The Global Positioning System (GPS) is a network of 24 Navstar satellites orbiting Earth at 11,000 miles. Originally established by the U.S. Department of Defence (DOD) at a cost of about US$13 billion, access to GPS is free to all users, including those in other countries. The system’s positioning and timing data are used for a variety of applications, including air, land and sea navigation, vehicle and vessel tracking, surveying and mapping, and asset and natural resource management. With military accuracy restrictions partially lifted in March 1996 and fully lifted in May 2000, GPS can now pinpoint the location of objects as small as a penny anywhere on the earth’s surface.
The satellite constellation consists of the nominal 24-satellite constellation (the first was launched in 1978 and the 24th in 1994). They transmit signals (at 1575.42 MHz) that can be detected by receivers on the ground. The satellites are positioned in six Earth-centred orbital planes with four satellites in each plane. This means that signals from six of them can be received 100 percent of the time at any point on earth. The nominal orbital period of a GPS satellite is one half of a sidereal day or 11 hr 58 min. The orbits are nearly circular and equally spaced about the equator at a 60° degree separation with an inclination relative to the equator of nominally 55° degrees. The orbital radius is approximately 26,600 km (i.e., distance from satellite to centre of mass of the earth).
GPS satellites transmit two low power radio signals, designated L1 and L2. Civilian GPS uses the L1 frequency of 1575.42 MHz in the UHF band.A GPS signal contains three different bits of information — a pseudo-random code, ephemeris data and almanac data. The pseudo-random code is simply an I.D. code that identifies which satellite is transmitting information.
Several different notations are used to refer to the satellites in their orbits. One particular notation assigns a letter to each orbital plane (i.e., A, B, C, D, E, and F) with each satellite within a plane assigned a number from 1 to 4. Thus, a satellite referenced as B3 refers to satellite number 3 in orbital plane B. A second notation used is a NAVSTAR satellite number assigned by the U.S. Air Force. This notation is in the form of a space vehicle number (SVN) 11 to refer to NAVSTAR satellite 11.
The basic idea behind GPS is to use satellites in space as reference points for locations on earth. With GPS, signals from the satellites arrive at the exact position of the user and are triangulated. This triangulation is the key behind accurate location determining and is achieved through several steps.
1, Determining your Position
2, Measuring the Distance
3, Error Correction
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