The

Global Positioning System

THE AEROSPACECORPORATION

Printed in U.S.A./AC/PAD/7M/Aug 97/RH0039

The Aerospace CorporationP.O. Box 92957Los Angeles, CA 90009-2957http://www.aero.org/

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Has anyone ever asked you what career you wanted to pursue?

Imagine being an archaeologist on an ex-pedition to the Yucatan Peninsula in Mexi-co. After preparing for your trip for months, youare certain that somewhere close by are theruins of villages once populated by MayanIndians. The forest is dense, the sun is hot,and the air is moist and humid. The only wayyou can record where you have been, or findyour way back to civilization, is by using thealmost magic power of your GPS receiver.

Or let’s suppose you are an oceanograph-er for the International Ice Patrol. You maybe responsible for finding icebergs that formin the cold waters of the north Atlantic Ocean.Some of these icebergs are 50 miles long.They are a major threat to the ships that trav-el those waters, and more than 300 of themform every winter.

Using a GPS receiver, you are able to helpships avoid disaster by zeroing in on the po-sition of the icebergs and notifying ship cap-tains of their locations, perhaps averting disaster.

There will probably be a time soon whenevery car on the road can be equipped witha GPS receiver, including a video screen in-stalled in the dashboard. The in-dash monitorwill be a full-color display showing your lo-cation and a map of the roads around you. Itwill probably monitor your car’s performanceand your car phone as well. Systems as amaz-ing as this one are already being tested onhighways in the United States.

1Cover: Photo illustration by The Aerospace Corporation/NASA

The men and women who created the Glob-al Positioning System (GPS) were once young peo-ple like yourself. Each had a special interest inthe marvelous order of mathematics and thephysical laws affecting all our lives. They pur-sued a career in science or engineering and de-veloped a navigation system that can tell youexactly where you are on Earth, any time ofday or night, in any type of weather. The Aero-space Corporation is pleased to provide thisbooklet as an aid to your understanding of GPS.

If you are interested in pursuing an excitingcareer in navigation or a related field, I en-courage you to study mathematics and sciencein junior high and high school. By becoming pro-ficient in these areas, you will lay the ground-work for an understanding of physics andastronomy—the building blocks of the scienceof navigation.

This booklet provides basic information onthe subject. It is intended to stimulate your in-terest. Your school librarian can help you locateadditional details on the many fascinating fieldssupporting the use and exploration of space.

E. C. “Pete” Aldridge, Jr.

President and

Chief Executive Officer

The Aerospace Corporation

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The next major developments in the questfor the perfect method of navigation were themagnetic compass and the sextant. The nee-dle of a compass always points north, so it isalways possible to know in what directionyou are going.

The sextant uses adjustable mirrors to mea-sure the exact angle of the stars, moon, andsun above the horizon. However, in the earlydays of its use, it was only possible to deter-mine latitude (the location on the Earth mea-sured north or south from the equator) from thesextant observations. Sailors were still unableto determine their longitude (the location onthe Earth measured east or west). This wassuch a serious problem that in the 17th cen-tury, the British formed a special Board ofLongitude consisting of well-known scien-tists. This group offered £20,000, equal toabout a million of today’s dollars, to anybodywho could find a way to determine a ship’s long-itude within 30 nautical miles.

The generous offer paid off. In 1761, a cab-inetmaker named John Harrison developeda shipboard timepiece called a chronometer,which lost or gained only about one seconda day—incredibly accurate for the time. For thenext two centuries, sextants and chronometerswere used in combination to provide latitudeand longitude information.

In the early 20th century several radio-based navigation systems were developed,which were used widely during World War II.Both allied and enemy ships and airplanesused ground-based radio-navigation systemsas the technology advanced.

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What is GPS?GPS, which stands for Global Positioning Sys-

tem, is the only system today able to showyou your exact position on the Earth anytime,in any weather, anywhere. GPS satellites, 24in all, orbit 11,000 nautical miles above theEarth. They are continuously monitored byground stations located worldwide. The satel-lites transmit signals that can be detected byanyone with a GPS receiver. Using the re-ceiver, you can determine your location withgreat precision.

GPS is one of history’s most exciting and rev-olutionary developments, and new uses for itare constantly being discovered. But before welearn more about GPS, it’s important to un-derstand a bit more about navigation.

What is Navigation?Since prehistoric times, people have been

trying to figure out a reliable way to tell wherethey are and to help guide them to wherethey are going. Cavemen probably used stonesand twigs to mark a trail when they set out hunt-ing for food. The earliest mariners followed thecoast closely to keep from getting lost. Whennavigators first sailed into the open ocean, theydiscovered they could chart their course byfollowing the stars. The ancient Phoenicians usedthe North Star to journey from Egypt andCrete. According to Homer, the goddessAthena told Odysseus to “keep the Great Bearon his left” during his travels from Calypso’sIsland. Unfortunately, the stars are only visi-ble at night—and only on clear nights.

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accurate. High-frequency radio waves (likeUHF TV) can provide accurate position lo-cation but can only be picked up in a small,localized area. Lower frequency radio waves(like AM radio) can cover a larger area, but arenot a good yardstick to tell you exactly whereyou are.

