GPS for Dummies.pdf - Engineering Surveyor

50 Part II: All About GPS A short history of GPSMilitary, government, and civilian users all overthe world rely on GPS for navigation and locationpositioning, but radio signals have been usedfor navigation purposes since the 1920s. LORAN(Long Range Aid to Navigation), a position-findingsystem that measured the time difference ofarriving radio signals, was developed duringWorld War II.The first step to GPS came way back in 1957when the Russians launched Sputnik, the firstsatellite to orbit the Earth. Sputnik used a radiotransmitter to broadcast telemetry information.Scientists at the Johns Hopkins Applied PhysicsLab discovered that the Doppler shift phenomenonapplied to the spacecraft — and almostunwittingly struck gold.A down-to-earth, painless example of theDoppler shift principle is when you stand on asidewalk and a police car speeds by in hot pursuitof a stolen motorcycle. The pitch of the policesiren increases as the car approaches you andthen drops sharply as it moves away.American scientists figured out that if they knewthe satellite’s precise orbital position, they couldaccurately locate their exact position on Earth bylistening to the pinging sounds and measuringthe satellite’s radio signal Doppler shift. Satellitesoffered some possibilities for a navigation andpositioning system, and the U.S. Department ofDefense (DoD) explored the concept.By the 1960s, several rudimentary satellitepositioningsystems existed. The U.S. Army,Navy, and Air Force were all working on independentversions of radio navigation systemsthat could provide accurate positioning and allweather,24-hour coverage. In 1973, the Air Forcewas selected as the lead organization to consolidateall the military satellite navigation effortsinto a single program. This evolved into theNAVSTAR (Navigation Satellite Timing andRanging) Global Positioning System, which is theofficial name for the United States’ GPS program.The U.S. military wasn’t just interested in GPS fornavigation. A satellite location system can beused for weapons-system targeting. Smartweapons such as the Tomahawk cruise missileuse GPS in their precision guidance systems.GPS, combined with contour-matching radarand digital image-matching optics, makes aTomahawk an extremely accurate weapon. Thepossibility of an enemy using GPS against theUnited States is one reason why civilian GPSreceivers are less accurate than their restrictedusemilitary counterparts.The first NAVSTAR satellite was launched in 1974to test the concept. By the mid-1980s, more satelliteswere put in orbit to make the system functional.In 1994, the planned full constellation of24 satellites was in place. Soon, the militarydeclared the system completely operational.The program has been wildly successful and isstill funded through the U.S. DoD.How GPS WorksThe intricacies of GPS are steeped in mathematics, physics, and engineering,but you don’t need to be a rocket scientist to understand how GPS works.GPS is composed of three parts (as shown in Figure 3-1):