Leica Geosystems GPS Basics - Manual (Page 11)

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GPS Basics -1.0.0en How GPS works
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3.1 Simple Navigation
This is the most simple technique
employed by GPS receivers to instanta-
neously give a position and height and/
or accurate time to a user. The accuracy
obtained is better than 100m (usually
around the 30-50m mark) for civilian
users and 5-15m for military users. The
reasons for the difference between
civilian and military accuracies are given
later in this section. Receivers used for
this type of operation are typically small,
highly portable handheld units with a
low cost.
A Handheld GPS Receiver
3.1.1 Satellite ranging
All GPS positions are based on measuring the distance from the satellites to the
GPS receiver on the earth. This distance to each satellite can be determined by the
GPS receiver. The basic idea is that of resection, which many surveyors use in their
daily work. If you know the distance to three points relative to your own position, you
can determine your own position relative to those three points. From the distance to
one satellite we know that the position of the receiver must be at some point on the
surface of an imaginary sphere which has it’s origin at the satellite. By intersecting
three imaginary spheres the receiver position can be determined.
Intersection of three imaginary spheres