Contents
- What is GPS?
- How does GPS work?
- What is the satellite network like?
- How does the GPS signal work?
- What are the applications for GPS?
- How can I learn more about GPS?
GPS is an acronym for Global Positioning System, a worldwide navigationand positioning system developed by the US Department of Defense for bothmilitary and civilian use. GPS can determine the location of an object onearth with an accuracy ranging from tens of meters down to millitmeters,depending on the receiver and signal processing technology. Thistechnology permits military, civilian and scientific users to obtainthree-dimensional locations near the Earth's surface with unprecedentedprecision. GPS has many applications; a navigational tool in boating andhiking, 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 broadcastingsatellites orbiting the earth at a height of more than 20,000km. GPS alsoconsists of receivers on the ground, which listen to and interpret thetransmissions of the satellites. Stations on the earth carefully monitorthe orbit of each satellite, maintaining a highly accurate record of thesatellites instantaneous position. The knowledge of the precise positionof the satellites allows them to be used as reference points, from whichGPS receivers on earth can determine their position. This technique ofdetermining 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 aperson holding a GPS receiver. Then the person's position is known tobe somewhere on a sphere 25,000 km in radius, centered on thesatellite. However, the exact location of the person on that sphereis yet unknown. If, at the same time, the distance from the person toa second satellite can be discovered to be 20,000 km, then a secondsphere of radius 20,000 km on which the person is positioned can bedetermined. Thus the person must be on the circle formed by theintersection of the two spheres of position. A third satelliteprovides yet a third sphere, which narrows down the location of theperson to exactly two points. One of these points is often animpossible solution, frequently several thousand kilometers off inspace, thus three satellite ranges can determine the precise positionof the person. Three satellites provide enough information to findthe x, y, and z coordinates (measured from the center of mass of theearth). However, in practice, four satellites are required topinpoint a position, for reasons that will soon become clear.
In the above model of ranging, the distance between onesatellite 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 GlobalPositioning System works by having each of the 21 active satellitesconstantly radiate microwaves. These microwaves are received by theGPS receiver, which can use the method of ranging to locate itsposition. The distance from the receiver to one satellite is measuredin the following way. The satellite and receiver are controlled byseparate clocks. The satellites are set as accurately as possiblewith an atomic clock, and are assumed to be synchronized with oneanother. At some known time a satellite emits a signal in the form ofmicrowaves. This signal reaches the receiver after a certain intervalhas passed. Since microwaves travel at the speed of light, a knownvelocity and a known time allows the receiver to determine thedistance to the satellite. Thus it is important that the time bemeasured precisely in order to accurately measure distance, as anerror of the synchronization of the two clocks of one microsecondcreates an error of 300 meters. This requires a fourth satellite,since a fourth variable, time, has been added to the unknowns whichpreviously included only the x, y, and z distances.
The 21 functioning satellites are evenly spaced in circularorbits inclined 55 degrees to the equatorial plane, approximately20,000 km above the surface of the earth. At a height of over three
times the radius of the earth, each satellite orbits the earth onceevery twelve hours. A particular satellite is visible to a user onearth for approximately five hours each rotation. Depending on thelocation of the user and the time of day, between four and tensatellites are visible at any time. Five control stations, spacedevenly around the world, control the satellites. These stations areresponsible for predicting and correcting the orbits of thesatellites, and for transmitting data up to the satellites, includingclock corrections.
The satellite broadcasts two carrier waves, the L1 carrier wave at1575.42 MHz, and L2 at 1227.60 MHz. These dual frequencies are chosen toeliminate ionospheric dispersion, one of the major sources of systematicrange error. The pseudoranges, which are derived from signal travel timeto the receiver, use two pseudorandom noise (PRN) codes. These codes aremodulated onto the carrier frequencies. The first code, which isavailable for civilians, is the C/A- code (Course/Acquisition-code), whichhas a wavelength of approximately 300 meters. The C/A-code is modulatedonly upon L1. Its omission from L2 allows the government to control thelevel of accuracy available to civilian users. The second code, theP-code (Precision-code) is available only to the military and a fewdesignated 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 tothe public since the system was declared fully operational in 1992. Thetechnique used to transmit the signals from the satellites involvestransmitting a carefully formulated code known as pseudo-random sequences. The received signals and the transmitted sequences are compared to oneanother, and the travel time for the signal is found by measuring when thetwo signals are most closely correlated.
In order to maintain control over the navigation system, themilitary wanted to limit access to the most accurate GPSmeasurements. The method they chose for this operation was totransmit inaccurate information to civilians about when the signalshad been sent from the satellites. By altering the satelliteUs clocksslightly according to a specific code, those with access to the codeare able to obtain accurate information while civilians are forced todeal with the inaccuracy in distance measurements resulting from thetime error. The modified signals allow non-military GPS users toobtain navigational readings that are accurate to approximately 100meters. However, civilians have found ways around this dithering ofthe clocks. By comparing the GPS-measured position of a knownlocation with its actual coordinates, it is possible to detect theamount of dithering. Once the amount of dithering is known, thecorrections can be broadcast to the GPS receiver, and an accuratecalculation of the receiverUs position can be achieved. Thus evenwithout access to the dithering codes kept by the military, it ispossible for a civilian to determine their position with an accuracyof millimeters.
Many varied uses for Global Positioning System exist. GPS is able tomeasure position very accurately, and positioning signals are available tousers around the world at all times. These features account for thepopularity of GPS with many diverse groups.
The Global Positioning System was designed for precise, real-timemilitary positioning. The military utilizes GPS in land, marine, andairborne navigation. In addition, GPS satellites are equipped withsensors to monitor and detect the detonations of nuclear weapons. However, navigation is the main function of GPS, with uses in all branchesof the military. Some examples are photo reconnaissance, low-levelnavigation, target acquisition, command and control, en route navigation,and missile guidance.
Although GPS was designed for military use, civilian use of thenavigation technology has dramatically increased with the advent of
affordable, portable GPS receivers and the ability to increase theaccuracy of civilian GPS readings. A major use of GPS is for surveyingand mapping, including land, marine, and airborne surveying, local andglobal deformation monitoring, and geodetic control. Applications intransportation and communication include automotive navigation aids, withan automated display of the vehicle position on an electronic map. Thisis particularly useful for emergency vehicles and search and rescuemissions. Monitoring the location and movement of vehicles such as taxis,trucks, and boxcars can also be achieved using GPS. Recreationalactivities have also become a large market for low-cost, portablereceivers. Boating, backpacking, biking, and horseback riding are a fewof the activities whose participants use fairly inexpensive, relativelylow accuracy (good to tens or hundreds of meters) GPS receivers.
The following sources (used as references for this article) areexcellent places to look for more in-depth information on the GlobalPositioning System.
Herring, Thomas A. "The Global Positioning System". ScientificAmerican, February 1996: p. 44-50.
Hofmann-Wellenhof, B. and others. GPS Theory and Practice. 2ndEdition. Wien: Springer-Verlag,1993.
Leick, Alfred. GPS Satellite Surveying. New York: John Wiley & Sons,1995.
Wells, David. Guide to GPS Positioning. New Brunswick: NewBrunswick Graphic Services, 1987.
