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GPS
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:
As illustrated in the previous figure, the position is calculated as the meeting
point of three spheres which are drawn around the three satellites with
diameters d1, d2 and d3.
9.2.2. Position Fixing Accuracy: HDOP
The GPS fix accuracy is due to the locations of 3 satellites in the sky. High
accuracy is obtainable when the satellites are widely scattered in the sky; on
the contrary, accuracy is reduced when the satellites have gathered in a
narrow space.
In the following figure, in both cases it is possible to obtain the GPS fix, but in
the left case the accuracy will be higher than the right: