This four-day course is designed for SONAR systems engineers, combat systems engineers, undersea warfare professionals, and managers who wish to enhance their understanding of passive and active SONAR or become familiar with the "big picture" if they work outside of either discipline. Each topic is presented by instructors with substantial experience at sea. Presentations are illustrated by worked numerical examples using simulated or experimental data describing actual undersea acoustic situations and geometries. Visualization of transmitted waveforms, target interactions, and detector responses is emphasized.
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Fundamentals of Passive and Active Sonar Technical Training Short Course Sampler
1. FUNDAMENTALS OF PASSIVE AND ACTIVE SONAR
Instructor:
Duncan Sheldon, Ph.D.
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3. DECISION: H1/H0 ?
ACTIVE SONAR DETECTION MODEL
POST-DETECTION
PROCESSOR
TRANSMISSION
DETECTOR
PRE-DETECTION
FILTER
DELAY AND
ATTENUATION AMBIENT NOISE RECEPTION
TIME-VARYING
MULTIPATH
TARGET
TIME-VARYING DELAY AND
MULTIPATH ATTENUATION
REVERBERATION 2
4. DEFINITIONS
Acoustic pressure, p: The difference between the total pressure, ptotal, and
the hydrostatic (or undisturbed) pressure po.
ptotal = ptotal(x,y,z;t) Pascals
po = po(x,y,z) or
Newtons/(meter)2
p = p (x,y,z;t) = ptotal - po
Acoustic intensity, I : A vector whose component I •n in the direction of any
unit vector n is the rate at which energy is being
transported in the direction n across a small plane
element perpendicular to n , per unit area of that plane
element.
I ≡ pu ; where u is the fluid velocity at (x,y,z;t).
I = I (x,y,z;t) Watts/(Meter)2
3
5. ACOUSTIC INTENSITY LEVELS EXPRESSED IN DECIBELS
(prms)2
For a plane wave, Iref = ρ c
ref
PRESSURE
1 Pascal = 1 Newton/(1 Meter)2
prms = Root -mean -square pressure
ENERGY ρ = Density
1 Joule = 1 Newton x 1 Meter c = Sonic velocity
POWER prms, ref water = 10- 6 Pascals
1 Watt = 1 Joule/(1Second)
ρ c water = 1.5 x 10 + 6 kgm/(meter 2second)
INTENSITY (or ENERGY FLUX)
Watts/(Meter)2 prms, ref air = 20 x 10- 6 Pascals
Joules/Second /(Meter)2
ρ c air = 430 kgm/(meter 2second)
Intensity level, I, usually refers to a normalized intensity magnitude,
( | I |average /Iref), expressed in decibels. For example, I = 60 dB means
60 dB = 10 log10 (| I | average/ Iref)
6 = log10 (| I | average/ Iref)
I = 10 +6 x Iref [ Watts/(meter2) ] 4
6. ACOUSTIC INTENSITY REFERENCE VALUES: AIR AND WATER
(prms)2
Average acoustic intensity magnitude for a plane wave, | I | =
ρc .
prms = Root -mean -square of acoustic pressure
ρ = Density
c = Sonic velocity
For water: Iref water ≡ | I |ref water
ρ c = 1.5 x 10 + 6 kgm/(meter 2 second)
Iref water ≡ [10 − 6 Pascals] 2 [ρ c]
/ = 0.67 x 10 −18 Watts / (meter)2
water
For air: Iref air ≡ | I |ref air
ρ c = 430 kgm/(meter 2 second)
Iref air ≡ [ 20 x 10 − 6 Pascals] 2 [ρ c]
/ ≈ 10 −12 Watts / (meter)2
air
5
7. DECIBELS MEASURED IN AIR AND WATER
X watts/meter2
100 dB water re 10-6 Pa = 10log10 ( I ref water watts/meter2
)
X watts/meter2
= 10log10 ( )
6.76 x 10-19 watts/meter2
Y watts/meter2
100 dB air re 20 x 10-6 Pa = 10log10 ( )
I ref air watts/meter2
Y watts/meter2
= 10log10 ( )
10-12 watts/meter2
For water: X watts/meter2 = 6.76 x 10-9 watts/meter2
For air: Y watts/meter2 = 10-2 watts/meter2
10-2 watts/meter2
10log10 ( ) = 61.7 dB difference in intensity level.
