Global Positioning Satellites
Global Positioning Orbits
The 24 satellites of the GPS are placed in orbits at about 3.75 times the radius of the Earth. A GPS receiver, which may be a small hand-held unit, can triangulate its position on the Earth's surface within 30 meters or less with signals from three of the satellites. The satellites are arranged in six orbital planes with four satellites in each plane.
GPS Navigation Message
The positioning information from GPS satellites in sent in the form of repeating codes which identify the sending satellite, give locations of the other satellites in the system, and give the navigation data. The codes which form the GPS signal structure are superimposed upon two carrier waves in the L-band , a frequency range set aside for satellite communication.
Both the carrier frequencies and the signal frequencies are derived directly from the onboard atomic clock oscillator frequency of 10.23 MHz. The coded signals are repeated regularly in epochs on the order of 15 seconds. The codes used are referred to as "pseudo-random codes", a name applied by early radio astronomers who were the first to make wide use of such codes.
The codes are well suited to decoding a message embedded in noise signals which may be orders of magnitude larger than the signal itself. Such techniques were valuable in picking up radar echoes off the moon and asteroids. Use of these codes facilitates the sorting out of signals which arrive simultaneously from several GPS satellites.
Global Positioning Triangulation
|A single satellite can establish range, locating the detector on a sphere.Two satellites can locate the detector on the intersection of two spheres.|
Adding a third as in the GPS system locates it at one of two discrete points where the three spheres intersect. The correct one is easily chosen to get a precise location. The 24 GPS satellites carry atomic clocks to give them the accuracy necessary for position measurement.
Rubidium Atomic Clock
Earth Orbit Velocity