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4.3 Receiver performance under CRI conditions
As mentioned before, in order to meet the stringent
eLoran performance standards, the impact of CRI
within the system must be greatly reduced. Several
strategies concerning how the receiver can reduce
the effects of CRI have been described in the litera-
ture (Pelgrum 2005). There are two prevalent CRI
mitigation techniques, commonly referred to as CRI
cancelling and CRI blanking.
eLoran employs all-in-view receivers capable of
simultaneously tracking signals of many rates. When
an eLoran signal is being tracked, a footprint of the
received pulse waveform is available. With cancel-
ling, the receiver uses this footprint to reconstruct
accurate replicas of the individual signals and sup-
press the signals of all unwanted rates (Esti-
mate & Subtract). This allows the receiver to miti-
gate the effects of CRI almost perfectly, however the
technique has its limitations, as will be shown short-
ly.
With CRI blanking, the receiver detects the puls-
es likely corrupted by CRI and discards them. The
interference is thus completely suppressed, but the
price we pay is a (sometimes excessive) loss of
tracking energy.
4.3.1 Simulator experiments
In order to assess the effects of CRI on a modern
eLoran receiver, a series of simulator experiments
were conducted in which signals of a selected chain
were disturbed by white Gaussian noise and inter-
fered with signals of another chain at different lev-
els. Figure 6 plots the ToA standard deviation versus
the Signal-to-Interference Ratio (SIR) for a GRI
6731 signal at 30 dB SNR, interfered with the sig-
nals of GRI 7001.
We can see from the plot that for high enough
SIR values, the errors are largely determined by the
Gaussian noise (see the dashed line in Figure 6,
Model 2) and can easily be modelled as described in
the previous subsection.
As the interference grows stronger, the measure-
ment errors gradually increase. This gradual increase
suggests that in the region of relatively weak inter-
ference (SIR above 10 dB) the receiver is using
some kind of cancelling algorithm to mitigate CRI.
Since the signal replicas used in the CRI cancelling
process are mere estimates of the true interfering
waveforms, there is always some residual effect on
our ToA measurements. This effect is more pro-
nounced as the SIR decreases. With SIR values ap-
proaching 10 dB the residual error rises sharply and
when the SIR is further decreased, the receiver ap-
parently switches to CRI blanking. A model for the
transitional region is currently being developed and
will be presented in a follow-up paper. We will now
concentrate solely on the CRI blanking.
4.3.2 Modelling the impact of CRI blanking
As explained above, with CRI blanking all the
colliding pulses are completely removed from the
signal processing. The task of quantifying the impact
on the ToA measurements thus reduces to estimating
the percentage of discarded pulses and decreasing
accordingly the number of samples per ToA meas-
urement in Models 1 to 3 above.
In the following considerations we will ignore the
influence of the ninth master Loran pulse, as well as
any data modulation of the signals. We will assume
that the receiver uses the same blanking strategy as
is used on Loran dual-rated transmitters, i.e. that it
discards all pulses that overlap any part of the blank-
ing interval of the cross-rating pulse groups (see
Figure 7). This is a different approach from the one
in our previous paper (Safar et al. 2010), where we
had assumed that blanking only occurred when indi-
vidual pulses overlap each other.
Let us first consider the case of two interfering
eLoran ground wave signals. It can easily be shown
(Safar et al. 2009) that the average portion of
blanked pulses of the desired signal, or the blanking
loss, can be calculated as:
, (4)
where w
d
is the pulse width for the desired signal, w
i
is the width of the blanking interval for the interfer-
ing signal, and T
GRI,i
is the length of the group repe-
tition interval of the interfering station. In our anal-
yses we set w
d
= 250 µsec, and w
i
= 9500 µsec.
Figure 7. CRI blanking. Dashed line shows the blanking inter-
val extending over the pulse group of the unwanted cross-
rating signal. In this example, samples of the first three pulses
of the second group will be discarded.
When analysing real-world eLoran systems we
also need to evaluate the blanking loss due to multi-
ple cross-rating stations, L
b,rx
. In this case, the eval-
uation needs to be broken down into two stages.
First, we calculate the blanking loss due to stations
of individual GRIs, L
b,rx,gri
, by summing the contri-
butions of individual stations, operating on a given