DANISH GPS CENTER
Code Tracking; MultipathGPS Signals And Receiver Technology MM13Darius Plauš[email protected]
DANISH GPS CENTERToday’s Subjects
• Code tracking – Delay Lock Loops (DLL)• Multipath and DLL
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Code TrackingThe Delay Lock Loop (DLL)
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DANISH GPS CENTERCode Tracking Task
• Enhance the accuracy of code phase obtained by acquisition
• Maintain exact alignment of the codes (local and received) – minimize the τi:– To maximum possible power of the received signal– In order to have accurate time of arrival
measurements
• Provide means to measure time of arrival of received signals
iIiiii
ii nDRT
NS
TfTfI +∆⋅⋅⋅⋅
∆∆
= )cos()(2)(
)sin(
0
ϕτππ
2009 4Danish GPS Center
DANISH GPS CENTERCode Tracking Idea
• General idea is the same as in general tracking loop: measure the tracking error, filter it, update local copy of the signal (update frequency of the generator)
• The error detector: the autocorrelation properties of the code are used to “compare” the incoming signal to an advanced and a delayed local copies of the signal
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Early
Prompt
Late
Received code
-1 -0.5 0 0.5 10
0.5
1
Corr
elat
ion
Delay in chips, time
Locally generated
copies of the code
Code Tracking Idea
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Code Tracking Loop Construction• Block diagram of the Early-Late correlators
Local oscillator
PRN code generator
Integrate& dump
IE
IP
IL
Incoming signal
E P L
I Integrate& dump
Integrate& dump
2009 7Danish GPS Center
DANISH GPS CENTERCode Discriminator
• Code discriminators:– The coherent discriminator is the simplest of all :
– A more complicated discriminators are signal amplitude independent e.g. normalized early minus late:
• Feedback the discriminator output to the PRN code generator (through a loop filter), which will adjust the code phase of the early, prompt and late codes
LE IID −=
( ) ( )( ) ( )2222
2222
LLEE
LLEE
QIQIQIQID
++++−+
=
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• Three outputs from the correlators are shown here (in-phase signal)
• The DLL tries to equalize power of the early and late signals by adjusting frequency of the code generator
• Second method is to move in time a pre-generated code– Can cause SNR loss– Low transmission time
tracking resolution
Coherent DLL
2009 9Danish GPS Center
DANISH GPS CENTERNoncoherent DLL
• The signal power is not in the In-phase arm, if the PLL has not locked on to the carrier. In such cases it is needed to track the code phase also in the quadrature arm.
• A plot will demonstrate this situation later• Next slide shows an improved DLL design,
which is insensitive to such problem
( ) ( )( ) ( )2222
2222
LLEE
LLEE
QIQIQIQID
++++−+
=
2009 10Danish GPS Center
DANISH GPS CENTERNoncoherent DLL
Local oscillator
PRN code generator
Integrate& dump IE
IP
IL
Incoming signal
E P L
I Integrate& dump
Integrate& dump
90 QE
QP
QL
E P L
Q
Integrate& dump
Integrate& dump
Integrate& dump
Inputs for the discriminator
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• Output from the 6 correlators, when the PLL is in locked state. It is sufficient to use only inphase correlators for DLL discriminator.
Noncoherent DLL
-6000 -4000 -2000 0 2000 4000 6000
-4000
-2000
0
2000
4000
Discrete-Time Scater Plot
I prompt
Q p
rom
pt
0 100 200 300 400 500 6000
0.5
1
1.5
2
2.5
3x 10
7 Inphase Code Correlators
Time (ms)
Am
plitu
de
0 100 200 300 400 500 6000
0.5
1
1.5
2
2.5
3x 10
7 Quadrature Code Correlators
Time (ms)
Am
plitu
de
IL2
IP2
IE2
QL2
QP2
QE2
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• Output from the correlators if the PLL is not in the lock state. The DLL must rely on outputs from all 6 correlators
Noncoherent DLL
-6000 -4000 -2000 0 2000 4000 6000
-4000
-2000
0
2000
4000
Discrete-Time Scater Plot
I prompt
Q p
rom
pt
0 20 40 60 80 100 1200
0.5
1
1.5
2x 10
7 Inphase Code Correlators
Time (ms)
Am
plitu
de
0 20 40 60 80 100 1200
0.5
1
1.5
2x 10
7 Quadrature Code Correlators
Time (ms)
Am
plitu
de
IL2
IP2
IE2
QL2
QP2
QE2
2009 13Danish GPS Center
DANISH GPS CENTERCode Tracking Loop
• The DLL error measurements are noisier than PLL – Therefore output form PLL is often used to aid DLL– The DLL noise bandwidth is reduced to minimize
noise (even more in case of an aided DLL)– Longer integration time can be used due to low
Doppler values• The new, BOC type signals have multiple
autocorolation peaks – the DLL must make shure that it tracks the main peak
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• Figures show signal autocorrelation and DLL discriminator output (E-L 0.5 chip spacing)
• BOC autocorrelation function and the discriminator have several minima and maxima. This is a problem for the traditional tracking therefore additional checks are introduced.
