MATLAB Code - A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach - page 139

rameters are used by many files—from acquisition to pseudorange computation.
Once the parameters are updated in the variable settings , all files will use the
changed values.
One more advantage is that the function parameter list does not depend on how
many parameters the function actually is using. Changes inside the function will
not affect the calling function code.
The most commonly used variables are

IFfrequency Intermediate frequency of the GPS signal, Hz
samplingFrequency Frequency at which the GPS signal is sampled, Hz
msToProcess This variable is set to 37,000 to ensure all five 6-s subframes are
processed and included in the output, the first 1000 ms can be excluded for
any transient response
processBlockSize Size of the block to be processed by the tracking function
numberOfChannels Sets the number of channels of the software receiver.

The function initSettings creates the structure settings . The very first time this
function is executed by the script init . The function should be executed every time
variables are changed. An experienced user might change some of the variables
directly in the settings structure. However, care must be taken as some variables
have internal dependencies and are recomputed automatically. So it is safest to
change variables in the function initSettings , which must be re-executed after-
ward.
The block-dependent variables are described in the following sections.

A.3 Acquisition Function

The function acquisition employs the parallel code phase search acquisition al-
gorithm described in Section 6.4. The purpose is to find signal parameters for all
available satellites in a few-ms-long data record. The implementation is based on
the block diagram shown in Figure 6.8. The flow diagram for the actual code is
shown in Figure A.2.
The acquisition function looks for a GPS signal in frequency steps of 0.5 kHz.
For each frequency step, a parallel code search is performed. The correlation re-
sults are saved and the function proceeds with the next frequency step. Thus, the
function steps through all frequency bands (user-defined Doppler space). Next the
function looks for a maximum correlation value (correlation peak) in results from
all frequency bins. After the peak is detected, the function looks for the second-
highest correlation peak in the same frequency bin of the highest peak. Then the
ratio of the two peaks is used for the signal detection rule. The ratio is compared
to the value preset in the receiver variable acq_threshold .
The detector does not depend on sampling frequency and therefore is not de-
pendent on the size of peak and noise level.