What is GPS?

GPS is an acronym for Global Positioning System, a worldwide navigation and positioning system developed by the US Department of Defense for both military and civilian use. GPS can determine the location of an object on earth with an accuracy ranging from tens of meters down to millitmeters, depending on the receiver and signal processing technology. This technology permits military, civilian and scientific users to obtain three-dimensional locations near the Earth's surface with unprecedented precision. GPS has many applications; a navigational tool in boating and hiking, while the military applications cover a wide range of operational, strategic and tactical disciplines.

How does GPS work?

The Global Positioning System consists of a network of 24 broadcasting satellites orbiting the earth at a height of more than 20,000km. GPS also consists of receivers on the ground, which listen to and interpret the transmissions of the satellites. Stations on the earth carefully monitor the orbit of each satellite, maintaining a highly accurate record of the satellites instantaneous position. The knowledge of the precise position of the satellites allows them to be used as reference points, from which GPS receivers on earth can determine their position. This technique of determining the position of an object is called ranging.

The concept of ranging is best illustrated by example. Consider one satellite that is a distance of 25,000 kilometers from a person holding a GPS receiver. Then the person's position is known to be somewhere on a sphere 25,000 km in radius, centered on the satellite. However, the exact location of the person on that sphere is yet unknown. If, at the same time, the distance from the person to a second satellite can be discovered to be 20,000 km, then a second sphere of radius 20,000 km on which the person is positioned can be determined. Thus the person must be on the circle formed by the intersection of the two spheres of position. A third satellite provides yet a third sphere, which narrows down the location of the person to exactly two points. One of these points is often an impossible solution, frequently several thousand kilometers off in space, thus three satellite ranges can determine the precise position of the person. Three satellites provide enough information to find the x, y, and z coordinates (measured from the center of mass of the earth). However, in practice, four satellites are required to pinpoint a position, for reasons that will soon become clear.

In the above model of ranging, the distance between one satellite and the person on earth is given to be 20,000 km. However, no mention was made as to how the distance was determined. The Global Positioning System works by having each of the 21 active satellites constantly radiate microwaves. These microwaves are received by the GPS receiver, which can use the method of ranging to locate its position. The distance from the receiver to one satellite is measured in the following way. The satellite and receiver are controlled by separate clocks. The satellites are set as accurately as possible with an atomic clock, and are assumed to be synchronized with one another. At some known time a satellite emits a signal in the form of microwaves. This signal reaches the receiver after a certain interval has passed. Since microwaves travel at the speed of light, a known velocity and a known time allows the receiver to determine the distance to the satellite. Thus it is important that the time be measured precisely in order to accurately measure distance, as an error of the synchronization of the two clocks of one microsecond creates an error of 300 meters. This requires a fourth satellite, since a fourth variable, time, has been added to the unknowns which previously included only the x, y, and z distances.

What is the satellite network like?

The 21 functioning satellites are evenly spaced in circular orbits inclined 55 degrees to the equatorial plane, approximately 20,000 km above the surface of the earth. At a height of over three times the radius of the earth, each satellite orbits the earth once every twelve hours. A particular satellite is visible to a user on earth for approximately five hours each rotation. Depending on the location of the user and the time of day, between four and ten satellites are visible at any time. Five control stations, spaced evenly around the world, control the satellites. These stations are responsible for predicting and correcting the orbits of the satellites, and for transmitting data up to the satellites, including clock corrections.

How does the GPS signal work?

The satellite broadcasts two carrier waves, the L1 carrier wave at 1575.42 MHz, and L2 at 1227.60 MHz. These dual frequencies are chosen to eliminate ionospheric dispersion, one of the major sources of systematic range error. The pseudoranges, which are derived from signal travel time to the receiver, use two pseudorandom noise (PRN) codes. These codes are modulated onto the carrier frequencies. The first code, which is available for civilians, is the C/A- code (Course/Acquisition-code), which has a wavelength of approximately 300 meters. The C/A-code is modulated only upon L1. Its omission from L2 allows the government to control the level of accuracy available to civilian users. The second code, the P-code (Precision-code) is available only to the military and a few designated users. P-code, with a wavelength of approximately 30 meters, is modulated on both L1 and L2. Access to the P-code has been denied to the public since the system was declared fully operational in 1992. The technique used to transmit the signals from the satellites involves transmitting a carefully formulated code known as pseudo-random sequences. The received signals and the transmitted sequences are compared to one another, and the travel time for the signal is found by measuring when the two signals are most closely correlated.

In order to maintain control over the navigation system, the military wanted to limit access to the most accurate GPS measurements. The method they chose for this operation was to transmit inaccurate information to civilians about when the signals had been sent from the satellites. By altering the satelliteUs clocks slightly according to a specific code, those with access to the code are able to obtain accurate information while civilians are forced to deal with the inaccuracy in distance measurements resulting from the time error. The modified signals allow non-military GPS users to obtain navigational readings that are accurate to approximately 100 meters. However, civilians have found ways around this dithering of the clocks. By comparing the GPS-measured position of a known location with its actual coordinates, it is possible to detect the amount of dithering. Once the amount of dithering is known, the corrections can be broadcast to the GPS receiver, and an accurate calculation of the receiverUs position can be achieved. Thus even without access to the dithering codes kept by the military, it is possible for a civilian to determine their position with an accuracy of millimeters.

What are the applications for GPS?

Many varied uses for Global Positioning System exist. GPS is able to measure position very accurately, and positioning signals are available to users around the world at all times. These features account for the popularity of GPS with many diverse groups.

The Global Positioning System was designed for precise, real-time military positioning. The military utilizes GPS in land, marine, and airborne navigation. In addition, GPS satellites are equipped with sensors to monitor and detect the detonations of nuclear weapons. However, navigation is the main function of GPS, with uses in all branches of the military. Some examples are photo reconnaissance, low-level navigation, target acquisition, command and control, en route navigation, and missile guidance.

Although GPS was designed for military use, civilian use of the navigation technology has dramatically increased with the advent of affordable, portable GPS receivers and the ability to increase the accuracy of civilian GPS readings. A major use of GPS is for surveying and mapping, including land, marine, and airborne surveying, local and global deformation monitoring, and geodetic control. Applications in transportation and communication include automotive navigation aids, with an automated display of the vehicle position on an electronic map. This is particularly useful for emergency vehicles and search and rescue missions. Monitoring the location and movement of vehicles such as taxis, trucks, and boxcars can also be achieved using GPS. Recreational activities have also become a large market for low-cost, portable receivers. Boating, backpacking, biking, and horseback riding are a few of the activities whose participants use fairly inexpensive, relatively low accuracy (good to tens or hundreds of meters) GPS receivers.

How can I learn more about GPS?

The following sources (used as references for this article) are excellent places to look for more in-depth information on the Global Positioning System.

Herring, Thomas A. "The Global Positioning System". Scientific American, February 1996: p. 44-50.

Hofmann-Wellenhof, B. and others. GPS Theory and Practice. 2nd Edition. Wien: Springer-Verlag, 1993.

Leick, Alfred. GPS Satellite Surveying. New York: John Wiley & Sons, 1995.

Wells, David. Guide to GPS Positioning. New Brunswick: New Brunswick Graphic Services, 1987.