HOW DOES GPS WORK?
GPS global positioning system is a space-age version of triangulation. Originally developed for military use, GPS has three components: satellites orbiting Earth, master control stations around the world, and receivers in stalled in locations ranging from naval destroyers to private golf carts.
In the U.S. GPS system, two dozen Navstar satellites orbit the planet every 12 hours, following six differ ent orbits. Three additional satellites orbit as backup. The satellites contain atomic clocks that send precise times with each signal. The control stations monitor the satellites, using remote controlled on-board thrusters to manage their positions.
When a GPS user on land or sea mine distance to a point on an imagi nary sphere, and the user's latitude
ADVANCES IN MAPPING
NAVSTAR BLOCK II NAVIGATION SATELLITE
Earthc NASA Goddard Space Flight Center
THE NAVSTAR SATELLITE SYSTEM contains 24 orbiting satellites, similar to Block II (above, right). Signals from Earth-based GPS equipment send and receive information from these satellites, thus ascertaining latitude and longitude position.
calls for location information, signals and longitude can be calculated by us- pass from orbiting satellites to that ing the mathematics of triangulation. user's receiver. The length of time Three satellites would suffice, but taken by the transmissions-usually more provide redundancy and com a fraction of a second-helps deterpensate for inaccuracies.
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GPS signals are broadcast on two different frequencies, one for military use and one for civilian use. Civilian augmentation can provide precise lo cation to within 0.4 of an inch.
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