6.76 x 10-9 watts/meter 2
6
8. REPLICA CORRELATION (1 of 6)
o A cross-correlation sequence can be calculated for any pair of causal
sequences x[k] and y[k], and there is no implied restriction on their relationship:
k = +∞
xcorr[x,y;n] = ∑x[k] y[k -n] ≡ g[n]
k = −∞
where g[n] is used to denote a cross-correlation sequence whose
constituent sequences are understood but not explicitly stated.
k = +∞
INPUT x[k] X ∑x[k] y[k -n] g[n] OUTPUT
k = −∞
REPLICA y[k-n] REPLICA CORRELATOR
o The term ‘replica correlator’ or ‘replica correlation’, is applied when there
is some relationship between the input x[k] and the replica y]k], for example:
x[k]=y[k], i.e, auto-correlation,
x[k] may be the result of applying a Doppler shift to y[k],
x[k] is the result of applying a (fixed) shift n to y[k], i.e., x[k]=y[k-n], or
x[k] = A y[k-n], where A is a constant and the same for all k. When
this is the case, it is helpful to think of x[k] and y[k] as structurally
matched or simply matched. 7
9. REPLICA CORRELATION (2 of 6)
40
x[k], 30 k(2) NON-ZERO
INPUT 20 VALUES OF x[k]
(SIGNAL) 10
k(2) = 4
k(1), FIXED BULK DELAY
k
0
k(1) k(1) + k(2)
y[k], k(2) NON-ZERO
REPLICA VALUES OF y[k]
4
3
2
1 k(2) = 4
k
0 1 2 3
y[k-n],
SHIFTED n INCREASES Each n > 0 produces a
4
REPLICA 2
3 different (shifted)
1
y[k-n] sequence.
k
0 n n+ k(2)
y[k-k(1)],
SHIFTED
REPLICA 4
3
ALIGNED 2
n = k(1) 1
WITH INPUT
k
0 k(1) k(1) + k(2) 8
10. REPLICA CORRELATION (3 of 6)
40
x[k], 30 k(2) NON-ZERO
INPUT 20 VALUES OF x[k]
(SIGNAL) 10
k(2) = 4
k(1) FIXED
k
k(1) k(1) + k(2)
y[k-n],
SHIFTED
n INCREASES 4
y[k-n] when n=k(1)
REPLICA 3 4
2 2 3
1 1
k
n n+ k(2) k(1) k(1) + k(2)
g[n],
k(1) 300
OUTPUT OF
REPLICA 200 200
CORRELATOR n = k(1)-k(2) 110 110 n = k(1)+k(2)
40
40
n
n= k(1)
The cross-correlation g[n] of x[k[ and y[k] is the output of the replica correlator, and
k ≥ k(1)+ k(2)
g[n] = ∑
k=
x[k]y[k − n] where n is the shift of y[k] with respect to x[k].
9
0
11. REPLICA CORRELATION (4 of 6)
40
x[k], 30 k(2) NON-ZERO
INPUT 20 VALUES OF x[k]
(SIGNAL) 10
k(2) = 4
k(1) FIXED
k
k(1) k(1) + k(2)
Instead of writing
k ≥ k(1)+ k(2)
g[n] = ∑
k=
x[k]y[k − n]
0
we can write
k = +∞
g[n] = ∑∞ y[k −n]
k=−
x[k]
Since x[k] = 0 if k < 0 or k > k(1) + k(2).
10
12. REPLICA CORRELATION (5 of 6)
SIGNAL k = +∞
INPUT +
NOISE
x[k] X ∑x[k] y[k -n] g[n] OUTPUT
k = −∞
REPLICA y[k-n] REPLICA CORRELATOR
SIGNAL+NOISE
REPLICA
2 k = +∞ 2
| g[n]| = | ∑∞ y[k −n] |
k=−
x[k]
o The instantaneous power output of the replica correlator |g[n]|2 is calculated
for each n and is the detection statistic, i.e., the statistic used to decide if a
signal matching, or nearly matching, the replica is embedded in the noise.
