GPS or Global Positioning System was developed in the 1970's by the U.S. Department of Defense. It is a system of 27 satellites (called Navigation Satellite Timing and Ranging, or NAVSTAR) and ground control stations that provides locational information for military, public and commercial use. These uses can include recreational wayfinding, aeronautical or maritime navigation, and surveying.
There are 24 active satellites (plus a few spares) placed in Earth orbit as illustrated above. These satellites transmit time-encoded radio signals to Earth, which are recieved by GPS units and converted into locational information.
How does it work? The concept behind GPS is really not that complicated. You can find the distance between two objects if you know the time that it takes to travel from one to the other at a fixed speed. The basic formula can be stated as "speed times the time equals distance," or s * t = d. For example, in GPS, a satellite sends a radio signal at the speed of light (approx. 186,000 miles/sec.), and extremely accurate atomic clocks report the time it was sent. A GPS recieving unit on Earth uses the information to calculate its distance from the satellite. Of course, just knowing your distance from one satellite isn't going to give you your location. Using multiple satellites (i.e. four or more) with known altitudes and orbital paths, a GPS receiving unit can triangulate its location and altitude above sea level. If the unit is in motion (such as in boats or airplanes) it can even calculate its travel speed.
The accuracy of the system depends upon several elements. The more satellites in your field of view, the more accurate your calculated position will be. Because the signals from the satellites are transmitted radio waves, they are subject to interference and scattering by the atmosphere and surrounding obstacles such as buildings or trees. The longer a GPS receiving unit stays in one location, the more point readings it has from which to calculate its average position. If you also use a base station with a known location, your position can be calculated (using differential correction) with an accuracy within two or three meters. More advanced (and expensive) systems can acheive an even greater degree of accuracy, much less than a meter!
Accuracy can also be affected by Selective Availablity (S/A). Prior
to May 2000, the GPS signal was deliberately downgraded for all but military
and authorized government users out of regard for national security. Since
that time, S/A has been turned off, granting civilian and commercial interests
access to all but the most classified GPS signals. S/A can be turned on
again, however, if security concerns so require.
GPS Development Timeline
(from GPSWorld)
Basic Timeline of GPS Development
- Late 1960s, concept development
- Early 1970s, program funding and establishment of a Joint Program Office within the Department of Defense
- December 1973, proposal for GPS approved by the Defense System Acquisition and Review Council (DSARC)
- Mid-1970s, ground testing
- 22 February 1978, launch of the first satellite
- 1986-1989, hiatus following the Challenger space shuttle disaster
- 1989, revived launch program and changes in the design of the satellite constellation
- 1989, Magellan Corporation introduces first hand-held GPS receiver
- January 1990, first issue of GPS World
- 1991, detection and fix of a major glitch that slowed progress
- January 1992, use of GPS in Operation Desert Storm
- December 1993, declaration of Initial Operational Capability (IOC) by the U.S. Secretary of Defense
- 2 May 2000, SA is turned off by presidential directive; inexpensive civilian GPS receivers increase their horizontal accuracy from "no worse than" 100 meters to 15-25 meters