Global Positioning System Reference
In-Depth Information
5.6 Slope-based multipath estimator
A simple slope-based multipath mitigation technique, named as Slope-Based Multipath
Estimator (SBME), in Bhuiyan, Lohan & Renfors (2010). Unlike the multipath mitigation
techniques discussed above, SBME does not require a huge number of correlators, rather it
only requires an additional correlator (as compared to a conventional DLL) at the late side
of the correlation function. In fact, SBME is used in conjunction with a nEML tracking loop
Bhuiyan, Lohan & Renfors (2010). It first derives a multipath estimation equation by utilizing
the correlation shape of the ideal normalized correlation function, which is then used to
compensate for the multipath bias of a nEML tracking loop. The derivation of the multipath
estimation equation for BPSK modulated GPS L1 C/A signal can be found in Bhuiyan, Lohan
& Renfors (2010). It is reported in Bhuiyan, Lohan & Renfors (2010) that SBME has superior
multipath mitigation performance than nEML in closely spaced two path channel model.
5.7 C/N
0
-based two-stage delay tracker
A C/N
0
based two-stage delay tracker is a combination of two individual tracking techniques,
namely nEML and HRC (or MGD). The tracking has been divided into two stages based on the
tracking duration and the received signal strength (i.e., C/N
0
). At the first stage of tracking
(for about 0.1 seconds or so), the two-stage delay tracker always starts with a nEML tracking
loop, since it begins to track the signal with a coarsely estimated code delay as obtained
from the acquisition stage. And, at the second or final stage of tracking (i.e., when the DLL
tracking error is around zero), the two-stage delay tracker switches its DLL discriminator from
nEML to HRC (or MGD), since HRC (or MGD) has better multipath mitigation capability
as compared to nEML. While doing so, it has to be ensured that the estimated C/N
0
level
meets a certain threshold set by the two-stage tracker. This is because of the fact that HRC (or
MGD) involves one (or two in case of MGD) more discrimination than NEML, which makes
its discriminator output much noisier than nEML. It has been empirically found that a C/N
0
threshold of 35 dB-Hz can be a good choice, as mentioned in Bhuiyan, Zhang & Lohan (2010).
Therefore, at this fine tracking stage, the two-stage delay tracker switches from nEML to HRC
(or MGD) only when the estimated C/N
0
meets the above criteria (i.e., C/N
0
threshold is
greater than 35 dB-Hz).
An example non-coherent S-curve is shown in Fig. 8 for CBOC(-) modulated signal in single
path static channel Bhuiyan, Zhang & Lohan (2010). The nearest ambiguous zero crossings for
HRC (around
0.54 chips)
in this particular case. This indicates the fact that the probability of locking to any of the side
peaks is much higher for HRC than that of nEML, especially in the initial stage of tracking
when the code delay may not necessarily be near the main peak of the correlation function.
This is the main motivation to choose a nEML tracking at the initial stage for a specific time
duration (for example, 0.1 seconds or so). This will eventually pull the delay tracking error
around zero after the initial stage.
±
0.16 chips) is much closer as compared to that of nEML (around
±
5.8 TK operator combined with a nEML DLL
A combined simplified approach with TK operator and a nEML DLL was implemented in
Bhuiyan & Lohan (2010), in order to justify the feasibility of having a nEML discrimination
after the TK operation on the non-coherent correlation function. In this combined approach,
TK operator is first applied to the non-coherent correlation function,
and then nEML
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