Leica Geosystems GPS Basics - Manual (Page 22)

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GPS Basics -1.0.0en How GPS works
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3.3 Differential Phase GPS
and Ambiguity Resolution
Differential Phase GPS is used mainly in
surveying and related industries to
achieve relative positioning accuracies
of typically 5-50mm (0.25-2.5 in). The
technique used differs from previously
described techniques and involves a lot
of statistical analysis.
It is a differential technique which means
that a minimum of two GPS receivers are
always used simultaneously. This is one
of the similarities with the Differential
Code Correction method described in
section 3.2.
The Reference Receiver is always
positioned at a point with fixed or known
coordinates. The other receiver(s) are
free to rove around. Thus they are known
as Rover Receivers. The baseline(s)
between the Reference and Rover
receiver(s) are calculated.
The basic technique is still the same as
with the techniques mentioned previ-
ously, - measuring distances to four
satellites and computing a position from
those ranges.
The big difference comes in the way
those ranges are calculated.
3.3.1 The Carrier Phase, C/A and P-codes
At this point, it is useful to define the various components of the GPS signal.
Carrier Phase. The sine wave of the L1 or L2 signal that is created by the satellite.
The L1 carrier is generated at 1575.42MHz, the L2 carrier at 1227.6 MHz.
C/A code. The Coarse Acquisition code. Modulated on the L1 Carrier at 1.023MHz.
P-code. The precise code. Modulated on the L1 and L2 carriers at 10.23 MHz.
Refer also to the diagram in section 2.1.
What does modulation mean ?
The carrier waves are designed to carry the binary C/A and P-codes in a process
known as modulation. Modulation means the codes are superimposed on the
carrier wave. The codes are binary codes. This means they can only have the values
1 or -1. Each time the value changes, there is a change in the phase of the carrier.
Carrier Modulation