o A signal is declared to be present if | g[n] |2 exceeds a threshold:
INSTANTANEOUS POWER
OUTPUT OF REPLICA THRESHOLD
CORRELATOR,
SIGNAL + NOISE
2
|g[n]|
n
n = k(1)
BULK DELAY, k(1) 11
13. REPLICA CORRELATION (6 of 6)
SIGNAL k = +∞
INPUT +
NOISE
x[k] X ∑x[k] y[k -n] g[n] OUTPUT
k = −∞
REPLICA y[k-n] REPLICA CORRELATOR
SIGNAL+NOISE
REPLICA
2 k = +∞ 2
| g[n]| = | ∑∞ y[k −n] |
k=−
x[k]
INSTANTANEOUS POWER THRESHOLD
OUTPUT OF REPLICA
CORRELATOR,
SIGNAL + NOISE
2
|g[n]|
n
n = k(1)
BULK DELAY, k(1)
If zero-mean, statistically independent Gaussian noise masks the
input signal, cross-correlation of the received data with a replica
matching the signal is the optimum receiver structure.
12
14. REPLICA CORRELATION FOR A CONTINUOUS CW WAVEFORM (1 of 4)
TRANSMISSION
s(t) t
BULK DELAY
t =τ BD ECHO
s ( t − τ BD ) t
AMBIENT NOISE
n (t ) t
RECEIVED DATA
s ( t − τBD ) + n ( t ) t
REPLICAS OF THE
t = τ RD
TRANSMISSION FOR
s(t-τRD)
(1)
(1) DIFFERENT REPLICA DELAYS
REPLICA
t
t = τ RD
DELAYS
s(t-τRD)
(2)
(2)
t
t=0 13
15. REPLICA CORRELATION FOR A CONTINUOUS CW WAVEFORM (2 of 4)
ECHO
t =τ BD RECEIVED DATA
s ( t − τBD ) + n ( t )
t
REPLICAS OF THE
t = τ RD
(1) TRANSMISSION FOR
DIFFERENT REPLICA DELAYS
s(t-τRD)
(1)
t
t = τ RD
REPLICA (2)
DELAYS
s(t-τRD)
(2) t
t=0
Shifted replicas, each with the same shape but different delays,
τRD, i=1,2, …, n,
(i)
are under consideration,
Only one record of the received data is under consideration.
14
16. REPLICA CORRELATION FOR A CONTINUOUS CW WAVEFORM (3 of 4)
INCREASING
REPLICA DELAY REPLICA OF THE
TRANSMISSION FOR
τ RD
Replica
SOME REPLICA DELAY
Time, t
τ BD
Signal
τd τd = τBD - τRD
Output of the replica correlator is the product of the
echo and the replica for time delays τd.
Note triangular
shape of upper
Peak value is peaks.
at τ d= 0
−τd Time delay, τd
Replica correlator output
and its envelope
τd =0 for a CW waveform 15
17. REPLICA CORRELATION FOR A CONTINUOUS CW WAVEFORM (4 of 4)
INCREASING
REPLICA DELAY REPLICA OF THE
TRANSMISSION FOR
τ
Replica
SOME REPLICA DELAY RD
Time, t
Received
data
τd
Time delay, τd
−τd
Replica correlator output
τd =0 for a CW waveform
plus ambient noise. 16
18. REPLICA CORRELATOR OUTPUT FOR A CW
WAVEFORM MASKED BY NOISE (1 of 3)
15
10
5 Independent normally
0 distributed noise only.
-5
INSTANTANEOUS
AMPLITUDES
-10
-15 0 200 400 600 800 1000 1200
SAMPLED DATA POINTS
Sine wave 240 samples
15
long with zero-padding on
10 each side.
5
0
Normally distributed
-5 noise plus sine wave.
-10
-15
Replica of sine wave
0 200 400 600 800 1000 1200
SAMPLED DATA POINTS (offset by -5).
150
Sine wave
CORRELATOR
100
σNOISE = 4.0 amplitude = 1.0
REPLICA
OUTPUT
50
0
Sine wave power
-50
= 0.516 = 132
-100 Noise power
-150
10 log( 132) = -15 dB
-600 -400 -200 0 200 400 600
17
TIME DELAY τd IN DATA SAMPLES
19. REPLICA CORRELATOR OUTPUT FOR A CW
WAVEFORM MASKED BY NOISE (2 of 3)
15
10
Independent
5
normally
0
distributed
-5 noise only.
INSTANTANEOUS
-10
AMPLITUDES
-15
0 500 1000 1500 2000 2500 Sine wave 480
SAMPLED DATA POINTS samples long with
15 zero-padding on
10 each side.
