GNSS Signal Simulation - A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach - page 147

Below we list the properties of a GPS signal from one satellite—see Chapter 2
for additional details about the satellite signals:

PRN The pseudorandom noise number corresponding to the satellite. This num-
ber indicates which of the C/A codes should be used.
Doppler The Doppler count is the frequency deviation from the IF. The Doppler
count is directly associated with line-of-sight dynamics between the satel-
lite and the receiver.
Code Phase The code phase is the time alignment of the PRN code in the received
data.
P(Y) Code In addition to the C/A code, the L1 signal contains the P(Y) code. This
code is modulated onto the carrier wave as a quadrature component while
the C/A code is present in the in-phase component.
Data Bits The navigation data bits are phase-modulated onto the carrier wave
with a frequency of 50 Hz.
Signal Strength Due to the long signal path from the satellite to the receiver in
combination with a low power transmitter at the satellite, the received signal
is very weak. An additional property of the GPS signal is the signal-to-noise
ratio (SNR), the ratio between the signal and the noise originating from the
signal path.

The signal simulator should meet the above description of the global parame-
ters and the GPS signal properties. For each satellite it should be possible to define
PRN, Doppler, and code phase. The P(Y) code is not relevant to the acquisition
and tracking algorithms. As a result of that, this component is simplified in the
simulator. The P(Y) code is simulated as a digital signal alternating between

1
and 1 with the P(Y) code rate of 10.23 MHz. In most cases, the navigation data are
not relevant to the acquisition and tracking algorithms. So by default the naviga-
tion data signal is also simulated as a digital signal alternating between

−

1and1
with the navigation data rate of 50 Hz. This gives a data bit transition each 20 ms.
If a real navigation data sequence is needed it should be possible to provide it to
the simulator.

−

B.2 Simulink Implementation

The GPS signal simulator could be implemented in any software development
tool. The only demands to the program, except the possibility of implementing
the needed functionalities, are

- It should be easy to change parameters of a simulation.
- It should be possible to save the simulated data in a file that can be read by
MATLAB.