Global Positioning System

The Global Positioning System (GPS) is a network of 24 Navstar1 satellites orbiting Earth at 11,000 miles up. Established by the U.S. Defense Department for military applications, access to GPS is now free to all users, including those in other countries.

The system’s positioning and timing data are used for a variety of applications, including air, land, and sea navigation; vehicle and vessel tracking; surveying and mapping; and asset and natural resource management. With military accuracy restrictions lifted in May 2000, the GPS can now pinpoint the exact location of people as they move about with their receivers powered on. This development has ushered in a wave of new commercial applications for GPS.

The first GPS satellite was launched in 1978. The first 10 satellites were developmental satellites. From 1989 to 1993, 23 production satellites were launched. The launch of the twenty-fourth satellite in 1994 completed the $13 billion constellation. The satellites are positioned so that signals from 6 of them can be received nearly 100 percent of the time at any point on earth. The GPS consists of satellites, receivers, and ground control systems.

The satellites transmit signals (1575.42 MHz) that can be detected by GPS receivers on the ground. These receivers can be portable or mounted in ships, planes, or cars to provide exact position information, regardless of weather conditions. They detect, decode, and process GPS satellite signals to give the precise position of the user.

The GPS control or ground segment consists of five unmanned monitor stations located in Hawaii, Kwajalein in the Pacific Ocean, Diego Garcia in the Indian Ocean, Ascension Island in the Atlantic Ocean, and Colorado Springs, Colorado. There is also a master ground station at Falcon Air Force Base in Colorado Springs, Colorado, and four large ground antenna stations that broadcast signals to the satellites. The stations also track and monitor the GPS satellites.

With GPS, signals from several satellites are triangulated to identify the exact position of the user. To triangulate, GPS measures distance using the travel time of a radio message from the satellite to a ground receiver. To measure travel time, GPS uses very accurate clocks in the satellites. Once the distance to a satellite is known, knowledge of the satellite’s location in space is used to complete the calculation.

GPS receivers on the ground have an “almanac” stored in their computer memory that indicates where each satellite will be in the sky at any given time. GPS receivers calculate for ionosphere and atmosphere delays to further tune the position measurement. To make sure both satellite and receiver are synchronized, each satellite has four atomic clocks that keep time to within 3 nanoseconds, or 3 billionths of a second.

For cost savings, the clocks in the ground receivers are not that accurate. To compensate, an extra satellite range measurement is taken. Trigonometry says that if three perfect measurements locate a point in three-dimensional space, then a fourth measurement can eliminate any timing offset. This fourth measurement compensates for the receiver’s imperfect synchronization. The ground unit receives the satellite signals, which travel at the speed of light.

Even at this speed, the signals take a measurable amount of time to reach the receiver. The difference between when the signals are sent and the time they are received, multiplied by the speed of light, enables the receiver to calculate the distance to the satellite. To measure precise latitude, longitude, and altitude, the receiver measures the time it took for the signals from several satellites to get to the receiver.

GPS uses a system of coordinates called the Worldwide Geodetic System 1984 (WGS-84). This is similar to the latitude and longitude lines that are commonly seen on large wall maps used in schools. The WGS-84 system provides a built-in, standardized frame of reference, enabling receivers from any vendor to provide exactly the same positioning information.

The GPS system has amply proven itself in military applications, most notably in Operation Desert Storm where U.S. and allied troops faced a vast, featureless desert. Without a reliable navigation system, sophisticated troop maneuvers could not have been performed. This could have prolonged the operation well beyond the 100 hours it actually took. With GPS, troops were able to go places and maneuver in sandstorms or at night when even the troops who were native to the area could not.

Initially, more than 1000 portable commercial receivers were purchased for their use. The demand was so great that before the end of the conflict, more than 9000 commercial receivers were in use in the Gulf region. They were carried by ground troops and attached to vehicles, helicopters, and aircraft instrument panels. GPS receivers were used in several aircraft, including F-16 fighters, KC-135 tankers, and B-52s. Navy ships used GPS receivers for rendezvous, minesweeping, and aircraft operations.