5
Normally
0 distributed noise
-5 plus sine wave.
-10
Replica of sine
-15
0 500 1000 1500 2000 2500 wave (offset -5).
SAMPLED DATA POINTS
REPLICA CORRELATOR
300
200 As on previous
slide,
OUTPUT
100
0 Sine wave power
-100 Noise power
-200
=> -15 dB
-300
0
-1000 -500 0 500 1000 18
TIME DELAY τd IN DATA SAMPLES
20. REPLICA CORRELATOR OUTPUT FOR A CW
WAVEFORM MASKED BY NOISE (3 of 3)
15
10
Independent
5
normally
0
distributed
-5 noise only.
INSTANTANEOUS
-10
AMPLITUDES
-15
0 500 1000 1500 2000 2500 Sine wave 480
SAMPLED DATA POINTS samples long with
15 zero-padding on
10 each side.
5
Normally
0 distributed noise
-5 plus sine wave.
-10 Note
Replica of sine
-15
0 500 1000 triangular 2000
1500 2500 wave (offset -5).
SAMPLED DATA POINTS shape
REPLICA CORRELATOR
300
200 As on previous
slide,
OUTPUT
100
0 Sine wave power
-100 Noise power
-200
=> -15 dB
-300
0
-1000 -500 0 500 1000 19
TIME DELAY τd IN DATA SAMPLES
21. RANDOM WALK SAMPLE AND A GENERAL RESULT (1 of 2)
INDEPENDENT NORMALLY DISTRIBUTED NOISE, σ = 1
3
NOISE VALUES
2
1
0
-1
-2
-3
-4
0 100 200 300 400 500 600 700 800 900 1000
SAMPLED DATA POINTS
N INCREASING N
STEPS REMOVED FROM
RANDOM WALK (SUM OF NOISE VALUES UP TO N DATA POINTS)
STARTING POSITION
20
10
0
-10
-20
-30
-40
-50
0 100 200 300 400 500 600 700 800 900 1000
NUMBER OF STEPS, N
Root-mean-square departure from starting position is N . 20
23. RANDOM WALK SAMPLE AND A GENERAL RESULT (2 of 2)
POINT-BY-POINT PRODUCTS OF REPLICA AND NOISE
(FROM PREVIOUS SLIDE)
2
PRODUCT
VALUES
1
0
-1
-2
0 100 200 300 400 500 600 700 800 900 1000
SAMPLED DATA POINTS
N INCREASING N
SUM OF ABOVE PRODUCT VALUES OUT TO N POINTS
SUM OF PRODUCT
10
5
0
VALUES
-5
-10
-15
-20
-25
-30
-35
0 100 200 300 400 500 600 700 800 900 1000
N, NUMBER OF PRODUCTS SUMMED
Root-mean-square departure from starting position (zero) is proportional to N .
22
24. PEAK REPLICA CORRELATOR OUTPUT WHEN ECHO MATCHES REPLICA
ECHOES OF INCREASING DURATION
1
0
N
-1
0 100 200 300 400 500 600 700 800 900 1000
SAMPLED DATA POINTS
REPLICAS OF INCREASING DURATION
1
0
N
CORRELATOR OUTPUT
-1
0 100 200 300 400 500 600 700 800 900 1000
PEAK REPLICA
SAMPLED DATA POINTS
500
400
300
200
100
0
0 100 200 300 400 500 600 700 800 900 1000
LENGTH OF REPLICA CORRELATION IN SAMPLED DATA POINTS, N
Peak replica correlator output with matched inputs is (nearly) proportional to N,
not N . 23
25. SUMMARY
If the number of ‘matched’ sampled data points in a received
signal and replica is N, and if the noise masking the signal is
uncorrelated from sample-to-sample, then:
1) The expectation of the root-mean-square noise
1
output of the replica correlator increases as N , 2
2) The peak output of the replica correlator due to
the signal increases (nearly) linearly with N.
3) The ratio of the peak signal output to the root-mean-square
N
noise output is expected to increase as 1 ,
N 2
4) The peak signal-to-noise instantaneous power output of
2
N
=
the replica correlator is expected to increase as
1
N.
