You’ve heard about GPS and probably know that it has something to do with handheld gadgets and satellites that tell you where you’re located. That’s great for starters, but to understand and use GPS, you need a bit more detailed knowledge. Follow this topic to go through some of the fundamentals of GPS so you have a better grasp of what it is and how you can use it.
GPS stands for Global Positioning System. A special radio receiver measures the distance from your location to satellites that orbit the earth broadcasting radio signals. GPS can pinpoint your position anywhere in the world. Pretty cool, huh? Aside from buying the receiver, the system is free for anyone.
You can purchase an inexpensive GPS receiver, pop some batteries in it, turn it on, and presto! Your location appears on the screen. No map, compass, sextant, nor sundial is required. Just like magic. It’s not really magic, though, but has evolved from some great practical applications of science that have come together over the last 50 years.
Other satellite Global Positioning Systems are either in orbit or planned, but this topic uses the term GPS for the Global Positioning System operated by the United States government.
A short history of GPS
Military, government, and civilian users all over the world rely on GPS for navigation and location positioning, but radio signals have been used for navigation purposes since the 1920s. LORAN (Long Range Aid to Navigation), a position-finding system that measured the time difference of arriving radio signals, was developed during World War II.
The first step to GPS came way back in 1957 when the Russians launched Sputnik, the first satellite to orbit the Earth. Sputnik used a radio transmitter to broadcast telemetry information. Scientists at the Johns Hopkins Applied Physics Lab discovered that the Doppler shift phenomenon applied to the spacecraft — and almost unwittingly struck gold.
A down-to-earth, painless example of the Doppler shift principle is when you stand on a sidewalk and a police car speeds by in hot pursuit of a stolen motorcycle. The pitch of the police siren increases as the car approaches you and then drops sharply as it moves away.
American scientists figured out that if they knew the satellite’s precise orbital position, they could accurately locate their exact position on Earth by listening to the pinging sounds and measuring the satellite’s radio signal Doppler shift. Satellites offered some possibilities for a navigation and positioning system, and the U.S. Department of Defense (DoD) explored the concept.
By the 1960s, several rudimentary satellite-positioning systems existed. The U.S. Army, Navy, and Air Force were all working on independent versions of radio navigation systems that could provide accurate positioning and all-weather, 24-hour coverage. In 1973, the Air Force was selected as the lead organization to consolidate all the military satellite navigation efforts into a single program. This evolved into the NAVSTAR (Navigation Satellite Timing and Ranging) Global Positioning System, which is the official name for the United States’ GPS program.
The U.S. military wasn’t just interested in GPS for navigation. A satellite location system can be used for weapons-system targeting. Smart weapons such as the Tomahawk cruise missile use GPS in their precision guidance systems. GPS, combined with contour-matching radar and digital image-matching optics, makes a Tomahawk an extremely accurate weapon. The possibility of an enemy using GPS against the United States is one reason why civilian GPS receivers are less accurate than their restricted-use military counterparts.
The first NAVSTAR satellite was launched in 1974 to test the concept. By the mid-1980s, more satellites were put in orbit to make the system functional. In 1994, the planned full constellation of 24 satellites was in place. Soon, the military declared the system completely operational. The program has been wildly successful and is still funded through the U.S. DoD.
