All About GPS
Developed by the United States Department of Defense, GPS is officially named Navstar GPS. Its name came from John Walsh, who was a key decision maker involving the budget for the technology program.
It is the only fully functional Global Navigation Satellite System Utilizing a constellation of at least 24 medium Earth Orbit satellites that transmit precise microwave signals, the system enables a GPS receiver to determine its location, speed, direction, and time.
The satellite constellation is managed by the United States Air Force 50th Space Wing. In 1983, President Ronald Reagan issued a directive making the system available for free for civilian use as a common good. Since then, the technology has become a widely used aid in navigation worldwide, and a useful tool for map making, land surveying, commerce, and scientific uses.
GPS - How it Works
A typical receiver calculates its position using the signals from four or more satellites. Four satellites are needed since the process needs a very accurate local time, more accurate than any normal clock can provide, so the receiver internally solves for time as well as position. The orbits are arranged so that at least six satellites are always within line of sight from almost everywhere on Earth's surface.
Each GPS satellite has an atomic clock, and continually transmits messages containing the current time at the start of the message. The signals travel at a known speed, the speed of light through outer space, and slightly slower through our atmosphere. The receiver uses the arrival time to compute the distance to each satellite, from which it determines the position of the receiver using geometry and trigonometry.
There are now around 33 actively broadcasting satellites in the GPS constellation. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements.
GPS - Receiver
GPS receivers are composed of an antenna, tuned to the frequencies transmitted by satellites, receiver-processors, and a highly stable clock. They may also include a display for providing location and speed to the user.
Conditions in the atmosphere affect the speed of the signals as they pass through the Earth's atmosphere, especially the ionosphere. Correcting these errors is very important to improving position accuracy.
Changes in receiver altitude also change the amount of delay, due to the signal passing through less of the atmosphere at higher elevations.
GPS - Time
While most clocks are synchronized to Coordinated Universal Time, the atomic clocks on the satellites are set to Global Positioning Satellite time. The difference is that time is not corrected to match the rotation of the Earth, so it does not contain leap seconds or other corrections which are periodically added to UTC.