While the GPS system was developed originally to meet the needs of the military community, new ways to use its capabilities are continually being found, from the exotic to the mundane. Among the former is the use of GPS for wildlife management. Endangered species such as Montana elk and Mojave Desert tortoises have been fitted with tiny GPS receivers to help determine population distribution patterns and possible sources of disease.

In Africa, GPS receivers are used to monitor the migration patterns of large herds for a variety of research purposes. Handheld GPS receivers are now used routinely in field applications that require precise information gathering, including field surveying by utility companies, mapping by oil and gas explorers, and resource planning by timber companies. GPS-equipped balloons are used to monitor holes in the ozone layer over the polar ice caps.

Air quality is being monitored using GPS receivers. Buoys tracking major oil spills transmit data using GPS. Archaeologists and explorers are using the system to mark remote land and ocean sites until they can return with proper equipment and funding. Vehicle tracking is one of the fastest-growing GPS applications. GPS-equipped fleet vehicles, public transportation systems, delivery trucks, and courier services use receivers to monitor their locations at all times.

GPS data are especially useful to consumers when they are linked with digital mapping. Accordingly, some automobile manufacturers are offering moving-map displays guided by GPS receivers as an option on new vehicles. The displays can even be removed and taken into a home to plan a trip. Some GPS-equipped vehicles give directions to drivers on display screens and through synthesized voice instructions.

These features enable drivers to get where they want to go more rapidly and safely than has ever been possible before. GPS receivers are also included in newer mobile phones, and add-on receivers are available for hand-held computers, such as the Palm III.

GPS is also helping save lives. Many police, fire, and emergency medical service units are using GPS receivers to determine the police car, fire truck, or ambulance nearest to an emergency, enabling the quickest possible response in life-ordeath situations. When GPS data are used in conjunction with geographic data collection systems, it is possible to instantaneously arrive at submeter positions together with feature descriptions to compile highly accurate geographic information systems (GIS).

When used by cities and towns, for example, GPS can help in the management of the geographic assets. Some government agencies, academic institutions, and private companies are using GPS to determine the location of a multitude of features, including point features such as pollutant discharges and water supply wells, line features such as roads and streams, and area features such as waste lagoons and property boundaries.

Before GPS, such features had to be located with surveying equipment, aerial photographs, or satellite imagery. With GPS, the precise location of these and other features can be determined with a hand-held GPS receiver.

GPS technology is even being used in conjunction with cellular technology to provide value-added services. With the push of a button on a cellular telephone, automobile drivers and operators of commercial vehicles in some areas can talk to a service provider and simultaneously signal their position, emergency status, or equipment failure information to auto clubs, security services, or central dispatch services.

This is possible with Motorola’s Cellular Positioning and Emergency Messaging Unit, for example, which offers mobile security and tracking to those who drive automobiles and/or operate fleets. The system is designed for sale to systems integrators that configure consumer and commercial systems that operate via cellular telephony.

The Cellular Positioning and Emergency Messaging Unit communicates GPS-determined vehicle position and status, making it suited for use in systems that support roadside assistance providers, home security monitoring firms, cellular carriers, rental car companies, commercial fleet operators, and auto manufacturers seeking a competitive advantage.

As an option, the OnStar system is available for select vehicles manufactured by General Motors, which uses a GPS receiver in conjunction with analog cellular phone technology to provide a variety of travel assistance services, including emergency response. At the push of a button, a cellular call is placed to an OnStar operator. Although digital technology is more advanced, OnStar uses analog cellular because it has the broadest geographic coverage in the United States.

Over 90 percent of the country is covered by the analog system, whereas digital coverage is less than 30 percent. OnStar has worked to “clear” the OnStar emergency button call through all analog cellular phone companies so that it will go through no matter which carrier is used locally. GPS comes into play by providing the OnStar operator with the precise location of the vehicle.

Because of its accuracy, GPS is rapidly becoming the location data-collection method of choice for a variety of commercial, government, and military applications. GPS certainly has become an important and cost-effective method for locating terrestrial features too numerous or too dynamic to be mapped by traditional methods.

Although originally funded by the U.S. Defense Department, access to the GPS network is free to all users in any country. This has encouraged applications development and created an entirely new consumer market, particularly in the area of vehicular location and highway navigation.