Si-tex NAVSTAR 10, Руководство по эксплуатации

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NAVSTAR 10/12 series Introduction
HOW GPS WORKS
Currently, the GPS constellation consists of 26 orbiting satellites (including 3 spares), but this number
will increase in the future.
The GPS receiver computes an accurate position by calculating the distance to the GPS satellites that
orbit the earth. Signals are required from 3 satellites for two dimensional (2D) position calculation
whilst 4 satellites are required for three dimensional (3D) position calculation.
As mentioned earlier, GPS satellites are not geostationary, but they are orbiting the earth as illustrated
on the following figure:
[The GPS constellation]
Note that position is repeatedly fixed through the following three steps while any 3 satellites are in line
of sight.
The position calculation procedure is indicated in the following three steps:
1. GPS satellites continuously transmit their own precise orbital data and the GPS receiver computes
their locations by receiving this data.
2. In this receiving process, the GPS receiver measures very accurate distances to the satellites, using
the "Spread Spectrum Modulation" method. Excellence in GPS's position-fixing accuracy is mainly due
to this technology.
3. When the satellite locations and their distances are known, the GPS receiver fixes its own position
by triangulation: