GNSS Signal Simulation - A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach - page 154

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0

-5

-10

0 10 20 30 40 50

Frequency [MHz]

FIGURE B.6. Spectrum for generated Galileo L1 signal.

wide lobes at the sides of the spectrum originate from the restricted access signal
BOC(15,2.5).
Our model does not save the signal from the generator. The output of the gener-
ator must be sampled and directed to a file or a variable in MATLAB workspace.
The technique is similar to the implementation of the FFT plot visible at the top
right corner of the diagram. A zero-order hold block is needed to sample the sig-
nal. Then one of the signal sinks must be inserted from the Simulink block library
depending on whether it has to be a file or a variable. The Additive White Gaus-
sian Noise (AWGN) block is used to add noise to the signal.
The current model only generates the signal for one satellite at a time. It can be
used as a subsystem in Simulink. So several such generators must be connected to
generate signals of a constellation of satellites. Section B.2.5 describes how that
can be implemented.
The current model is very basic and must be updated comprehensively in order
to account for properties and propagation environment of the real signals. First
a more complicated signal propagation model must be added which accounts for
signal amplification at the satellite, signal losses in free space, atmosphere, an-
tennas, and the front end. Next the model must be modified to include Doppler
effects. The rate of all generators (data, PRN, subcarriers, and the carrier) must
be controlled simultaneously and all intersignal phases must be preserved. This
requires custom implementations of these blocks. An excellent example of an ad-
vanced generator is explained in the Mathworks webinar “Implementing a GPS
Receiver with DSP and FPGA Hardware Using Simulink and Related Tools.”

B.4 Differences in Processing GPS and Galileo

Signals

Originally this text was planned to deal with both GPS and Galileo on equal
terms. It soon became clear that this ideal goal had to be modified. It turned out
to be difficult to get reliable detailed information about Galileo signals; in addi-
tion, the necessary testing of the software implementation also became difficult to
carry out.