Global Positioning System
The Global Positioning System (GPS) consists of a group of 24 satellites that orbit Earth once every 12 hours. Designed and controlled by the United States military, this system is widely used by civilians to determine their exact longitude, latitude, and altitude. Any person that owns a relatively inexpensive hand-held GPS receiver can access this position information from any location on Earth. This device should be operated outdoors, and it may perform poorly in heavily wooded areas or dense cities.
A GPS receiver weighs about 8 ounces and measures about 6 inches by 2 inches. It contains a small screen, several buttons, and a small antenna. When turned on, a GPS receiver must communictate with, or "lock on" to any 4 satellites that are located above Earth's horizon. At any given time there are usually between 6 and 9 satellites within this "field of view" orbiting Earth at an altitude of about 11,000 miles. Once the GPS receiver has locked on to 4 satellites, a person's position information usually can be determined to within about 100 feet. Because satellite signals travel at the speed of light, this information is received almost immediately. Most GPS receivers are able to store position information and track movements as they happen. Because of this "real time" feature, GPS systems are increasingly being used as navigation tools in automobiles. These sophisticated navigation systems also contain road maps that are preprogrammed into the unit. This feature allows the user to actually track his or her progress on a road map or similar diagram.
The accuracy of GPS systems is largely dependent on the atomic clocks located onboard each of the 24 orbiting satellites. These clocks establish a precise time base that allows position information to be accurately determined, despite the constant movement of the satellites and Earth. The receiving devices are programmed with information about each of the 24 satellites' locations at any given time. GPS receivers "look up" the position of each satellite and determine how long it took for the signal to reach the receiver. The satellite signals arrive at the GPS receiver at slightly different times because some satellites are further away than others. From this information, exact position can be determined. Military receivers are accurate to within 66 feet while commercial devices are accurate to within 330 feet.
You can find out more about the Global Positioning System (GPS) from the
National Air and Space Museum