II-1 17 May 2004 ICASSP Tutorial Sensor Networks, Aeroacoustics, and Signal Processing Part II:...
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Transcript of II-1 17 May 2004 ICASSP Tutorial Sensor Networks, Aeroacoustics, and Signal Processing Part II:...
17 May 2004 ICASSP Tutorial II-1
Sensor Networks, Aeroacoustics,
and Signal Processing
Part II: Aeroacoustic Sensor Networks
Brian M. SadlerRichard J. Kozick
17 May 2004
17 May 2004 ICASSP Tutorial II-2
Heterogeneous Network of Acoustic Sensors
CentralNode?
Source
One mic.
Sensorarray
10’s to 100’s of mrangeIssues:
•Centralized versus distributed processing•Minimize comms., energy•Triangulation, TDOA?•Effects of acoustic prop.•(Node localization, sensor layout & density, Classification, tracking)
17 May 2004 ICASSP Tutorial II-3
Acoustic Source & Propagation
SPECTROGRAM OF SENSOR 1 (dB)
TIME (sec)
FR
EQ
UE
NC
Y (
Hz)
120 125 130 135 140 145 150 155
0
50
100
150
200
250
Source Turbulence scatters wavefronts
Signalat sensor(mic.)
Source:•Loud•Spectral lines
One sensor:•Detection•Doppler estimation•Harmonic signature
DSP
17 May 2004 ICASSP Tutorial II-4
More Sophisticated Node:Sensor Array
DSP
1 m apertureor (much) less
Issues:•Array size/baseline
Coherence decreases with larger spacing•AOA estimation accuracy with turbulent scattering•What can we do with a hockey-puck sensor?
Small, covert, easily deployablePerformance? Wavelength ~ 1 to 10 m
17 May 2004 ICASSP Tutorial II-5
Why Acoustics?• Advantages:
– Mature sensor technology (microphones)– Low data bandwidth: ~[30, 250] Hz
Sophisticated, real-time signal proc.– Loud sources, difficult to hide: vehicles, aircraft, ballistics
• Challenges:– Ultra-wideband regime: 157% fractional BW– λ in [1.3, 11] m Small array baselines– Propagation: turbulence, weather, (multipath)– Minimize network communications
• We bound the space of useful system design and performance [Kozick2004a]
– with respect to SNR, range, frequency, bandwidth, observation time, propagation conditions (weather), sensor layout, source motion/track
– evaluate performance of algorithms (simulated & measured data)
Distinct fromRF arrays
17 May 2004 ICASSP Tutorial II-6
Brief History of Acoustics in Military [Namorato2000]
• Trumpeting down walls of Jericho• Chinese, Roman (B.C.)• Civil War: Generals used guns to
signal attacks• World War 1:
– Science of acoustics developed– Air & underwater acoustic systems
• World War 2: Mine actuating, submarine detection
• Many developments …• Army Research Laboratory, 1991-
present [Srour1995]– Hardware and software for
detection, localization, tracking, and classification
– Extensive field testing in various environments with many vehicle types
17 May 2004 ICASSP Tutorial II-7
• A little more background:– Source characteristics– Sound propagation through turbulent atmosphere
• Detection of sources (Saturation, • Array processing
– Coherence loss from turbulence (Coherence, )– Array size: 2 sensors source AOA– Array of arrays source localization
Distributed processing? Data to transmit?Triangulation/TDOA Minimize comms.
Remainder of Tutorial
Experimentallyvalidatedmodels
17 May 2004 ICASSP Tutorial II-8
Source Characteristics• Ground vehicles (tanks, trucks), aircraft (rotary,
jet), commercial vehicles, elephant herds LOUD
• Main contributors to source sound:– Rotating machinery: Engines, aircraft blades– Tires and “tread slap” (spectral lines)– Vibrating surfaces
• Internal combustion engines: Sum-of-harmonics due to cylinder firing
• Turbine engines: Broadband “whine”• Key features: Spectral lines and high SNR
Distinct fromunderwater
17 May 2004 ICASSP Tutorial II-9
+/- 350 m from Closest Point of Approach (CPA)
TIME (sec)
HighSNR
17 May 2004 ICASSP Tutorial II-10
SPECTROGRAM OF SENSOR 1 (dB)
TIME (sec)
FR
EQ
UE
NC
Y (
Hz)
60 65 70 75 80 85 90 95
0
50
100
150
200
250
+/- 85 m from CPA
CPA = 5 m15 km/hr
Time
Fre
que
ncy
-20 -15 -10 -5 0 5 10 15 2013
13.5
14
14.5
15
15.5
16Gv2c1007: FUNDAMENTAL FREQUENCY ESTIMATE
FR
EQ
. (H
z)
BLOCK FROM CPA
Doppler shift:0.5 Hz @ 14.5 Hz 3 Hz @ 87 Hz
Harmonic lines
17 May 2004 ICASSP Tutorial II-11
Overview of Propagation Issues
• “Long” propagation time: 1 s per 330 m
• Additive noise: Thermal, Gaussian(also wind and directional interference)
• Scattering from random inhomogeneities (turbulence) temporal & spatial correl.