N2
24
26. HYPOTHETICAL TRANSMISSION, RECEIVED DATA, AND
INSTANTANEOUS POWER OUTPUT OF A REPLICA CORRELATOR
ARBITRARY N SAMPLES
UNITS REPLICA OF TRANSMISSION
FOR TIME SHIFT τd
TIME
RECEIVED DATA,
SIGNAL MATCHING REPLICA PLUS NOISE
ARBITRARY
UNITS
TIME
τd
OUTPUT OF REPLICA OUTPUT AT τd = 0
ARBITRARY
CORRELATOR DUE TO SIGNAL ~ N
UNITS
TIME DELAY
- τd +τd
OUTPUT DUE TO ROOT-MEAN SQUARE VALUE
NOISE ALONE OF NOISE ALONE ~ N1/2
OUTPUT AT τ=0 INSTANTANEOUS POWER OUTPUT OF
ARBITRARY
DUE TO SIGNAL ~ N2 REPLICA CORRELATOR AND ITS PEAK
UNITS
POWER ENVELOPE
TIME DE;AY
- τd +τd
τd = 0 MEAN VALUE OF NOISE
25
POWER ALONE ~ N
(Too weak to be seen on this scale.)
27. THRESHOLD SETTING DETERMINES (WITHIN LIMITS) THE
RESULTING PROBABILITIES OF DETECTION AND FALSE ALARM
INSTANTANEOUS POWER OUTPUT OF A REPLICA
CORRELATOR FOR A CW WAVEFORM MASKED BY NOISE
REPLICA CORRELATOR
ARBITRATY UNITS
POWER OUTPUT,
TOO HIGH A THRESHOLD LEADS
TO MISSED DETECTIONS TOO LOW A THRESHOLD
LEADS TO EXCESSIVE
FALSE ALARMS
0
-100
-1000 -500 0 500 1000
-200 SAMPLED DATA POINTS FORWARD AND BACKWARD IN TIME
FROM EXACT OVERLAP OF CW WAVEFORM AND ITS REPLICA
26
28. PROBABILITIES OF FOUR POSSIBLE OUTCOMES
SEPARATE AND DISTINCT
ENSEMBLES
pd is the probability a threshold
crossing will occur when a target
TRUTH
is actually present. COLUMNS
pfa is the probability a threshold
crossing will occur when a target
TARGET IS TARGET IS
is not present. PRESENT ABSENT
THRESHOLD IS CORRECT FALSE
CROSSED,
DECIDE TARGET DETECTION ALARM
INSTANTANEOUS
PRESENT pd pfa
POWER OUTPUT OF
REPLICA THRESHOLD NOT NO
CORRELATOR
MISSED
CROSSED, ACTION
DECIDE TARGET
DETECTION
ABSENT 1 - pd 1 - pfa
For example, a detector might be designed to provide a 50% probability
of detection while maintaining a probability of false alarm below 10-6.
27
29. DECISION: H1/H0 ?
ACTIVE SONAR DETECTION MODEL
(DISCUSSED SO FAR, ALONG WITH SONAR EQUATION)
POST-DETECTION
PROCESSOR
REPLICA
TRANSMISSION FOR CW
CORRELATOR,
TRANSMISSIONS DFT DETECTOR
PRE-DETECTION
FILTER
(Beamformer)
DELAY AND RECEPTION
ATTENUATION AMBIENT NOISE
(Receive array)
TIME-VARYING
MULTIPATH
TARGET
TIME-VARYING DELAY AND
MULTIPATH ATTENUATION
REVERBERATION 28
30. DECISION: H1/H0 ?
ACTIVE SONAR DETECTION MODEL
(NEXT STEPS)
POST-DETECTION
PROCESSOR
TRANSMISSION FOR FREQUENY REPLICA
MODULATED CORRELATOR,
TRANSMISSIONS DFT DETECTOR
PRE-DETECTION
FILTER
(Beamformer)
DELAY AND RECEPTION
ATTENUATION AMBIENT NOISE
(Receive array)
TIME-VARYING
MULTIPATH
TARGET
TIME-VARYING DELAY AND
MULTIPATH ATTENUATION
REVERBERATION 29
31. REPLICA CORRELATION FOR A CONTINUOUS CW WAVEFORM
Replica matches echo
Replica
PRESSURE except for time delay.
Time, t
Echo
+τ d
Replica correlator output
and its envelope
for a CW waveform.
