The GPS System
	See also: 'Why Do You Need Celestial in These Days of GPS?' The Global Positioning System (GPS) is a highly accurate, worldwide navigation and positioning system that can be used 24 hours a day. Designed at the impetus of the US Department of Defense and primarily for military use, GPS is available to a variety of users worldwide, including recreational boaters, fishing and shipping fleets, general and commercial aviation, surveyors, and engineers. The system is based on a constellation of satellites that, when complete, will consist of 21 satellites and 3 working spares orbiting the Earth twice a day in six orbital planes. Each satellite is in a fixed orbit approximately 10,900 nautical miles above the Earth, and inclined at 55 degrees from the equator.
	Data Transmission Each satellite continuously transmits two types of orbit data Almanac and Ephemeris. Almanac data contains the health and approximate location of every satellite in the system. A GPS receiver gathers Almanac data from any available satellite; using information from the Almanac, the receiver then determines which set of satellites will give the best geometries for a position fix. Ephemeris data contains the precise orbital parameters of each satellite. The satellites also transmit two codes. The encrypted code (P-Code) is the more accurate of the two, and is reserved for military use. The unprotected code (Coarse Acquisition, or C/A Code) is intended for public access. It is also used to determine the precise range of the user from each satellite, which is the first step in calculating a position fix. All transmissions from the satellites are in real time.
	How a Position Fix is Obtained
	Navigation with the Global Positioning System and a well-designed GPS receiver is very simple. The receiver uses data collected from three or four satellites to solve a fundamental geometric equation and presents it in navigation displays. First, the unit determines which satellites to use to obtain the position fix, and then the receiver obtains Ephemeris data from those satellites. (Three satellites are used in 2D; a fourth satellite is used in 3D to determine altitude.) The receiver then assesses the transmission time and signal quality from each satellite, and multiplies the difference in the transmission time by the speed of light (186,000 miles per second) to arrive at an estimate of the satellite's distance from the receiver (range). Next, the unit calculates (by triangulation) and displays the position fix. Although accuracy varies slightly with satellite constellation geometry, a position fix accuracy of 25 meters or better is typical with C/A Code receivers.
	Interference GPS uses a high frequency radio signal (1575.42MHz) that operates in a radio band where there is little interfering radiation. Also, GPS uses spread spectrum technology to protect its navigation signals. The GPS signal is therefore extremely resistant to conditions that disturb other electronic navigation systems. In general, weather conditions, on-board electronics, passing ships, on-shore electronic installations, on-board engine ignition, and portable radio receivers do not affect the GPS signal.
	Monitoring and Controlling GPS GPS is operated by the US Air Force from a master control station in Colorado, USA. The facility is equipped for satellite monitoring, telemetry, tracking, command and control, data uploading, and navigation message generation. Monitor stations and ground antennas throughout the world passively track the GPS satellites and relay data to the master control station. Exact satellite position and signal-data accuracy can therefore be constantly updated and maintained. Minor discrepancies between where the satellite "thinks" it is and where the monitor station "knows" it is can also be adjusted. If any satellite emits erroneous data or is otherwise not operating properly, a ground station marks it "unhealthy." The affected satellite broadcasts its status to the GPS receiver, which is programmed to ignore an unhealthy satellite and use the next best satellite to obtain a position fix. The master control station can selectively degrade satellite data. This degradation, or Selective Availability (SA), can cause positioning errors of 100 meters (2D RMS).