Theglobal 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 one 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.
Fast Fact
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.
Fast Fact
Tracking
GPS technology is used to track animals as they migrate. Animals, from humpback whales (Megaptera novaeangliae) to arctic terns (Sterna paradisaea) to grizzly bears (Ursus arctos horribilis), 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.