- τd +τd
τd = 0
τd is the time delay of the echo with respect
to the replica established in the correlator. 30
32. EFFECT SHIFTING A CW WAVEFORM’S FREQUENCY BY φ
USING THE ‘NARROWBAND’ APPROXIMATION
Frequency = fo + φ
Echo
PRESSURE
Time, t
Replica
Frequency = fo
Replica correlator output
Envelope and its envelope
narrows
for a CW waveform
- τd +τd
τd = 0
φ is the frequency difference (shift) of the echo with respect to
the replica established in the correlator. The greater | φ |, the
narrower the envelope of the correlator’s output
31
33. TRANSMITTED AND RECEIVED CW FREQUENCIES
AFTER APPLYING A NARROWBAND DOPPLER SHIFT
Instantaneous frequency
g6
Received CW frequencies
f6 g1 , g 2 , g 3 , … , g 6
g5
are each the result
f5 of the same narrowband
g4
Doppler shift with respect
f4 to equally spaced transmitted
g3
CW frequencies
f
g2 3 f1 , f 2 , f 3 , … , f 6
g1 f 2
f1 T TRANSMIT
Time
T RECEIVE
32
34. TRANSMITTED AND RECEIVED CW FREQUENCIES
FOR AN ECHO PRODUCED BY A CLOSING TARGET,
A WIDEBAND DOPPLER TRANSFORMATION
g6
Instantaneous frequency
g5 Received CW frequencies
f6 g1 , g 2 , g 3 , … , g 6
are each the result
g 4 f5 • of a different narrowband
Doppler shift with respect
f4 to equally spaced transmitted
g3
CW frequencies
f3 f1 , f 2 , f 3 , … , f6
g2
f2
g1
f1 T TRANSMIT
Time
T RECEIVE
33
35. DOPPLER PARAMETER S AND THE
WIDEBAND DOPPLER TRANSFORMATION
o s ≡ (1 − ν / c) / (1 + ν / c) = 1 − (2ν / c ) if ν / c << 1
where
ν = Constant range-rate of a reflector, positive closing, and
c = Sonic velocity in the medium.
o Each closing range-rate νi maps into a ‘stretch’ parameter s : νi si
i
1 to 1
o Each transmitted CW pulse of frequency fk becomes a
received CW pulse of frequency gki that depends upon si:
gki = f 1 + 2ν i / c = f k /si k = 1,2, …, N
k
o The difference between the received and transmitted
CW frequencies depends on both si and fk:
s
gki _ fk = fk 1 - i
si 34
36. EFFECT SHIFTING A CW WAVEFORM’S FREQUENCY BY φ
USING THE WIDEBAND DOPPLER TRANSFORMATION
C
L
Frequency = fo + φ
Echo
PRESSURE
Time, t
- τd
Replica
Frequency = fo
C
L
Envelope of replica
correlator output
- τd +τd
τd = 0
φ is the frequency shift of the echo with respect to the replica
established in the correlator. In the ‘wideband’ case the echo
undergoes a Doppler transformation and not simply a Doppler shift. 35
37. REPLICA CORRELATOR OUTPUT FOR A
FREQUENCY-MODULATED WAVEFORM (1 of 3)
Echo
PRESSURE
No
frequency
Time, t
Replica
shift
- τd
Replica correlator output
and its envelope for a
frequency-modulated waveform
- τd +τd
τd = 0
τd is the time delay of the echo with respect
to the replica established in the correlator. 36
38. REPLICA CORRELATOR OUTPUT FOR A
FREQUENCY-MODULATED WAVEFORM (2 of 3)
Echo
PRESSURE
No
frequency
Time, t
Replica
shift
- τd
Replica correlator output
and its envelope for a
frequency-modulated waveform
- τd +τd
Time resolution of the
envelope is ~W-1 where
τd = 0 W is the bandwidth of
the waveform.
τd is the time delay of the echo with respect
to the replica established in the correlator. 37
39. REPLICA CORRELATOR OUTPUT FOR A
FREQUENCY-MODULATED WAVEFORM (3 of 3)
C
L
Echo
PRESSURE
With echo
+τˆd frequency
Replica
shift
τˆd is the time delay when C
L
Replica correlator output
the replica correlator’s output +τˆd and its envelope for a
envelope is a maximum. frequency-modulated echo
experiencing a frequency shift.