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A few ground-based radio-navigation sys-tems are still in use today. One drawback ofusing radio waves generated on the ground isthat you must choose between a system thatis very accurate but doesn’t cover a wide area,or one that covers a wide area but is not very

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Map courtesy of Dr. Seymour Schwartz

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The GPS system can tell you your locationanywhere on or above the Earth to withinabout 300 feet. Even greater accuracy, usuallywithin less than three feet, can be obtained withcorrections calculated by a GPS receiver at aknown fixed location.

To help you understand the GPS system,let’s take the three parts of the system—the satel-lites, the receivers, and the ground control—and discuss them in more detail. Then we’lllook more closely at how GPS works.

Satellites in SpaceAs we’ve said, the complete GPS space

system includes 24 satellites, 11,000 nauticalmiles above the Earth, which take 12 hours togo around the Earth once (one orbit). Theyare positioned so that we can receive signalsfrom six of them nearly 100 percent of thetime at any point on Earth. You need thatmany signals to get the best position information.

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Scientists, therefore, decided that the onlyway to provide coverage for the entire worldwas to place high-frequency radio transmittersin space. A transmitter high above the Earthsending a high-frequency radio wave with aspecial coded signal can cover a large area andstill overcome much of the “noise” encoun-tered on the way to the ground. This is one ofthe main principles behind the GPS system.

GPS ElementsGPS has 3 parts: the space segment, the user

segment, and the control segment. The spacesegment consists of 24 satellites, each in itsown orbit 11,000 nautical miles above theEarth. The user segment consists of receivers,which you can hold in your hand or mountin your car. The control segment consists ofground stations (five of them, located aroundthe world) that make sure the satellites areworking properly.

One trip around the Earth in space equalsone orbit. The GPS satellites each take 12hours to orbit the Earth. Each satellite isequipped with an accurate clock to let itbroadcast signals coupled with a precise timemessage. The ground unit receives the satel-lite signal, which travels at the speed of light.Even at this speed, the signal takes a mea-surable amount of time to reach the receiver.The difference between the time the signal issent and the time it is received, multiplied bythe speed of light, enables the receiver to cal-culate the distance to the satellite. To measureprecise latitude, longitude, and altitude, the re-ceiver measures the time it took for the signalsfrom four separate satellites to get to the receiver.

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GPS consists of three major segments: the space seg-ment, the user segment, and the control segment.

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Ground Control StationsThe GPS control, or ground, segment con-

sists of unmanned monitor stations locatedaround the world (Hawaii and Kwajalein in thePacific Ocean; Diego Garcia in the IndianOcean; Ascension Island in the Atlantic Ocean;and Colorado Springs, Colorado); a masterground station at Falcon Air Force Base inColorado Springs, Colorado; and four largeground antenna stations that broadcast sig-nals to the satellites. The stations also track andmonitor the GPS satellites.

ReceiversGPS receivers can be hand carried or installed

on aircraft, ships, tanks, submarines, cars,

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Satellites are equipped with very precise clocksthat keep accurate time to within three nano-seconds—that’s 0.000000003, or three bil-lionths, of a second. This precision timing isimportant because the receiver must knowexactly how long it takes for its signal to getto each satellite and return. By knowing theexact amount of time the signal has taken toget back from each satellite, it can calculateits position.

The first GPS satellite was launched in1978. The first 10 satellites were develop-mental satellites, called Block I. From 1989 to1993, 23 production satellites, called Block II,were launched. The launch of the 24th satel-lite in 1994 completed the system.

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nThe 24-satellite constellation orbits 11,000 nautical milesabove Earth. Each satellite completes its orbit every 12hours. This diagram gives you an idea of satellite posi-tions and is not to scale.

GPS Block II is a production satellite first launched in 1989.Block II consists of 24 satellites, the last one launchedin 1994.

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something like this: If we know our exact dis-tance from a satellite in space, we know weare somewhere on the surface of an imaginarysphere with radius equal to the distance tothe satellite radius. If we know our exact dis-tance from two satellites, we know that we arelocated somewhere on the line where the twospheres intersect. And, if we take a third mea-surement, there are only two possible pointswhere we could be located. One of these isusually impossible, and the GPS receivershave mathematical methods of eliminatingthe impossible location.

An Amazing SystemWe now have a fairly clear picture of the GPS

system. We know that it consists of satelliteswhose paths are monitored by ground sta-

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and trucks. These receivers detect, decode,and process GPS satellite signals. More than100 different receiver models are already inuse. The typical hand-held receiver is aboutthe size of a cellular telephone, and the newermodels are even smaller. The hand-held unitsdistributed to U.S. armed forces personnelduring the Persian Gulf war weighed only 28ounces.

How GPS WorksSo you can more easily understand some of

the scientific principles that make GPS work,let’s discuss the basic features of the system. Theprinciple behind GPS is the measurement ofdistance (or “range”) between the receiverand the satellites. The satellites also tell usexactly where they are in their orbits. It works

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GPS is used by surveyors, utility companies, and oiland gas explorers for precise positioning.

