The global positioning system (GPS) is a network of satellites and receiving devices used to determine the location of something on Earth. Some GPS receivers are so accurate they can establish their location within 1 centimeter (0.4 inches). GPS receivers provide location in latitude, longitude, and altitude. They also provide the accurate time.

GPS includes 24 satellites that circle Earth in precise orbits. Each satellite makes a full orbit of Earth every 12 hours. These satellites are constantly sending out radio signals.

GPS receivers are programmed to receive information about where each satellite is at any given moment. A GPS receiver determines its own location by measuring the time it takes for a signal to arrive at its location from at least four satellites. Because radio waves travel at a constant speed, the receiver can use the time measurements to calculate its distance from each satellite.

Using multiple satellites makes the GPS data more accurate. If a GPS receiver calculates its distance from only one satellite, it could be that exact distance from the satellite in any direction. Think of the satellite as a flashlight. When you shine it on the ground, you get a circle of light. With one satellite, the GPS receiver could be anywhere in that circle of light. With two more satellites, there are two more circles. These three circles intersect, or cross, in only one place. That is the location of the GPS receiver. This method of determining location is called trilateration.

Aircraft, ships, submarines, trains, and the space shuttle all use GPS to navigate. Many people use receivers when driving cars. The GPS receiver plots the car's constantly-changing location on an electronic map. The map provides directions to the person's destination. Both the location and the vehicle are plotted using satellite data. Some hikers use GPS to help them find their way, especially when they are not on marked trails.

Sometimes there are obstacles to getting a clear GPS signal. Gravity can pull the GPS satellites slightly out of orbit. Parts of Earth's atmosphere sometimes distort the satellite radio signals. Trees, buildings, and other structures can also block the radio waves. GPS control and monitoring stations around the world track the satellites and constantly monitor their signals. They then calculate corrections that are broadcast to GPS receivers. These corrections make GPS much more accurate.

The original GPS system began as a project of the U.S. military. The first experimental satellite was launched in 1978. By 1994, a full 24 GPS satellites were orbiting Earth. At first, GPS available for civilian, or nonmilitary, use was not very accurate. It would only locate a GPS receiver within about 300 meters (1,000 feet). Today, an accurate signal is free and available to anyone with a GPS receiver.

GPS is American. Russia has its own version of a GPS system, called GLONASS (Global Orbiting Navigation Satellite System). China and the European Union are currently creating systems of their own.

GPS
GPS receivers can be handheld or fitted to a vehicle's instrument panel, like a car or airplane dashboard.

Tracking
GPS technology is used to track animals as they migrate. Animals, from humpback whales to arctic terns to grizzly bears, are fitted with GPS receivers. These receivers let researchers know where that animal is as it moves. Biologists can track animals as they migrate to another habitat for a season, move in search of food or shelter, or are forced out of their ecosystem by human activity such as construction.

Early Warning
Scientists are using GPS to quickly determine the size of earthquakes. First, scientists plant GPS receivers in the ground. By measuring how far these GPS receivers move, scientists can sometimes measure the strength of an earthquake in as little as 15 minutes.

Knowing the size of an earthquake is central to predicting whether it can produce dangerous ocean waves known as a tsunamis. By the time a tsunami reaches land, it can be a huge, destructive wall of water. Early warning is crucial in saving lives because tsunami waves move faster than people can run.

aircraft
Noun

vehicle able to travel and operate above the ground.

Noun

the distance above sea level.

arctic tern
Noun

small bird that migrates from the Arctic to the Antarctic.

Noun

layers of gases surrounding a planet or other celestial body.

biologist
Noun

scientist who studies living organisms.

broadcast
Verb

to transmit signals, especially for radio or television media.

calculate
Verb

to reach a conclusion by mathematical or logical methods.

civilian
Noun

person who is not in the military.

construction
Noun

arrangement of different parts.

data
Plural Noun

(singular: datum) information collected during a scientific study.

destructive
Adjective

harmful.

determine
Verb

to decide.

distort
Verb

to deform or misrepresent.

Noun

our planet, the third from the Sun. The Earth is the only place in the known universe that supports life.

earthquake
Noun

the sudden shaking of Earth's crust caused by the release of energy along fault lines or from volcanic activity.

Noun

community and interactions of living and nonliving things in an area.

Global Positioning System (GPS)
Noun

system of satellites and receiving devices used to determine the location of something on Earth.

GLONASS
Noun

(Global Orbiting Navigation Satellite System) Russian GPS technology.

GPS receiver
Noun

device that gets radio signals from satellites in orbit above Earth in order to calculate a precise location.

gravity
Noun

physical force by which objects attract, or pull toward, each other.

grizzly bear
Noun

large mammal native to North America.

Noun

environment where an organism lives throughout the year or for shorter periods of time.

hike
Verb

to walk a long distance.

humpback whale
Noun

marine mammal native to all of Earth's oceans.

intersect
Verb

to cross paths with.

Noun

distance north or south of the Equator, measured in degrees.

light wave
Noun

electromagnetic radiation visible to the human eye. Also called visible light.

Noun

position of a particular point on the surface of the Earth.

Noun

distance east or west of the prime meridian, measured in degrees.

Noun

symbolic representation of selected characteristics of a place, usually drawn on a flat surface.

migrate
Verb

to move from one place or activity to another.

Noun

armed forces.

monitor
Verb

to observe and record behavior or data.

navigate
Verb

to plan and direct the course of a journey.

network
Noun

series of links along which movement or communication can take place.

obstacle
Noun

something that slows or stops progress.

Verb

to move in a circular pattern around a more massive object.

plot
Verb

to form a path based on calculations.

precise
Adjective

exact.

predict
Verb

to know the outcome of a situation in advance.

radio wave
Noun

electromagnetic wave with a wavelength between 1 millimeter and 30,000 meters, or a frequency between 10 kilohertz and 300,000 megahertz.

satellite
Noun

object that orbits around something else. Satellites can be natural, like moons, or made by people.

space shuttle
Noun

vehicle used to transport astronauts and instruments to and from Earth.

sphere
Noun

round object.

submarine
Noun

vehicle that can travel underwater.

tracker
Noun

device, usually attached to an animal, that follows its movements.

train
Noun

connected railroad cars pulled by a single engine.

tsunami
Noun

ocean waves triggered by an earthquake, volcano, or other movement of the ocean floor.