# What Is Trilateration?

How GPS devices use mathematics to determine positioning

Trilateration is a mathematical technique used by a global positioning system (GPS) device to determine user position, speed, and elevation. By constantly receiving and analyzing radio signals from multiple GPS satellites and applying the geometry of circles, spheres, and triangles, a GPS device can calculate the precise distance or range to each satellite being tracked.

## How Trilateration Works

Trilateration is a sophisticated version of triangulation, though it does not use the measurement of angles in its calculations. Data from a single satellite provides a general location of a point within a large circular area on the Earth's surface. Adding data from a second satellite allows the GPS to narrow the specific location of that point down to a region where the two areas of satellite data overlap. Adding data from a third satellite provides an accurate position of the point on the Earth's surface.

All the GPS devices require three satellites for an accurate calculation of position. Data from a fourth satellite—or even more than four satellites—further enhance the precision of the point's location, and also allows factors such as elevation or, in the case of aircraft, altitude to also be calculated. GPS receivers routinely track four to seven satellites simultaneously and use trilateration to analyze the information.

## GPS Satellites

The U.S. Department of Defense maintains the 24 satellites that relay data worldwide. Your GPS device can remain in touch with at least four satellites no matter where you are on earth, even in wooded areas or major metropolises with tall buildings. Each satellite orbits the earth twice a day, regularly sending signals to Earth from an altitude of about 12,500 miles. Satellites run on solar energy and have backup batteries.

## When a GPS Fails

When a GPS navigator receives insufficient satellite data because it's not able to track enough satellites, trilateration fails. Obstructions such as large buildings or mountains may also block weak satellite signals and prevent the accurate calculation of location. The GPS device will alert the user in some way that that it is unable to provide correct position information.

Satellites may also fail temporarily. Signals may move too slowly due to factors in the troposphere and ionosphere, for example. Signals might also ping off certain formations and structures on the Earth, causing a trilateration error.

## Government GPS Technologies and Systems

GPS was introduced in 1978 with the launch of the first global positioning satellite. It was controlled and used solely by the U.S. government until the 1980s. The full fleet of 24 active satellites controlled by the U.S. did not come into use until 1994.

A GPS device does not send data to satellites. GPS devices, such as smartphones equipped with the technology, can also use telephonic systems, such as cell phone towers and networks, and internet connections to further enhance location accuracy. When using these latter two systems, a GPS device may send data out to these systems.

Because the GPS satellite system is owned by the U.S. government, and it can selectively deny or limit access to the network, other countries have developed their own GPS satellite networks. These include:

• China's BeiDou Navigation Satellite System
• Russia's Global Navigation Satellite System (GLONASS)
• The European Union's Galileo positioning system
• India's Indian Regional Navigation Satellite System (IRNSS), also known as NAVIC
FAQ
• How do I turn off GPS on my phone?

To disable the GPS on your device, turn off location services. When you turn off location services, you may not be able to use certain apps. You cannot turn off Enhanced 911, the feature in all phones that emergency dispatchers use to find people in case of an accident.

• How do I find my GPS coordinates on my phone?

In Google Maps, set a location marker to see the GPS coordinates on the map. Unfortunately, Apple Maps doesn't show GPS coordinates, but there are GPS hiking apps that provide location information.

• What is a GPS tracker?

GPS trackers are devices that track and report location data. Businesses use them to keep track of company vehicles, and there are commercial ones available for parents to track teenage drivers. Some GPS trackers even give real-time alerts when a driver speeds or deviates from a specific area.

• What is assisted GPS?

Assisted GPS, also known as AGPS, collects location information from cell towers (instead of satellites) to help improve the accuracy of GPS on mobile devices. Buildings can obstruct GPS satellite signals, so AGPS is sometimes necessary to determine your location.