach a tower, this method is often less precise than GPS-based measurements.
Understanding GPS Receivers
Similar to a cell phone, a GPS receiver operates using radio waves. However, instead of communicating with ground-based towers, it connects with satellites orbiting the Earth. Currently, 27 GPS satellites are in orbit — 24 are actively in use, while 3 serve as backups in case any satellite fails.
To accurately determine your location, a GPS receiver must calculate the following:
- The positions of at least three satellites above you
- Your location relative to those satellites
The receiver then applies trilateration to pinpoint your exact position. Essentially, it creates a sphere around each of the three satellites it detects. These three spheres intersect at two points — one in space and one on the Earth's surface. The intersection point on the ground is where you are located.
For a GPS receiver to function properly, it must have an unobstructed view of the satellites. Therefore, thick tree cover and tall buildings can prevent the receiver from acquiring an accurate location fix.
GPS receivers and cell phones share many similarities, and both are widely used. In the following section, we will explore some of the features of GPS-enabled cell phones.
Some GPS-enabled phones utilize wireless-assisted GPS to determine the user's location. In these systems, the phone combines signals from orbiting GPS satellites with information from the phone’s cell signal. Often referred to as enhanced GPS, wireless-assisted GPS can typically determine a user's location faster than a GPS-only receiver. It can also function in areas where traditional GPS may struggle, such as inside buildings, under dense tree cover, and in urban environments. Learn more about wireless-assisted GPS from Lucent Technologies.
GPS Phones
Almost all modern cell phones sold in the U.S. come with built-in GPS capabilities. For those that don't, they can connect to a server that uses techniques described earlier to analyze the phone's signals and pinpoint its location. This feature enables phones to transmit a caller's location to a Public Safety Answering Point (PSAP) when dialing 911. However, that's usually the extent of their GPS functionality.
Some phones, however, are equipped with a full GPS receiver or can connect to one via wires or a Bluetooth connection. These GPS-enabled devices can interpret programming languages such as Java, offering services like turn-by-turn navigation or information on nearby businesses and attractions. Some phones can even function as tracking devices. To take advantage of these features, you need to have:
- A GPS-enabled phone or a compatible GPS receiver
- A calling plan that allows for the transmission of maps and GPS data
- A service plan or software that delivers actual maps and directions or supplies information on the phone's location
Some common applications for GPS phones include:
- Location Tracking: Some employers track their employees' whereabouts using GPS-enabled phones, while certain businesses offer location-tracking services for such phones. For instance, the Wherifone locator phone provides precise GPS coordinates and can also dial emergency numbers. Parents and guardians can monitor the phone's location either via phone or online and receive alerts if the device exits a designated "safe zone." The Dog @ Watch by Wearable Environmental Information Networks of Japan is another example, a GPS watch phone designed specifically for children.
- Turn-by-Turn Directions: GPS-enabled phones equipped with screens can display turn-by-turn navigation and announce directions aloud through the phone's speaker. Typically, these services require a monthly subscription and rely on a database of maps to deliver directions. The accuracy of these services depends on the quality of the map database — outdated maps may lead to wrong directions. Notable turn-by-turn navigation services include: TeleNav, ViaMoto, MapQuest Find Me, smart2Go (which requires a separate Bluetooth GPS receiver and a memory card), and Destinator SP, a smartphone-compatible software package.
- Outdoor Location Services: Trimble Outdoors provides location-based services and maps tailored for outdoor activities such as hiking, mountain biking, and geocaching.
- Other Location-Based Services: Various companies aim to deliver targeted news, discounts, advertisements, and other content to mobile phone users based on their current location.
Some additional GPS-enabled phones include:
- Mio A701 Smart Phone
- Various Motorola models available through Sprint/Nextel
Not everyone uses GPS phones for good purposes. For example, law enforcement was able to quickly trace a teenager's location in Tennessee after he made several prank 911 calls from his GPS-enabled phone [ref]. In another case, a California man used a GPS phone to stalk his ex-girlfriend, hiding it under the hood of her car [ref].
The Federal Communications Commission's (FCC) Enhanced 911 (E911) program mandates that all cell phones transmit both their phone number and location when dialing 911. The FCC set a deadline of 2005 for phone manufacturers, service providers, and PSAPs to comply with this regulation. This is one of the reasons that many newer phones include GPS receivers, even if they can't provide turn-by-turn navigation.
While the E911 program helps ensure assistance during emergencies, privacy concerns have been raised. Additionally, many PSAPs still lack the capability to receive location data from mobile phones.
Learn more about E911 from the FCC or Consumer Reports.