- τd +τd
τd = 0
When a frequency-modulated echo experiences a frequency shift
with respect to the replica, the peak output of the correlator is
diminished and his peak output is no longer at τd = 0. 38
40. THE LINEAR FREQUENCY MODULATED (LFM)
WAVEFORM OF UNIT AMPLITUDE
Instantaneous frequency, Hz
Bandwidth, W fc , Carrier
frequency >> W
-t +t
-T/2 +T/2
Time, Seconds
39
41. TIME-FREQUENCY DIAGRAM REFERRED TO REPLICA WAVEFORM (1 of 2)
Instantaneous
frequency
Echo φ , frequency shift
Replica
~ ~
-t ~
+t
t=0 Time
T
Echo
~
PRESSURE
+t
Replica
~
+t
In the case of the narrowband assumption, φ is the
uniform upward frequency shift of the echo with
respect to the replica established in the correlator.
40
42. TIME-FREQUENCY DIAGRAM REFERRED TO REPLICA WAVEFORM (2 of 2)
Instantaneous Frequency shift is not uniform
frequency (closing target produces
Echo
(no narrowband increased echo frequencies)
assumption) Replica
~ ~
-t +t
~
t=0 Time
T Contraction if
frequency increases
Echo
~
PRESSURE
+t
Replica
~
+t
In the wideband case, the echo’s frequency shift is not uniform
over its duration, and any frequency shift brings about a dilation
or contraction in the duration of the waveform. 41
43. TIME DELAY DIAGRAMS
Echo data moves with
Instantaneous respect to replica
frequency Replica established in correlator
(narrowband
assumption)
Increasing clock time
τd
~ ~
-t +t
~
t=0 Time scale referred to replica waveform
BULK DELAY
t =τ BD ECHO
s ( t − τ BD ) t
τd > 0 Clock time
s(t-τRD )
(1) t = τ RD
(1) REPLICA
REPLICA
t
DELAY t=0
42
44. TIME DELAY DIAGRAMS
Echo data moves with
Instantaneous respect to replica
frequency Replica established in correlator
(narrowband
assumption)
Increasing clock time
~ ~
-t +t
~
t=0 Time scale referred to replica waveform
BULK DELAY
t =τ BD ECHO
s ( t − τ BD ) t
τd > 0 Clock time
s(t-τRD )
(1) t = τ RD
(1) REPLICA
REPLICA
t
DELAY t=0
43
45. TIME DELAY DIAGRAMS
Echo data moves with
Instantaneous respect to replica
frequency Replica established in correlator
(narrowband
assumption)
Increasing clock time
~ ~
-t +t
~
t=0 Time scale referred to replica waveform
BULK DELAY
t =τ BD ECHO
s ( t − τ BD ) t
τd > 0 Clock time
s(t-τRD )
(2) t = τ RD
(2) REPLICA
t
REPLICA
DELAY t=0
44
46. TIME DELAY DIAGRAMS
Echo data moves with
Instantaneous respect to replica
frequency Replica established in correlator
(narrowband
assumption)
Increasing clock time
~ ~
-t +t
~
t=0 Time scale referred to replica waveform
BULK DELAY
t =τ BD ECHO
s ( t − τ BD ) t
τd > 0 Clock time
s(t-τRD )
(3) t = τ RD
(3) REPLICA
t
REPLICA
DELAY t=0
45
47. TIME DELAY DIAGRAMS
Echo data moves with
Instantaneous τ d = τˆd when overlap is greatest respect to replica
frequency established in correlator
(narrowband
assumption) Replica
Increasing clock time
~ ~
-t +t
~
t=0 Time scale referred to replica waveform
BULK DELAY
t =τ BD ECHO
s ( t − τ BD ) t
τ d = τˆd > 0 Clock time
s(t-τRD )
(4) t = τ RD
(4) REPLICA
t
REPLICA
DELAY t=0
46
48. TIME DELAY DIAGRAMS
Echo data moves with
Instantaneous respect to replica
frequency established in correlator
(narrowband
assumption) Replica
Increasing clock time
~ ~
-t +t
~
t=0 Time scale referred to replica waveform
BULK DELAY
t =τ BD ECHO
s ( t − τ BD ) t
τd > 0 Clock time
s(t-τRD )
(5) t = τ RD
(5) REPLICA
t
REPLICA
DELAY t=0
47
49. TIME DELAY DIAGRAMS
Echo data moves with
Instantaneous respect to replica
frequency established in correlator
(narrowband
assumption) Replica
Increasing clock time
