Leica Geosystems GPS Basics - Manual (Page 13)

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GPS Basics -1.0.0en How GPS works
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3.1.2 Calculating the distance to the satellite
Calculating the Time
The satellite signal has two codes modulated upon it, the C/A code and the
P-code (see section 2.1). The C/A code is based upon the time given by a
very accurate atomic clock. The receiver also contains a clock that is used to
generate a matching C/A code. The GPS receiver is then able to “match” or
correlate the incoming satellite code to the receiver generated code.
The C/A code is a digital code that is ‘pseudo random’ or appears to be
random. In actual fact it is not random and repeats one thousand times
every second.
In this way, the time taken for the radio signal to travel from the satellite to
the GPS receiver is calculated.
In order to calculate the distance to each
satellite, one of Isaac Newton’s laws of
motion is used:
Distance = Velocity x Time
For instance, it is possible to calculate
the distance a train has traveled if you
know the velocity it has been travelling
and the time for which it has been
travelling at that velocity.
GPS requires the receiver to calculate
the distance from the receiver to the
satellite.
The Velocity is the velocity of the radio
signal. Radio waves travel at the speed
of light, 290,000 km per second
(186,000 miles per second).
The Time is the time taken for the radio
signal to travel from the satellite to the
GPS receiver. This is a little harder to
calculate, since you need to know when
the radio signal left the satellite and
when it reached the receiver.