– Random fluctuations in amplitude: – Spatial coherence loss (>1 sensor):
• Transmission loss: Attenuation by spherical spreading and other factors
17 May 2004 ICASSP Tutorial II-12
Transmission Loss
• Energy is diminished from Sref (at 1 m from source) to S at sensor:
– Spherical spreading– Refraction (wind, temp. gradients)– Ground interactions– Molecular absorption
• We model S as a deterministic parameter: Average signal energy
LowPassFilter
NumericalSolution[Wilson2002]
+/- 125 m from CPA
[Embleton1996]
17 May 2004 ICASSP Tutorial II-13
Measured Aeroacoustic Data
17 May 2004 ICASSP Tutorial II-14
Fluctuations due to turbulence
Measured SNR vs. Range & Frequency
Aspect dependence
SNR ~ 50 dBat CPA
Time
dB
17 May 2004 ICASSP Tutorial II-15
Outline
• Detection of sources– Saturation, – Detection performance
• Array processing – Coherence loss from
turbulence, – Array size: 2 sensors
Source AOA– Array of arrays:
Source localization
Triangulation/TDOA, minimize comms.
Sinusoidal signal emitted by moving source:
tfSts o2cos)( refref
)()()( twtstz Signal at the sensor:
1. Propagation delay, 2. Additive noise 3. Transmission loss4. Turbulent scattering
17 May 2004 ICASSP Tutorial II-16
Sensor Signal: No Scattering
• Sensor signal with transmission loss,propagation delay, and AWG noise:
• Complex envelope at frequency fo
– Spectrum at fo shifted to 0 Hz
– FFT amplitude at fo
n timepropagatio ,2cos)(
),()(
tfSts
Tttttwtstz
o
oo
)(~exp
)(~exp)(~
twjS
twjStz o
17 May 2004 ICASSP Tutorial II-17
Sensor Signal: With Scattering• A fraction, , of the signal energy is scattered
from a pure sinusoid into a zero-mean, narrowband, Gaussian random process, :
• Saturation parameter, in [0, 1] – Varies w/ source range, frequency, and
meteorological conditions (sunny, windy)– Based on physical modeling of sound propagation
through random, inhomogeneous medium
• Easier to see scattering effect with a picture:
)(~exp)(~exp1)(~ twjtvSjStz
)(~ tv
[Norris2001, Wilson2002a]
17 May 2004 ICASSP Tutorial II-18
AWGN, 2No
-B/2 B/20
(1- )S
Bv
Area= S
-B/2 B/20
(1- )S
Bv
S
• Study detection of source, w/ respect to– Saturation, (analogous to Rayleigh/Rician fading in comms.)– Processing bandwidth, B, and observation time, T– SNR = S / (2 No B)
– Scattering bandwidth, Bv < 1 Hz (correlation time ~ 1/Bv > 1 sec)
– Number of independent samples ~ (T Bv) often small
• Scattering ( > 0) causes signal energy fluctuations
Weak Scattering: ~ 0 Strong Scattering: ~ 1
Freq.
Power SpectralDensity (PSD)
17 May 2004 ICASSP Tutorial II-19
Probability Distributions
• Complex amplitude has complex Gaussian PDF with non-zero mean:
• Energy has non-central -squared PDF with 2 d.o.f.
