Global Positioning System Reference
In-Depth Information
values as input, and generate the estimated LOS delay as output, which is then smoothed by
a loop filter. A loop filter is generally used to improve the code delay estimate, reducing the
noise present at the output of the discriminator, and to follow the signal dynamics. The order
of a loop filter determines the ability of the filter to respond to different types of dynamics,
whereas the filter bandwidth ensures that a low bandwidth leads to a good filtering with a
high amount of noise filtered, but also requires that the dynamics of the signals are not too
high. The loop bandwidth is usually determined by the coefficients of the filter, and can thus
be considered as a design parameter for the filter.
In accordance with Kaplan & Hegarty
(2006), the code loop filter is a 1
st
order filter, which can be modeled as:
τ
(
+
)=
τ
(
)+
ω
0
d
(
)
k
1
k
k
,
(2)
where
ω
0
is calculated based on the loop filter bandwidth,
B
n
. As shown in Bhuiyan & Lohan
(2010), the multi-correlator based tracking structure being used by the advanced multipath
mitigation techniques, offers a superior tracking performance to the traditional nEML DLL at
the cost of higher number of correlators.
4. State-of-the-art multipath mitigation techniques
The GNSS community has started the correlation-based multipath mitigation studies since
early 1990s with the advent of Narrow Correlator (NC) or narrow Early-Minus-Late (nEML)
DLL Dierendonck et al. (1992). This section highlights some of the most prominent
state-of-the-art techniques, which have gained a lot of interest in the research community by
now.
4.1 Early-minus-late delay lock loop
The classical correlation-based code tracking structure used in a GNSS receiver is based on a
feedback delay estimator and is implemented via a feedback loop. The most known feedback
delay estimator is the Early-Minus-Late (EML) DLL, where two correlators spaced at one chip
from each other, are used in the receiver in order to form a discriminator function, whose zero
crossings determine the path delays of the received signal Baltersee et al. (2001); Bischoff et al.
(2002); Chen & Davisson (1994); Fine & Wilson (1999); Fock et al. (2001); Laxton (1996); Lohan
(2003). The classical EML usually fails to cope with multipath propagation Dierendonck
et al. (1992). Therefore, several enhanced EML-based techniques have been introduced in the
literature for last two decades in order to mitigate the impact of multipath, especially in closely
spaced path scenarios. A first approach to reduce the influences of code multipath is based on
the idea of narrowing the spacing between the early and late correlators, i.e., nEML or narrow
correlator Dierendonck et al. (1992); Fenton (1995); Fenton & Dierendonck (1996). The choice
of correlator spacing depends on the receiver's available front-end bandwidth along with the
associated sampling frequency Betz & Kolodziejski (2000). Correlator spacings in the range of
0.05 to 0.2 chips are commercially available for nEML based GPS receivers Braasch (2001).
4.2 Double delta (
ΔΔ
) technique
Another family of discriminator-based DLL variants proposed for GNSS receivers is the
so-called Double Delta (
) technique, which uses more than three correlators in the tracking
loop (typically, five correlators: two early, one in-prompt and two late) Irsigler & Eissfeller
ΔΔ
Search WWH ::
Custom Search