Global Positioning System Reference
In-Depth Information
•
Carrier phase measurements are no longer possible (or in reality certainly very complex
to carry out) since the skips that are the basis of the method, mean that the phases are
lost at each transition, leading to the need for a new search for the integer ambiguity
number at each transition. Thus a few meters of accuracy is the goal of this system:
enough for the continuity of service, but improvement directions will not be easy to
find.
•
The sequential scheme is a problem when one wants to address dynamic positioning
since the time the cycle takes should be taken into account in the displacement. This is
quite complex to implement and only slow movements can be dealt with. This is
acceptable for pedestrians in a commercial mall, but not for a car in a tunnel. This
sequential technique is very interesting for time based double differencing, but not for
dynamics where additional errors are present.
In addition, the multipath problem (Kaplan 2006) is not solved by the repeater concept and
since code phase measurements are typically carried out, it has to be solved: this is the topic
dealt with in the next section.
4.5 The multipath mitigation technique developed
This paragraph addresses a “short multipath insensitive code loop” (SMICL) mitigation
technique, developed in the context of the repeater based positioning system: the goal is to
mitigate multipath (Jardak and Samama 2010). For this, a new discriminator function has
been proposed which is insensitive to multipath signals having relative delays of less than
146.5 m, equivalent to half a chip length. The standard discriminator used by the Standard
DLL (SDLL, the DLL having a correlator spacing of 1 chip), has a non zero steady state error
in the presence of multipath signals. This is due to the non-symmetrical behaviour of the
composite ACF. As a result, when the early autocorrelation value equals the late
autocorrelation value, the prompt replica is not synchronized with the direct signal, but
rather with the composite signal. Consequently, another discriminator function was found:
the proposed code discriminator compares the early correlation value to an adjusted version
of the prompt one. The result is that the new discriminator expression yields zero when the
prompt reaches the delay of the direct signal component even in presence of multipath rays
of relative delays less than half a chip.
The proposed new expression of the discriminator is given by:
(
) (
)
2
2
2
2
DIEQE
=
+
−
IP QP
'
+
'
(3)
Where
Δ
IE
+
IL
⎧
IP
'
=−
IP
⎪
22
−
Δ
(4)
⎨
Δ
QE
+
QL
⎪
QP
'
=−
QP
⎪
⎩
22
−
Δ
Δ is the correlator spacing and IE, QE, IL, QL, IP and QP are respectively the in phase and
in-quadrature phase of the Early, Late and Prompt classic correlators. Note that modified
prompt correlators are introduced, IP' and QP', as described. Expression (3) is based on the
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