• has Rice PDF
2,-1CN~~ SSez j
-20 -15 -10 -5 0 5 100
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
ENERGY, 10 log10
(P) (dB)
PR
OB
AB
ILIT
Y D
EN
SIT
Y
PDF OF RECEIVED ENERGY (S=1, SNR = 30 dB)
= 0.02
0.04
0.08
0.20
0.50
1.00
2~zP
2,-1CN~~ SSez j
P
(Experimental validation in [Daigle1983, Bass1991, Norris2001])
17 May 2004 ICASSP Tutorial II-20
Saturation vs. Frequency & Range• Saturation depends on [Ostashev2000]:
– Weather conditions (sunny/cloudy), but varies little with wind speed
– Source frequency and range do
21
27
27
weather
Hz]500,30[2
,
cloudymostly ,2
1042.1
sunnymostly ,2
1003.4
2exp1
(rad/sec)frequency (m), source of range
o
o
d
d
Theoretical forms
Constants from numerical evaluation of particular conditions
17 May 2004 ICASSP Tutorial II-21
50 100 150 200 250 300 350 400 450 5000
0.5
1
SA
TU
RA
TIO
N,
MOSTLY SUNNY
do = 10 m50 m
100 m
200 m500 m
50 100 150 200 250 300 350 400 450 5000.2
0.4
0.6
0.8
1
FREQUENCY (Hz)
SA
TU
RA
TIO
N,
MOSTLY SUNNY
1 km
2 km
5 km
10 km
50 100 150 200 250 300 350 400 450 5000
0.5
1
SA
TU
RA
TIO
N,
MOSTLY CLOUDY
do = 10 m
50 m100 m200 m500 m
50 100 150 200 250 300 350 400 450 5000.2
0.4
0.6
0.8
1
FREQUENCY (Hz)
SA
TU
RA
TIO
N,
MOSTLY CLOUDY
1 km
2 km
5 km
10 km
Turbulence effects are small only for very short range and low frequency
Fully scattered
Saturation variesover entire range[0, 1] for typicalrange & freq.values
17 May 2004 ICASSP Tutorial II-22
Detection Performance
-5 0 5 10 15 20 25 300
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
SNR (dB)
PD
PROBABILITY OF DETECTION, PFA
= 0.01
= 0 = 0.1 = 0.3 = 1
PD = probability of detectionPFA = probability of false alarm
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10.985
0.99
0.995
1
PD
PROBABILITY OF DETECTION, PFA
= 0.01
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0.7
0.8
0.9
1
SATURATION,
PD
SNR = 20 dBSNR = 15 dBSNR = 10 dB
SNR = 30 dBSNR = 25 dB
Noscattering
Fullscattering
Scattering beginsto limit performance
17 May 2004 ICASSP Tutorial II-23
102
103
104
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
RANGE (m)P
D
PROB. OF DETECTION WITH SNR 1/RANGE2
CLOUDY, 40 Hz SUNNY, 40 HzCLOUDY, 200 Hz SUNNY, 200 Hz
101
102
103
104
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
RANGE (m)
SA
TU
RA
TIO
N,
SATURATION VARIATION WITH WEATHER AND FREQUENCY
CLOUDY, 40 Hz SUNNY, 40 HzCLOUDY, 200 Hz SUNNY, 200 Hz
200 Hz40 Hz
Lower frequenciesdetected at largerranges
Detection Performance with Range
SNR = 50 dB at 10 m rangeSNR ~ 1/(range)2
Saturation increases with•Range•Frequency•Temperature (sunny)
1 km
17 May 2004 ICASSP Tutorial II-24
Detection Extensions• Sensor networks: Detection queuing
(wake-up) of more sophisticated sensors• Multiple snapshots & frequencies
– Source motion & nonstationarities– Coherence time of scattering
• Multiple sensors with different SNR and – Distributed detection, what to communicate?– Required sensor density for reliable detection– Source localization based on energy level at the
sensors [Pham2003]
Physical models for cross-freq coherenceand coherence timeare in preliminary stage[Norris2001, Havelock1998]
17 May 2004 ICASSP Tutorial II-25
Outline
• Detection of sources– Saturation, – Detection performance
• Array processing – Coherence loss from
turbulence, – Array size: 2 sensors
Source AOA– Array of arrays:
Source localization
Triangulation/TDOA, minimize comms.
