Early Edition: Report on Netted Radar Clock Distribution Measurements J.S. Sandenbergh PhD Student...

Early Edition: Report on Netted Radar Clock Distribution Measurements

J.S. Sandenbergh

PhD Student

Radar Remote Sensing Group, UCT

with the help of Prof. Inggs, Marc Brooker,

Lance Williams, Regine Lord, Shaun Doughty

and many others.

© CSIR 2006 www.csir.co.za

Overview

• Netted Radar: Advantages & Challenges

• RRSG’s GPS Disciplined Oscillators

• Collaboration with UCL

• UCL’s Experimental Netted Radar

• Experiment Objective: Characterizing Clock Stability

• Experimental Setup

• Processing of Data

• Preliminary Results

• Conclusion


Netted Radar: Advantages & Challenges

• Theoretical advantages over mono-static radar:

• Increased detection probability• Better classification / identification• Reduced vulnerability / delectability• Larger illuminating area• etc

#3

#1

#2

• Netted radar requires an accurate knowledge of relative space and time amongst the nodes

• Phase and frequency synchronization across the network is required for full coherent operation [Hume & Baker, 2003]


RRSG’s GPS Disciplined Oscillators

10MHz LVDS

Phase Detector

OCXO

FPGA

DAC Circuitry

GPS Rx

Clk Tx

10x10MHz Sine (100ohm diff)

100MHz LVDS100MHz LVPECL

1Hz Sync Pulse

RS-232 Port to User Interface

GPS Antenna

Power Distribution System

10M

Hz

Sin

e (D

iffe

ren

tial

)

GP

S R

F S

ign

al

DC

Po

wer

220V / 50Hz (35W)

10x Multiplier and Clock Distribution

10MHz OCXO and DAC Steering

TDC


RRSG’s GPS Disciplined Oscillators

[ J. S. Sandenbergh , M. R. Inggs “A Common View GPSDO to Synchronize Netted Radar”, In Proceedings of the International Radar Conference, (2007).]


Collaboration with UCL• UCL developed an experimental netted radar

• 2.4GHz pulse radar consisting of 3 nodes

• [T. Derham, S. Doughty, K. Woodbridge, and C. J. Baker. “Realisation and evaluation of a low cost netted radar system”, In Proceedings of the CIE International Conference on Radar, pp 16–19, (2006).]

• Currently the clock is distributed using 50m long twisted pair cables• This limits the possible geometrical configurations of the radar• Reduces mobility• Hinders quick deployment

• RRSG visited UCL during October/November 2007• Twisted pair cables were replaced with UCT’s GPSDOs• Initial experiments were conducted to compare the performance of the different clocking schemes


Collaboration with UCL

#3

Rx

Tx

#2

Rx

Tx

#1

Rx

Tx100MHz

Tw

isted Pair (50m

)

Tw

isted Pair

(50m)

#3

Rx

Tx

#2

Rx

Tx

#1

Rx

Tx

GPSDO

GPSDO

GPSDO

100MHz

100MHz

100MHz

A. Twisted Pair Clk Distribution B. GPSDO Clk Distribution


UCL’s Experimental Netted Radar

Rx

x

ADC

DDSTx

Clk100MHz

2.4GHz

A. Three netted radar nodes B. Radar Front-end


Objective of ExperimentAllan Deviation is a measure of clock stability

[Allan, D. ”Time and Frequency (Time-Domain) Characterization, Estimation, and Prediction of Precision Clocks and Oscillators” NIST Technical Note 1337, (1987)]

