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

    Photograph by Scott S. Warren

    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.

    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.

    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 triangulation.

    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.

  • Term Part of Speech Definition Encyclopedic Entry
    aircraft Noun

    vehicle able to travel and operate above the ground.

    altitude Noun

    the distance above sea level.

    Encyclopedic Entry: altitude
    arctic tern Noun

    small bird that migrates from the Arctic to the Antarctic.

    atmosphere Noun

    layers of gases surrounding a planet or other celestial body.

    Encyclopedic Entry: atmosphere
    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


    determine Verb

    to decide.

    distort Verb

    to deform or misrepresent.

    Earth Noun

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

    Encyclopedic Entry: Earth
    earthquake Noun

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

    ecosystem Noun

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

    Encyclopedic Entry: ecosystem
    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.

    habitat Noun

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

    Encyclopedic Entry: habitat
    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.

    latitude Noun

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

    Encyclopedic Entry: latitude
    light wave Noun

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

    location Noun

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

    Encyclopedic Entry: location
    longitude Noun

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

    Encyclopedic Entry: longitude
    map Noun

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

    Encyclopedic Entry: map
    migrate Verb

    to move from one place or activity to another.

    military 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.

    orbit Verb

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

    Encyclopedic Entry: orbit
    plot Verb

    to form a path based on calculations.

    precise Adjective


    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.

    triangulation Noun

    method of determining distance or placement of two points by calculating their distance from a third point whose position is known.

    tsunami Noun

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

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