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transcript
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.
Slide 2 © 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
Slide 3 © CSIR 2006 www.csir.co.za
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]
Slide 4 © CSIR 2006 www.csir.co.za
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
Slide 5 © CSIR 2006 www.csir.co.za
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).]
Slide 6 © CSIR 2006 www.csir.co.za
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
Slide 7 © CSIR 2006 www.csir.co.za
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
Slide 8 © CSIR 2006 www.csir.co.za
UCL’s Experimental Netted Radar
Rx
x
ADC
DDSTx
Clk100MHz
2.4GHz
A. Three netted radar nodes B. Radar Front-end
Slide 9 © CSIR 2006 www.csir.co.za
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
Slide 10 © CSIR 2006 www.csir.co.za
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
Slide 11 © CSIR 2006 www.csir.co.za
Experimental Setup
A. GPSDO connected to a node B. Ready to go!
Slide 12 © CSIR 2006 www.csir.co.za
Processing of Data)(1t
Nod
e
t
PRI Pulse Length
t
)(2t
Nod
e
t
Slide 13 © CSIR 2006 www.csir.co.za
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
Slide 14 © CSIR 2006 www.csir.co.za
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
Slide 15 © CSIR 2006 www.csir.co.za
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
3000Phase Difference vs. Time (Frequency offset removed)
Time [s]
Pha
se [d
egre
es]
Node12
Node13Node23
Slide 16 © CSIR 2006 www.csir.co.za
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
100Phase Difference vs. Time (Frequency offset removed)
Time [s]
Pha
se [d
egre
es]
Node12
Slide 17 © CSIR 2006 www.csir.co.za
The Team
•Pulse-to-pulse phase difference
A. Arrival in the UK B. Setting up camp
Slide 18 © CSIR 2006 www.csir.co.za
The Team
•Pulse-to-pulse phase difference
A. Some smiles: we finally have lift-off B. The UCL Team
Slide 19 © CSIR 2006 www.csir.co.za
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
Slide 20 © CSIR 2006 www.csir.co.za
Thank you for your attention.
Slide 21 © CSIR 2006 www.csir.co.za
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
Slide 22 © CSIR 2006 www.csir.co.za
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)
Slide 23 © CSIR 2006 www.csir.co.za
Timing Performance of a Low Cost GPS Receiver
Slide 24 © CSIR 2006 www.csir.co.za
GPSDO Phase Noise Performance