)(tV

t

)(tx

)( tx


Experimental Setup

#3

Rx

Tx

#2

Rx

Tx

#1

Rx

Tx

Splitter

≈ 2.4GHz

#3

Rx

Tx

#2

Rx

Tx

#1

Rx

Tx

Splitter

≈ 2.4GHz

#3

Rx

Tx

#2

Rx

Tx

#1

Rx

Tx

Splitter

≈ 2.4GHz

1. Effect of Receiver 2. Effect of Transmitter 3. Tx/Rx Feed through


Experimental Setup

A. GPSDO connected to a node B. Ready to go!


Processing of Data)(1t

Nod

e

t

PRI Pulse Length

t

)(2t

Nod

e

t


Processing of Data

)(tA

t

)(t

t

)(t

t

)(t

2

)(t

t)(t

•Beat frequency

•Hilbert transform•Extract phase

•Unwrap•Average phase diff

•Pulse-to-pulse phase difference


Preliminary Results : Twisted Pair

0 100 200 300 400 500 600 700 800-30

-25

-20

-15

-10

-5

0

5

10

15Phase Difference vs. Time (Frequency offset removed)

Time [s]

Pha

se [d

egre

es]

Node12Node13Node23

0 100 200 300 400 500 600 700 800-50

0

50

100

150

200

250

300Phase Difference vs. Time

Time [s]

Pha

se [d

egre

es]

Node12Node13Node23


Preliminary Results: GPSDO (free running)

0 100 200 300 400 500 600 700 800-2

0

2

4

6

8

10x 10

4 Phase Difference vs. Time

Time [s]

Pha

se [d

egre

es]

Node12Node13Node23

0 200 400 600 800-3000

-2000

-1000

0

1000

2000


Time [s]

Pha

se [d

egre

es]

Node12

Node13Node23


Preliminary Results: GPSDO (free running)

0 100 200 300 400 500 600 700 800-200

0

200

400

600

800

1000

1200

1400

1600Phase Difference vs. Time

Time [s]

Pha

se [d

egre

es]

Node12

0 100 200 300 400 500 600 700 800-80

-60

-40

-20

0

20

40

60

80


Time [s]

Pha

se [d

egre

es]

Node12


The Team


A. Arrival in the UK B. Setting up camp


The Team


A. Some smiles: we finally have lift-off B. The UCL Team


Conclusion• Measurement data were successfully recorded at UCL

• Rigorous analysis of data has not yet been done (only preliminary)• Initial analysis of the data shows that the twisted pair cables give reliable long term frequency stability • GPSDOs were running non-disciplined which resulted in significant frequency offsets• Initial results indicate that crystals should be well aged to obtain sufficient stability• Results will be compared to results obtained from the FERS simulator

• Valuable experience was gained during initial experiments• Some interfacing problems were identified (some of which are already solved)• Some work is required to get an autonomous ‘sync pulse’ system working

• Future planned measurements• Actual field measurements are planned for the near future

• Aim of research• Test the feasibility of a GPS synchronized quartz frequency standard as a viable solution to low-cost timing distribution in

network based radar


Thank you for your attention.


GPS Time Transfer

#3#1

#2

#4

Technique Time* Frequency* Post-processing

One-way <20ns <2.10-13 None

Common-view Single-Channel <10ns ≈1.10-13 Some

Common-view Multi-Channel <5ns <5.10-14 Some

Carrier phase common-view <500ps <5.10-15 Complex

Achievable Performance for Different GPSTT Techniques

[Lombardi, Nelson, Novick, Zhang] *relative to absolute time


UCT’s GPSDO – Functional Diagram

FPGA(Xilinx Spartan 3)

RS

-232

Por

t

GPS Receiver(Motorola M12+T)

Antenna(Motorola Timing2000)

Time-to-Digital Converter

(ACAM GP2)

20-bit DAC(TI DAC1220)

OCXO(model 8788)

Serial Bus

1Hz

S

PI B

us

EFC

SPI Bus

20MHz

3× LVPECL

2× LVDS

User COMMs

×10(Wenzel LNEOM)

AD9512(optional freq division)

SPI Bus

10MHz (sine)

10MHz (CMOS)

AD9513(optional freq division)

100MHz (sine)


Timing Performance of a Low Cost GPS Receiver


GPSDO Phase Noise Performance

Early Edition: Report on Netted Radar Clock Distribution Measurements J.S. Sandenbergh PhD Student...

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