Coherence, , depends on
•Sensor separation, •Source frequency, •Source range, do
•Weather conditions: sunny/cloudy, wind speed
17 May 2004 ICASSP Tutorial II-26
Signal Model forTwo Sensors
[0,1] Coherence ,1
1
[0,1] Saturation
sin)/(phase
arcsinAOA,
exp
1
,-1CN~~
~~ 2
2
1
Γ
a
IaaΓaz
o
o
Hj
c
c
j
SSez
z
sensor spacing
AOA
Turbulence effects
Perfect plane wave: = 0 or 1 = 1
17 May 2004 ICASSP Tutorial II-27
Model for Coherence,
• Assume AOA = 0, freq. in [30, 500] Hz
• Recall saturation model:
• Coherence model [Ostashev2000]:
sensor spacing
do = range
od21 weather2exp1
2
2
2
2
22
3/522
3
22137.0weather
10
,1weatherexp
o
v
o
T
o
effo
c
C
T
C
c
Ld
Velocityfluctuations (wind)
Temperaturefluctuations
0 with freq., sensorspacing, and range
17 May 2004 ICASSP Tutorial II-28
2
2
2
2
22
3/522
3
22137.0weather
10
,1weatherexp
o
v
o
T
o
effo
c
C
T
C
c
Ld
Velocityfluctuations (wind)
Temperaturefluctuations
Depends on wind leveland sunny/cloudy
(From [Kozick2004a], based on[Ostashev2000, Wilson2000])
17 May 2004 ICASSP Tutorial II-29
Assumptions for Model Validity [Kozick2004a]
• Line of sight propagation (no multipath) appropriate for flat, open terrain
• AWGN is independent from sensor to sensor ignores wind noise, directional interference
• Scattered process is complex, circular, Gaussian [Daigle1983, Bass1991, Norris2001]
• Wavefronts arrive at array aperture with near-normal incidence • Sensor spacing, , resides in the inertial subrange of the
turbulence:
smallest turbulent eddies << << largest turbulent eddies = Leff
effo
effo
Ld
Ld
for 1weatherexp
for 01weatherexp
3/522
3/522
17 May 2004 ICASSP Tutorial II-30
50 100 150 200 250 300 350 400 450 5000.97
0.98
0.99
1
CO
HE
RE
NC
E,
MOSTLY SUNNY, MODERATE WIND
50 m100 m
200 m
500 m
50 100 150 200 250 300 350 400 450 5000.5
0.6
0.7
0.8
0.9
FREQUENCY (Hz)
CO
HE
RE
NC
E,
MOSTLY SUNNY, MODERATE WIND
do = 10 m
1 km2 km
5 km
10 km
50 100 150 200 250 300 350 400 450 5000.97
0.98
0.99
1
CO
HE
RE
NC
E,
MOSTLY CLOUDY, MODERATE WIND50 m
100 m200 m
500 m
50 100 150 200 250 300 350 400 450 500
0.7
0.8
0.9
1
FREQUENCY (Hz)
CO
HE
RE
NC
E,
MOSTLY CLOUDY, MODERATE WIND
do = 10 m
1 km2 km
5 km
10 km
Coherence, , versus frequencyand range forsensor spacing = 12 inches
> 0.99 for range < 100 m.Is this “good”?
Curves shiftup w/ less wind,down w/ more wind
17 May 2004 ICASSP Tutorial II-31
Outline
• Detection of sources– Saturation, – Detection performance
• Array processing – Coherence loss from
turbulence, – Array size: 2 sensors
Source AOA– Array of arrays:
Source localization
Triangulation/TDOA, minimize comms.
SenTech HE01 acoustic sensor [Prado2002]
• Freq. in [30, 250] Hz in [1.3, 11] m
• Angle of arrival (AOA) accuracy w.r.t.– Array aperture size
– Turbulence (, )
• Small aperture:– Easier to deploy
– More covert
– Better coherence
– How small can we go?
17 May 2004 ICASSP Tutorial II-32
Impact on AOA Estimation
• How does the turbulence (, ) affect AOA estimation accuracy? [Sadler2004]
– Cramer-Rao lower bound (CRB), simulated RMSE– Achievable accuracy with small arrays?
