Global Positioning System Reference
In-Depth Information
The next block is a look-up table much similar to that of the C/A part of the
simulator. In this case the look-up table contains a two-dimensional array, where
the first dimension selects whether or not there should be any navigation data tran-
sitions in the data. The second dimension is the index of the navigation bit to be
the output from the look-up table. In case no navigation bit transitions are wanted,
the look-up table always outputs the value 1. In the other case where navigation
bit transitions are wanted, the look-up table outputs alternating
1's and 1's.
The output from the navigation data part is provided to the C/A and the P code
part through a multiplicator.
B.2.3 P Code Generation
The generation of the P code component of the GPS signal is indicated within
Part C in Figure B.1. As mentioned before, the P code component is not relevant
to the acquisition and tracking algorithms and should thus only be simulated as a
squared pulse alternating between
1 and 1. In the GPS signal simulator this sig-
nal is implemented as a squared pulse generator with a frequency corresponding
to the chipping rate of the P code, which is 10.23 MHz.
In the second block of the P code part, this signal is combined with the naviga-
tion data sequence. This is done in a similar way as the C/A part by multiplication
of the two signals.
The carrier generation part in the lower left corner is also almost similar to the
carrier generation part in the C/A part. The only difference here is that the VCO
has a 90 phase shift compared to the other VCO. The result of this is that the P
code VCO generates a sine wave compared to the C/A VCO's cosine wave. The
combined P code and navigation data signal is BPSK modulated onto the carrier
in the multiplication block.
The final block of the P code part is a gain block. This block decreases the mod-
ulated P code signal by 3 dB in the same way as it is done in the signal generator
in the GPS satellites.
B.2.4 Combining the Signal Components
The last part of the GPS signal simulator is located at the right-most part of
Figure B.1. Here, the two modulated codes are combined, resulting in a com-
plete GPS signal. The two components are simply added together as in-phase and
quadrature components of the final signal.
The last part of the signal simulator is the addition of noise. This noise gener-
ator block is indicated with green in Figure B.1. The amount of noise is selected
from an input to the simulation. The resulting noise from the noise generator is
added to the GPS signal.
B.2.5 Upper-Level Implementation
When the lower level of the Simulink implementation of the GPS signal genera-
tor has been implemented, the upper-level implementation can be designed. The
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