Code Tracking; Multipath - Aalborg Universitetkom.aau.dk/~dpl/courses/mm13_slides.pdf · 2009-10-16 · Code Tracking Task DANISH GPS CENTER • ... Next slide shows an improved DLL

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

2009 2Danish GPS Center

DANISH GPS CENTER

Code TrackingThe Delay Lock Loop (DLL)


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


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


DANISH GPS CENTER

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


DANISH GPS CENTER

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


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

++++−+

=


DANISH GPS CENTER

• 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


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

++++−+

=


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


DANISH GPS CENTER

• 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


DANISH GPS CENTER

• 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


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


DANISH GPS CENTER

• 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


DANISH GPS CENTER

Multipath


DANISH GPS CENTER

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


DANISH GPS CENTER

• 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


DANISH GPS CENTER

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


DANISH GPS CENTER

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


DANISH GPS CENTER

Methods To Minimize Multipath Errors• DLL with narrow correlator spacing


DANISH GPS CENTERWide vs. Narrow Bandwidth


This slide contents is only available to the listeners of our

courses

DANISH GPS CENTER

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


DANISH GPS CENTER

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)


DANISH GPS CENTER

Combining Tracking Tasks


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


DANISH GPS CENTER

GPS Signal Tracking Movie


DANISH GPS CENTER

Questions and Exercises


DANISH GPS CENTERExercise

• Make a DLL tracking loop. Block process in 1ms.


Documents

Code Tracking; Multipath - Aalborg Universitetkom.aau.dk/~dpl/courses/mm13_slides.pdf · 2009-10-16 · Code Tracking Task DANISH GPS CENTER • ... Next slide shows an improved DLL