Global Positioning Systems

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The Global Positioning System (GPS) is undoubtedly one of the most practical of all satellite projects. It provides navigation and location information to other satellites, commercial airliners, cruise ships, land surveyors, map makers, bicyclists, and hikers. GPS is based on a very simple theoretical principle that is complex to achieve practically.

The basic principle behind GPS is the measurement of distance—in this case, the distance between satellites and receivers on the ground. The satellites transmit a radio message. Their distance from the receiver can be easily calculated using the speed at which the message travels (the speed of light) and the time it takes to complete its travel. By comparing the time the signal was sent and received, the distance from the satellite can be calculated as: Distance = Speed (C) × Time Difference. If the satellite were directly overhead, the time for the signal to travel to the receiver would be 0.6 seconds. However, the real world's dynamic satellite orbits, ionospheric distortion of the radio signal, and inaccuracies in timing measurements complicate the realization of this simple principle.

In order to achieve accurate, real-world measurements, the GPS is composed of three segments: space, control, and user. The space segment consists of GPS satellites in orbit around the Earth. The control segment is made up of ground stations that monitor the satellites. The user segment is comprised of the receivers used to make the measurements.

The space segment is a constellation of twenty-four NAVigation Satellite Timing and Ranging (NAVSTAR) satellites orbiting the Earth in circular orbits at 20,278 kilometers (12,600 miles). The constellation is composed of six orbital planes, each tilted 55 degrees with respect to the equator. Each orbital plane has four satellites spaced 60 degrees apart so that a minimum of five satellites are viewable from any location on the Earth. Although the constellation is comprised of twenty-four satellites, there are currently twenty-nine satellites in orbit (five are spares). The most recent satellite was launched on January 30, 2001.

Each satellite transmits precise time, position, and orbit information. In order to minimize or eliminate ionospheric distortion, the satellites transmittwo signals with binary codes at different frequencies. The binary signals use a pseudo-random code (i.e., a binary signal with random noise-like properties), which obviates the need for great power or large antennas.

The control segment, consisting of a master control station in Colorado Springs, Colorado, and four additional ground stations around the world, keeps the space segment operational and accurate. This segment, properly known as the Operational Control System (OCS), is a vital part of the GPS system but is basically invisible to the GPS user. The control segment constantly measures and calculates detailed orbits, monitors satellite clock accuracy, and assesses the health status of all satellites to determine if any repositioning is required. Updated satellite orbital (ephemeris) information, clock information, and routine maintenance commands are transmitted once or twice a day to the satellites from uplink antennas at three of the ground stations.

The user segment consists of users and their receivers. The GPS receiver contains a processor that calculates the location based on the satellite signals. The user does not transmit anything to the satellite and the satellite does not know the user is there. There is no limit to the number of users who can use the system at any time.

The GPS receiver detects and converts the signals transmitted by the satellites into useful information. It measures the time it takes for the signal to reach the receiver. This information is used together with satelliteephemeris data to compute position in three dimensions. In a perfect world, spherical trigonometry would require only three measurements to locate a point in three-dimensional space. However, four or more measurements are used in the GPS to eliminate any timing error. Dimensions are computed in Earth-Centered, Earth-Fixed X, Y, and Z coordinates. Position in XYZ is converted in the receiver to geodetic latitude, longitude, and elevation.

It should be noted that GPS signals are very weak, subject to atmospheric distortion, subject to signal bounce and diffusion, and difficult to access in dense forests and dense urban environments.

A simple hand-held receiver can provide a measurement of location that is accurate to approximately 50 meters (164 feet). However, this is not accurate enough for many applications such as mapmaking and surveying. More complex, and expensive, receivers and alternative measurement techniques can be used to improve locational accuracies to less than 1 meter (3.3 feet), and even to the centimeter (0.4 inch) range.

One of the most common techniques for achieving improved accuracies is differential GPS measurements. This is a method of eliminating errors in a GPS receiver to make the output more accurate. This technique is based on the principle that most of the errors seen by GPS receivers in a local area are common errors such as clock deviation or changing radio propagation conditions in the ionosphere. In the differential approach, two receivers are employed, with one called a base station placed at a location for which the coordinates are known and accepted. The base station constantly monitors the difference between the known coordinates and the GPS-calculated coordinates to provide a measure of the error.

There are two ways of utilizing the base station data. In post processing, data at the base station and the surveying receiver, or rover, are recorded and processed together at a later time. In real-time processing, data are transmitted from the base station to the rover and the error is calculated in real time.

Many commercial, private, and government base stations have data that can be used in either post processing or real-time processing. Commercial satellite services also provide correction signals for real-time processing of data.

A discussion of GPS would not be complete without mentioning GLOSNASS, which is made up of twenty-four satellites, eight in each of three orbital planes. GLOSNASS was deployed by the Russian Federation and is a system that has much in common with the NAVSTAR GPS.

Robert D. Regan

Database Management Software; Geographic Information Systems; Telecommunications.

Dana, Peter H. Global Positioning System Overview. Department of Geography, University of Texas. <http://www.colorado.edu/geograp hy/gcraft/notes/gps/gps.html>

GPS Elements. The Aerospace Corporation. <http://www.aero.org/publication s/GPSPRIMER/GPSElements.html>

Time Services Department, U.S. Naval Observatory, Washington, D.C. Current GPS Constellation. <http://tycho.usno.navy.mil/gpsc urr.html>

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