Principles behind the Global Positioning System aresimple. The system is based on the measurement ofthe distance between the receiver and the satellites.

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dition, it is used on satellites to obtain highlyaccurate orbit data and to control spacecraftorientation.

GPS is based on a system of coordinatescalled the Worldwide Geodetic System 1984(WGS-84), similar to the latitude and longitudelines you see on wall maps in school. TheWGS-84 system provides a built-in frame ofreference for all military activities, so unitscan synchronize their maneuvers.

GPS Uses in Everyday LifeThe GPS system was developed to meet

military needs of the Department of Defense,but new ways to use its capabilities are con-tinually being found. As you have read, the sys-tem has been used in aircraft and ships, but thereare many other ways to benefit from GPS.We’ll mention just a few.

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tions. Each satellite generates radio signalsthat allow a receiver to estimate the satellitelocation and distance between the satelliteand the receiver. The receiver uses the mea-surements to calculate where on or abovethe Earth the user is located.

Now that we have an idea about how theGPS functions, let’s see how we can put it towork for us. As you might imagine, GPS hasmany uses in both military and civilian life.

Military Uses for GPSAlthough the GPS system was completed

only recently, it has already proved to be a mostvaluable aid to U.S. military forces. Picture thedesert, with its wide, featureless expanses ofsand. The terrain looks much the same formiles. Without a reliable navigation system,U.S. forces could not have performed the ma-neuvers of Operation Desert Storm. With GPS,the soldiers were able to go places and ma-neuver in sandstorms or at night when eventhe troops who lived there couldn’t. Initially,more than 1,000 portable commercial re-ceivers were purchased for their use. The de-mand was so great that, before the end of theconflict, more than 9,000 commercial re-ceivers were in use in the Gulf region. Theywere carried by foot soldiers and attached tovehicles, helicopters, and aircraft instrumentpanels. GPS receivers were used in severalaircraft, including F-16 fighters, KC-135 aer-ial refuelers and B-52 bombers; Navy ships usedthem for rendezvous, minesweeping, and air-craft operations.

GPS has become important for nearly all mil-itary operations and weapons systems. In ad-

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More than 9,000 GPS receivers were used by U.S. andcoalition forces during Operation Desert Storm.

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help determine population distribution patternsand possible sources of disease.

GPS-equipped balloons are monitoringholes in the ozone layer over the polar re-gions, and air quality is being monitored usingGPS receivers. Buoys tracking major oil spillstransmit data using GPS. Archaeologists andexplorers are using the system. Anyoneequipped with a GPS receiver can use it as areference point to locate or find another lo-cation. With a basic knowledge of math andscience, plus a hand-held GPS receiver, youcould be an instant hero if you and friends gottemporarily lost on a camping trip.

The future of GPS is as unlimited as yourimagination. New applications will contin-ue to be created as technology evolves. TheGPS satellites, like handmade stars in the sky,will be guiding you well into the 21st century.

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During contruction of the tunnel under theEnglish Channel, British and French crewsstarted digging from opposite ends: one fromDover, England, one from Calais, France. Theyrelied on GPS receivers outside the tunnel tocheck their positions along the way and tomake sure they met exactly in the middle.Otherwise, the tunnel might have been crooked.

Remember the example of the car with avideo display in the dashboard? Vehicle track-ing is one of the fastest-growing GPS appli-cations. GPS-equipped fleet vehicles, publictransportation systems, delivery trucks, andcourier services use receivers to monitor theirlocations at all times.

GPS is also helping to save lives. Many po-lice, fire, and emergency medical serviceunits are using GPS receivers to determinethe police car, fire truck, or ambulance near-est to an emergency, enabling the quickestpossible response in life-or-death situations.

Automobile manufacturers are offeringmoving-map displays guided by GPS receiversas an option on new vehicles. The displays canbe removed and taken into a home to plan atrip. Several Florida rental car companies aredemonstrating GPS-equipped vehicles thatgive directions to drivers on display screens andthrough synthesized voice instructions. Nomore getting lost on the way to Disney World!

Mapping and surveying companies useGPS extensively. In the field of wildlife man-agement, endangered species such as Montanaelk and Mojave Desert tortoises are being fit-ted with GPS receivers and tiny transmitters to

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A biologist in Brazil uses a hand-held GPS receiv-er in wildlife research projects.

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The Aerospace Corporation was one of the GPSteam members honored with the 1992 RobertJ. Collier Trophy, awarded by the NationalAeronautic Association for the “greatestachievement in aeronautics in America.”

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An Air Force-launched Delta II carried the 18th GPS satellite intoorbit in February 1993. By March 1994, all 24 GPS satellites wereorbiting the Earth.

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The

Global Positioning System

THE AEROSPACECORPORATION

Printed in U.S.A./AC/PAD/5M/Aug 97/RH0039

The Aerospace Corporation2350 East El Segundo Blvd.El Segundo, CA 90245-4691Mail: P.O. Box 92957Los Angeles, CA 90009-2957

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