1-2
2
2
22
SNR-1SNR2
-1-1SNR2
-1SNR2SNR1
-1SNR2
12
ˆCRBˆCRB,1ˆCRB
J
cJoLarger sensor
spacing, :DESIRABLE
BAD!
phase
arcsinAOA
exp
1
oc
ja
17 May 2004 ICASSP Tutorial II-33
Special Cases of CRB
• No scattering (ideal plane wave model):
• High SNR, with scattering:
SNR2J
SNR21 and 1 SNRfor -1
1-1
2
-1
-1-12
-1SNR2
-12
22
2
2
2
22
J
SNR-limitedperformance
Coherence-limitedperformance
If SNR = 30 dB, then < 0.9989995 limits performance!
17 May 2004 ICASSP Tutorial II-34
Phase CRB with Scattering ( , )
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.5
1
1.5
2
2.5
3
COHERENCE,
sqrt
[C
RB
()]
(ra
d)
SNR = 30 dB
= 00.05
0.25
0.5
0.75
0.9
0.95
1.0
Idealplanewave
Coherence loss < 1 is significantwhen saturation > 0.1
17 May 2004 ICASSP Tutorial II-35
50 100 150 200 250 300 350 400 450 5000
5
10
15
20
25
30
35
40
FREQUENCY (Hz)
sqrt
[C
RB
()]
(d
eg)
MOSTLY SUNNY, STRONG WIND
do = 10 m
100 m
500 m
1 km
2 km
5 km
10 km
CRB on AOA Estimation
SNR = 30 dB
for all ranges
Sensor spacing
= 12 in.Increasingrange (fixed SNR)
Aperture-limitedat low frequency
Ideal plane wave modelis accurate for very shortranges ~ 10 m
Coherence-limited at larger ranges
17 May 2004 ICASSP Tutorial II-36
50 100 150 200 250 300 350 400 450 5000
2
4
6
8
10
12
14
16
18
20
FREQUENCY (Hz)
sqrt
[C
RB
()]
(d
eg)
MOSTLY CLOUDY, LIGHT WIND
do = 10 m
100 m
500 m1 km2 km
5 km
10 km
Cloudy and Less WindSNR = 30 dB
for all ranges
Sensor spacing
= 12 in.
Aperture-limitedat low frequency
Atmospheric conditionshave a large impact onAOA CRBs
Plane wave model isaccurate to 100 m range
17 May 2004 ICASSP Tutorial II-37
Coherence vs. Sensor Spacing
100
101
102
0.995
0.9955
0.996
0.9965
0.997
0.9975
0.998
0.9985
0.999
0.9995
1
SENSOR SPACING, (in.)
CO
HE
RE
NC
E,
SOURCE FREQ. = 100 Hz, RANGE = 200 m, SNR = 30 dB
Omega = 0.80, SUNNY
Omega = 0.43, CLOUDY
MOSTLY SUNNY, LIGHT WIND MOSTLY SUNNY, MODERATE WIND MOSTLY SUNNY, STRONG WIND MOSTLY CLOUDY, LIGHT WIND MOSTLY CLOUDY, MODERATE WINDMOSTLY CLOUDY, STRONG WIND
Lightwind
Strongwind
= 0 for ~ 1,000 in = 25 m
17 May 2004 ICASSP Tutorial II-38
CRB on AOA vs. Sensor Spacing
101
102
0
5
10
15
SENSOR SPACING, (in.)
sqrt
[C
RB
()]
(d
eg)
SOURCE FREQ. = 100 Hz, RANGE = 200 m, SNR = 30 dB
MOSTLY SUNNY, LIGHT WIND MOSTLY SUNNY, STRONG WIND MOSTLY CLOUDY, LIGHT WIND MOSTLY CLOUDY, STRONG WIND IDEAL PLANE WAVE ( = 0, = 1)
= 5 inches
17 May 2004 ICASSP Tutorial II-39
Coherence vs. Sensor Spacing
100
101
102
103
0.8
0.82
0.84
0.86
0.88
0.9
0.92
0.94
0.96
0.98
1
SENSOR SPACING, (in.)
