 GPS Explained
In a nutshell, the Global Positioning System (GPS) is a space-based navigation system, consisting of 24 satellites orbiting about 11,000 miles above the earth. The system is maintained by the US Department of Defense. A GPS receiver will show you your position, velocity, direction and time anywhere on the planet. There are no fees associated with using the signal.
History
In ancient times, navigation was a difficult and less than exact science. Sailors guided themselves using a compass and the stars. A good navigator who could get you within a few miles of your destination, was worth his weight in gold. Bad ones, who got lost, were often executed. In the 17th century accurate time keeping was added to the equation and navigation became more accurate but there was still a long way to go.
Fast forward to the 1960s. In the height of the cold war, scientists were looking for a better and more precise way to guide ICBMs. After years of development and testing, GPS was born. On Feb. 22, 1978, the first Navstar Global Positioning System satellite was launched from Vandenberg Air Force Base, California. It was the first of four GPS satellites to be launched that year.
It took 16 years for the system to become fully operational with 24 satellites (and some back-ups). Each satellite weighs about 2,000 pounds and is about 17 feet across with the solar panels extended. The satellites come equipped with an atomic clock, accurate to within 10-billionths of a second, and a radio transmitter of about 50 watts. They are solar powered and have a battery backup for the times when the sun is eclipsed.
 How It Works
The satellites (SVs) transmit a signal using their own unique ID code, ephemeris data and almanac data. The ephemeris data contains information about the position of the satellite, along with the current date and time. This part of the signal is essential for determining a position. The almanac data indicates where all the SVs are supposed to be in orbit at that given moment. This information is constantly updated and synchronized by land stations throughout the globe.
The GPS receiver takes this information and, making dozens of calculations per second, finds your position. The receiver determines how long it takes the signal to get from space and then can determine a ÒsphereÓ where it is located. The point at which the spheres intersect is the location.
The Future
The GPS industry is still very much in its childhood. Engineers and scientists are working to make GPS navigation even better. Using corrections from Ground stations (see DGPS and WAAS Explained), GPS receivers are becoming more accurate. The Department of Defense is working on systems with stronger signals that can penetrate walls and will resist an enemy's ability 'jam' the signal.
Additionally, industry leaders like Garmin and Magellan are working to make smaller receivers that have greater functionality. Look for more integration with devices like the Garmin Rino, which combines a GPS and a radio, or the Navtalk GMS cell phone. And look for other manufacturers to emerge with new and unique uses for the Global Positioning System.
Who knows? In the future your car may drive itself guided by the satellites!
References
For more information about GPS, have a look at some of these web resources:
FAA: http://gps.faa.gov/gpsbasics/index.htm
Trimble Navigation: http://www.trimble.com/gps/index.htm
Garmin: http://www.garmin.com/aboutGPS/
The Aerospace Corp. http://www.aero.org/general/faq.html#Primer
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