Updated June 01, 2013

The **GPS** is a satellite positioning system, publicly available, that locates a point at any time and anywhere on Earth and this with a precision ranging from one hundred meters to millimeters depending on the type of equipment and technique used. The Earth moves around the ecliptic around the sun at an average speed of 107 218.8 km/h (between 29.291 km/s and 30.287 km/s). At the same time the Earth rotates on itself around an axis at the speed (at the equator) of 1674.364 km/h (465 m / s). Our natural satellite (moon) and artificial turn at different speeds, 1.17 km/s for the Moon, 3.88 km/s for the GPS satellites or 7.68 km/s for ISS. In a waltz orbital GPS satellites measure with great precision geolocation of a point on Earth.

A person with a GPS receiver can locate and orient themselves on land, sea, air or space near the Earth. For this we use the Global Positioning System (GPS). The 31 GPS satellites orbit almost circular orbits of 6 at an average altitude of 20,200 km at a speed of 14 000 km/h (3.88 km/s). On each orbit there are 5 or 6 satellites that circle the Earth in 11 hours 58 min 2 s, or half a sidereal day, each satellite takes the same position every two towers, seen from the earth's surface. The distribution of 31 NAVSTAR satellites has been optimized to obtain a sufficient number (four) of visible satellites at each point of the globe.

Three ground stations control the **atomic clocks** of the satellites. Einstein predicted that gravity is more significant, as time unfolds slowly and gravity is low, the more time passes quickly. The further away from Earth, the more gravity is low, the gravity at 20 000 km altitude is different than on Earth. Like gravity distorts time, in space time goes a little faster than on Earth. This has an impact on the GPS satellites.

The GPS works very accurately describes the curvature of space-time. For GPS to work properly, the clocks on GPS satellites, must be perfectly synchronized with time on Earth.

A 20 200 km altitude, if the clocks of the satellites are not synchronized, the values given by the GPS derived from 10 to 12 km per day.

The satellite clocks forward by 38 microseconds per day relative to a clock placed on earth.

A time correction is made to fit the GPS to the effects of gravity. GPS satellites transmit signals to Earth brought on two frequencies (227.6 and 1 1 575.42 MHz) that are modulated by one or more pseudo-random codes.

The GPS uses the WGS 84 (World Geodetic System 1984), a benchmark for mapping. When four satellites are in view, the user has enough information to find its position on Earth.

GPS satellites follow a near-circular orbit at an altitude of 20,200 km and an average speed of 3.88 km/s.

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