MATLAB Code - A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach - page 140

acquisition.m

find PRN code
phase, carrier
frequency bin, and
magnitude of
maximum peak

preallocate
arrays

convert PRN
code to the
frequency
domain (DFT)

is there a
signal
present?

no

yes

generate sine/
cosine local
oscillator signals
for baseband
conversion

remove PRN code
modulation and
refine frequency
estimate via DFT

mix raw data
to baseband &
transform to
frequency
domain

compute
acquisition
metric and
record results

multiple baseband
data and PRN code
in the frequency
domain

no

All PRNs
tested?

yes

convert to time
domain via
inverse DFT and
record result

return

all frequency
bins tested?

no yes

FIGURE A.2. Flow diagram of the parallel code phase acquisition algorithm.

If the value of the peak ratio is larger than a specified value, the fine carrier
frequency is found via a postcorrelation FFT approach. This must be done to help
the PLL in the tracking loop to start tracking the signal. A frequency accuracy of
0.5 kHz is too coarse for the PPL to start tracking.
The function parameters are an initial data record, a table with pregenerated
C/A codes, and the settings structure.
The list of acquisition-specific variables contained in the structure settings is

acq_satelliteList A set of satellite PRNs can be specified. Acquisition will be
performed only for the specified satellites. An empty list (default) starts
a search for all available satellites 1-32.