CO
HE
RE
NC
E,
SOURCE FREQ. = 100 Hz, RANGE = 1,000 m, SNR = 20 dB
Omega = 1.00, SUNNY
Omega = 0.94, CLOUDY
MOSTLY SUNNY, LIGHT WIND MOSTLY SUNNY, MODERATE WIND MOSTLY SUNNY, STRONG WIND MOSTLY CLOUDY, LIGHT WIND MOSTLY CLOUDY, MODERATE WINDMOSTLY CLOUDY, STRONG WIND
17 May 2004 ICASSP Tutorial II-40
CRB on AOA vs. Sensor Spacing
101
102
103
0
5
10
15
20
25
30
SENSOR SPACING, (in.)
sqrt
[C
RB
()]
(d
eg)
SOURCE FREQ. = 100 Hz, RANGE = 1,000 m, SNR = 20 dB
MOSTLY SUNNY, LIGHT WIND MOSTLY SUNNY, STRONG WIND MOSTLY CLOUDY, LIGHT WIND MOSTLY CLOUDY, STRONG WIND IDEAL PLANE WAVE ( = 0, = 1)
Coherence lossesdegrade AOA perf.for > 8 feet
Ideal planewave modelis optimistic(poor weather)
Plane waveis OK for good weather
(First noted in[Wilson1998],[Wilson1999])
/2
17 May 2004 ICASSP Tutorial II-41
CRB Achievability
50 100 150 200 250 300 350 400 450 5000
0.5
1
FREQUENCY (Hz)
SA
TU
RA
TIO
N,
SNR = 40 dB, = 3 in, RANGE = 50 m
50 100 150 200 250 300 350 400 450 5000.9997
0.9998
0.9999
1
FREQUENCY (Hz)
CO
HE
RE
NC
E,
50 100 150 200 250 300 350 400 450 5000
5
10
15
FREQUENCY (Hz)
RM
SE
& s
qrt
[CR
B]
ON
AO
A,
(d
eg)
SNR = 40 dB, = 3 in, RANGE = 50 m
PD ESTIMATEML ESTIMATECRB
Coherence is high: > 0.999
Saturation issignificant for most offrequency range
AOA estimators break away from CRB approx.when > 0.1
Aperture-limited
Phase difference estimator:
PDo
PD
PD
c
zz
ˆarcsinˆ :AOA
ˆ :Phase 12
Turbulence prevents performance gain from larger aperture
Scenario:Small Sensor Spacing: = 3 in., SNR = 40 dB, Range = 50 m
17 May 2004 ICASSP Tutorial II-42
AOA Estimation for Harmonic Source
20 40 60 80 100 120 140 160 180 2000
5
10
15
20
25
30
35
AO
A (
deg
)
COMBINED AOA ESTIMATES AT FREQS. 50, 100, 150 Hz
RANGE (m)
RMSE, = 6 insqrt[CRB], = 6 inRMSE, = 3 insqrt[CRB], = 3 in
Equal-strength harmonicsat 50, 100, 150 Hz
SNR = 40 dB at 20 mrange, SNR ~ 1/(range)2
(simple TL)
Sensor spacing = 3 in. and 6 in.
Mostly sunny,moderate wind
One snapshot
= 6 in.
= 3 in.
RMSE
CRB
Achievable AOA accuracy ~ 10’s of degrees for this case
17 May 2004 ICASSP Tutorial II-43
Turbulence Conditions for Three-Harmonic Example
Coherence is ~ 1,but still limits performance.
Strongscattering
17 May 2004 ICASSP Tutorial II-44
Summary of AOA Estimation• CRB analysis of AOA estimation
– Tradeoff: larger aperture vs. coherence loss– Ideal plane wave model is overly optimistic for
longer source ranges– Performance varies significantly with weather cond.– Important to consider turbulence effects
• AOA algorithms do not achieve the CRB in turbulence ( > 0.1) with one snapshot
• Similar results obtained for circular arrays with > 2 sensors [Sadler2004]
17 May 2004 ICASSP Tutorial II-45
Outline
• Detection of sources– Saturation, – Detection performance
• Array processing – Coherence loss from
turbulence, – Array size: 2 sensors
Source AOA– Array of arrays:
Source localization
Triangulation/TDOA, minimize comms.
• Issues:– Comm. bandwidth
– Distributed processing
– Exploit long baselines?
