|
|
How GPS Work
The distance to the GPS satellites can be determined by estimating the amount of time it takes for their signals to reach the receiver. When the receiver estimates the distance to at least four GPS satellites, it can calculate its position in three dimensions. The satellites are currently operated by the U.S. Department of Defence (See Galileo) and orbit with a period of 12 hours and two orbits per day at a height of about 11,500 miles. Ground stations are used to precisely track each satellite's orbit. A receiver knows the location of the GPS satellites, because the information is included in the satellite transmissions. By estimating how far away a satellite is the receiver also knows where it is located on the surface of an imaginary sphere centred at the satellite. It then determines the sizes of several spheres, one for each satellite. The receiver is located where these spheres intersect. |
Each
GPS satellite transmits data that indicates its location and the current
time. All GPS satellites synchronize operations so that these repeating
signals are transmitted at the same instant. The signals, moving at the
speed of light, arrive at a GPS receiver at slightly different times because
some satellites are further away than others.