High Sensitivity Techniques for GNSS Signal Acquisition - Global Navigation Satellite Systems: Signal,Theory and Applications - page 23

Fig. 9. Test bed architecture: reference chain (green) and test chain (blue)

5.2 Acquisition procedure

• Step1-Preliminary fast detection of the strongest PRN

In this step, the FFT-based circular correlation stage is used to quickly detect the best PRN
selected on the basis of the predicted C / N 0 and elevation. The value of T coh is 10ms and;
the number of M correlator outputs is then non-coherently combined to achieve a sufficient
post-correlation SNR; 100 Hz (trade-off between frequency resolution and search

complexity), while the code-phase search space spans over a full code period.

f D =

• Step 2 - Determination of the assistance offsets

Code-phase and frequency offsets are caused by: (i) space displacement of test and reference
antennas (mostly code-phase offset); (ii) the time offset between the reference receiver and test
receiver clocks (code-phase offsets); and (iii) the uncertainty on the test receiver LO frequency
(Doppler frequency offset). In this step, these offsets, which are the same for all the PRNs,
can be computed by considering the difference of the preliminary estimates (from step 1) with
those provided by the assistance data.

• Step 3 - Aided long coherent correlation with data wipe-off on weaker PRNs

The offsets obtained with the strong PRN can be used to correct the assistance predictions
and finely determine the code-phase/Doppler frequency of other PRNs at the last step (aided
long coherent correlation), ensuring the best achievable post-correlation SNR by means of
a low-complexity data wipe-off technique.

At this step, the frequency bin size of f D ,3 =

1/ T coh (e.g. 1 s) is used, over a frequency search space 100 Hz wide

(i.e. the residual uncertainty from step 1), and a code-phase search range 6 chips wide. The
knowledge of aiding data would allow for a narrower search space, but the acquisition has to
account for possible residual errors between the true and predicted code phases.

f D ,3 = 1Hzif T coh =

The code-phase resolution is as low as 1 sample (for both step 1 and 3). The reference signal
bandwidth is B = 2.046 MHz chosen to match the main lobe (two-sided) of the GPS signal

spectrum. Therefore the performed tests have been run with a sampling rate f S = 4.092 MHz