High Sensitivity Techniques for GNSS Signal Acquisition - Global Navigation Satellite Systems: Signal,Theory and Applications - page 3

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High Sensitivity Techniques
for GNSS Signal Acquisition

Fabio Dovis 1 and Tung Hai Ta 2
1 Politecnico di Torino
2 Hanoi University of Science and Technology
1 Italy
2 Vietnam

1. Introduction

The requirements of location based and emergency caller localization services spurred by
the E-911 mandate (USA) and the E-112 initiative (EU) have generated the demand for the
availability of Global Navigation Satellite Systems (GNSS) in harsh environments like indoors,
urban canyons or forests where low power signals dominate. This fact has pushed the

development of High Sensitivity (HS) receivers

To produce positioning and timing information, a conventional GNSS receiver must go
through three main stages: code synchronization; navigation data demodulation; and
Position, Velocity and Time (PVT) computation. Code synchronization is in charge of
determining the satellites in view, estimating the transmission code epoch and Doppler shift.
This stage is usually divided into code acquisition and tracking. The former reduces the code
epoch and Doppler shift uncertainties to limited intervals while the latter performs continuous
fine delay estimation. In particular, code acquisition can be very critical because it is the first
operation performed by the receiver. This is the reason for lots of endeavors having been
invested to improve the robustness of the acquisition process toward the HS objective.

Basically, the extension of the coherent integration time is the optimal strategy for improving
the acquisition sensitivity in a processing gain sense. However, there are several limitations to
the extension of the coherent integration time T int . The presence of data-bit transitions, as the
50bps in the present GPS Coarse-Acquisition (C/A) service, modulating the ranging code is
the most impacting. In fact, each transition introduces a sign reversal in successive correlation
blocks, such that their coherent accumulation leads to the potential loss of the correlation peak.
Therefore, the availability of an external-aiding source is crucial to extend T int to be larger
than the data bit duration T b (e.g. for GPS L1 C/A, T b = 20 ms). This approach is referred

as the aided (or assisted) signal acquisition, and it is a part of the Assisted GNSS (A-GNSS)
positioning method defined by different standardization bodies (3GPP, 2008a;b; OMA, 2007).

However, without any external-aiding source, the acquisition stage can use the techniques
so-called post-correlation combination to improve its sensitivity. In general, there are 3

post-correlation combination techniques, namely: coherent, non-coherent and differential