-0.5
0
0.5
1
1.5
nato
r out
put/c
orre
latio
n
CorrelationDiscriminatoroutput
Differences Between C/A And BOC Tracking
-0.5
0
0.5
1
1.5
nato
r out
put/c
orre
latio
nCorrelationDiscriminatoroutput
2009 15Danish GPS Center
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Multipath
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Tracking Errors Due To Multipath• The multipath signal is a delayed and
attenuated copy of the direct signal. There can be several (M) multipath signals.
• The figures show the constructive and destructive interference of just one multipath signal
Corr
elat
ion
Delay in chips0
0.5
1
Delay in chips
0
1Co
rrel
atio
n
)())())((2cos())(())(()()( 01
tnttvfttCttDtAtx iiii
M
ii +++−−=∑
=
ϕπττ
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• The video shows how the delay of the multipath signal affects correlation results in the receiver
• Blue line – the direct signal• Red line – a multipath
(delayed) signal • Black line – combined result
of the two signals
Multipath Video
Late and early
correlator positions
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0 50 100 150 200 250 300 350 400 450 500-80
-60
-40
-20
0
20
40
60
80
Multipath delay δ [m]
Ran
ging
erro
r τ [m
]
d = 1d = 0.5d = 0.1d = 0.1 (C/A)
• Multipath envelope shows what is the expected ranging error due to a multipath signal (50% of the direct signal power)
• The figure shows multipath envelopes for BOC(1, 1) signal using correlators with different spacing d and one envelope for GPS signal
Multipath Envelope
2009 19Danish GPS Center
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Methods To Minimize Multipath Errors• Antenna design – does not receive signals
that have reflected once• Special antenna design that blocks potential
multipath signals from low elevation angles• Special DLL, discriminator designs• RAIM enables to detect erroneous
measurements
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Methods To Minimize Multipath Errors• DLL with narrow correlator spacing
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DANISH GPS CENTERWide vs. Narrow Bandwidth
2009 22Danish GPS Center
This slide contents is only available to the listeners of our
courses
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Methods To Minimize Multipath Errors• DLL with many correlators – monitors the
(auto)correlation shape
4 4.005 4.01 4.015 4.02 4.025 4.03 4.035
x 104
-2000
0
2000
4000
6000
Samples (time)
Cor
rela
tion
Real correlation result from GNSS SDR
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Methods To Minimize Multipath Errors• Signal design:
– High chipping rate signals have narrow autocorrelation functions – potential for very precise tracking• High chipping rate yields high bandwidth requirements
– BOC signals have narrow autocorrelation functions
0 50 100 150 200 250 300 350 400 450 500-80
-60
-40
-20
0
20
40
60
80
Multipath delay δ [m]
Ran
ging
erro
r τ [m
]
d = 1d = 0.5d = 0.1d = 0.1 (C/A)
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Combining Tracking Tasks
2009 25Danish GPS Center
DANISH GPS CENTERComplete Receiver Channel
Output(nav data bit
stream)
Output(code phase)
Output(carrier phase)
Incoming signal
Local oscillator
PRN code generator
Integrate& dump
IE
IP
IL
E P L
I Integrate& dump
Integrate& dump
90
QE
QP
QL
E P L
Q
Code loop filter
Code loop discriminator
Integrate& dump
Integrate& dump
Integrate& dump
Carrier loop filter
Carrier loop discriminator
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GPS Signal Tracking Movie
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Questions and Exercises
2009 28Danish GPS Center
DANISH GPS CENTERExercise
• Make a DLL tracking loop. Block process in 1ms.
2009 29Danish GPS Center