– Time sync. among arrays
• Model assumptions:– Individual arrays:
* Perfect coherence* Far-field
– Between arrays:* Partial coherence* Different power spectra* Near-field
17 May 2004 ICASSP Tutorial II-46
Fusion Node
Source
AOA
AOA
AOA
TransmitAOAs
Noncoherenttriangulationof AOAs
Fusion Node
Source
Transmitraw data from all sensors
Optimum,coherentprocessing
Fusion Node
Source
AOA &TDOA AOA &
TDOA
AOA
TransmitAOAs & TDOAs
Triangulationof AOAs andTDOAs
Transmit raw data from one sensorto other nodes
1) Triangulate AOAs:
•Comms.: Low•Distributed processing•Coarse time sync.
2) Fully centralized
•Comms.: High•Centralized processing •Fine time sync. req’d•Near-field w.r.t. arrays
3) AOAs & TDOAs:
•Comms.: Medium•Distributed processing•Fine time sync. req’d•Alt.: Each array xmits raw data from 1 sensor
Three Localization Schemes
17 May 2004 ICASSP Tutorial II-47
1. Triangulation of AOAs Minimal comms.
2. Fully centralized Maximum comms.
3. #1 + time delay estimation (TDE) Raw data from 1
sensor•Schemes 2 & 3 have same CRB! •Results are coherence sensitive:
coherence improves CRB over
AOA triangulation•Are the CRBs achievable?
Localization Cramer-Rao Bounds [Kozick2004b]
AOAtriangulation
Parameters:3 arrays, 7 elements, 8 ft. diam.Narrowband (49.5 to 50.5 Hz)SNR = 16 dB at each sensor0.5 sec. observation time
17 May 2004 ICASSP Tutorial II-48
Ziv-Zakai Bound on TDE
Threshold coherence to attain CRB:
•Function(SNR, % BW, TB product, coherence)
•Extends [WeissWeinstein83] to TDE with
partially-coherent signals TB product
Fract BW
SNR
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Threshold Coherence Simulation
Breakaway from CRB is accurately predictedby threshold coherence value
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Threshold Coherence
Narrowband sourcerequires perfect coherence
Doubling fractional BW time-BW product reduced by factor of ~ 10f0 = 50 Hz, f = 5 Hz T>100 s
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TDE Experiment – Harmonic Source
Moderate coherence,small bandwidth
TDE is not possible between arrays
TDE works within an array(sensor spacing < 8 feet)
17 May 2004 ICASSP Tutorial II-52
TDE Experiment – Wideband Source
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TDE Experiment – Wideband Source
Measured coherenceexceeds thresholdcoherence ~ 0.8 for this case Accurate localization from TDEs
17 May 2004 ICASSP Tutorial II-54
Summary of Array of Arrays
• Threshold coherence analysis tells conditions when joint, coherent processing improves localization accuracy
• Pairwise TDE between arrays captures localization information reduced comms.
• Incoherent triangulation of AOAs is optimum for narrowband, harmonic sources
17 May 2004 ICASSP Tutorial II-55
Odds & EndsOther Sensing Approaches
• Infrasonics: f < 30 Hz, λ > 11 m [Bedard2000]
– Over the horizon propagation via ducting from temperature gradients
– Wind noise is very high– Propagation models may be lacking
• Fuse acoustics with other low-BW sensor modalities
– Seismic has proven useful for heavy, loud vehicles and aircraft (also footsteps)
– Vector magnetic sensors are emerging
17 May 2004 ICASSP Tutorial II-56
Odds & EndsOther Processing
• Doppler estimation: [Kozick2004c]– Localize based on differential Doppler from multiple
sensors (combine with AOAs?)– Not sensitive to coherence– Simple, and exploits spectral lines in source– We have analyzed CRBs and algorithms
• Tracking of moving sources [see Biblio.]
• Classification based on harmonic ampls. [see Biblio.]
– Harmonic amplitudes fluctuate– Exploit aspect angle differences of sources?
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Summary
• Source characteristics:Spectral lines, ultra-wideband, high SNR
• Propagation: dominated by turbulent scattering
– Amplitude & phase fluctuations (saturation, )
– Spatial coherence loss (coherence, )
– Depends on freq., range, weather, sensor spacing
• Signal processing:– Coherence losses limit AOA and
TDE performance Ideal plane wave is overly optimistic
– Implications for detection and array aperture size
– Sensor network: comms. & distributed processing
CentralNode?
Source
Provided a detailed case-study ofa particular sensor network.
17 May 2004 ICASSP Tutorial II-58
Questions?
Thanks for attending!