~ ~
-t +t
~
t=0 Time scale referred to replica waveform
BULK DELAY
t =τ BD ECHO
s ( t − τ BD ) t
τd = 0 Clock time
s(t-τRD )
(6) t = τ RD
(6) REPLICA
t
REPLICA
DELAY t=0
48
50. TIME DELAY DIAGRAMS
Echo data moves with
Instantaneous respect to replica
frequency established in correlator
(narrowband
assumption) Replica
Increasing clock time
~ ~
-t +t
~
t=0 Time scale referred to replica waveform
BULK DELAY
t =τ BD ECHO
s ( t − τ BD ) t
τd< 0 Clock time
s(t-τRD )
(7) t = τ RD
(6) REPLICA
t
REPLICA
DELAY t=0
49
51. USING THE TIME-FREQUENCY DIAGRAM TO ESTIMATE THE MAXIMUM
OUTPUT OF A REPLICA CORRELATOR WHEN THE ECHO
HAS A FREQUENCY SHIFT φ
Instantaneous
frequency τd = 0 τd > τ^d
Replica Echo data moves with
data respect to replica data
established in correlator
−~
t ~ =0 +~
t
t Time scale referred to replica waveform
Instantaneous
frequency τd = τd
^
φ Overlap region producing
maximum correlator power
output for frequency shift φ
−~
t ~ =0 +~
t
t Time scale referred to replica waveform
The greater the overlap region, the larger the maximum replica correlator output.
In this example an LFM waveform has experienced a narrowband Doppler shift.
50
52. TIME-FREQUENCY DIAGRAM REFERRED TO REPLICA WAVEFORM
Instantaneous Frequency shift is not uniform
frequency (closing target produces
Echo
(no narrowband increased echo frequencies
assumption) Replica at higher frequencies)
~ ~
-t +t
~
t=0 Time
T Contraction if
frequency increases
Echo
~
PRESSURE
+t
Replica
~
+t
In the wideband case, the echo’s frequency shift is not uniform
over its duration, and any frequency shift brings about a dilation
or contraction in the duration of the waveform. 51
53. USING THE TIME-FREQUENCY DIAGRAM TO ESTIMATE THE
MAXIMUM OUTPUT OF A REPLICA CORRELATOR
Instantaneous
frequency
(no narrowband
assumption)
Replica Echo data moves with
~ respect to replica ~
-t +t
~ Time scale referred to replica waveform
t=0
Instantaneous
frequency
τd
Overlap region producing
maximum correlator output
~ ~
-t +t
~ Time scale referred to replica waveform
t=0
Here an LFM echo has experienced a wideband Dopper transformation
rather than a narrowband Doppler shift. The overlap with the replica is
reduced, and the corresponding replica correlator output is reduced. 52
54. HYPERBOLIC FREQUENCY MODULATED (HFM) WAVEFORMS,
THEIR TIME-PERIOD AND TIME-FREQUENCY DIAGRAMS
Instantaneous period τ (t), linear
τ1
∆τ το
seconds
per cycle
τ2
-t +t
-T/2 0 +T/2
Instantaneous frequency (hyperbolic)
F2 = (τ2 )−1
W
F1 = (τ1 )−1
-t +t
-T/2 0 +T/2
HFM waveforms and linear period modulated
53
(LPM) waveforms are the same.
55. USING THE TIME-FREQUENCY DIAGRAM TO ESTIMATE THE
MAXIMUM OUTPUT OF A REPLICA CORRELATOR
Instantaneous Higher frequency echo
frequency data moves with
(no narrowband τd respect to replica
assumption)
Overlap region producing
maximum correlator output
Replica
~ ~
-t +t
~ Time scale referred to replica waveform
t=0
Here an HFM echo has experienced a wideband Dopper transformation
and the HFM’s time-frequency distribution adjusts itself to remain ‘more’
overlapped than in a similar case for an LFM waveform.
54
56. ANALYTIC SIGNAL OF A REAL WAVEFORM (1 of 2)
ο If U(f) is the Fourier transform of waveform u(t), U(f) can be folded
over as shown below to obtain Fourier transform Ψ(f).
ο The inverse Fourier transform of Ψ(f), ψ(t), is a complex waveform
in the time domain and provides a convenient way to describe the
envelope and phase of the real waveform u(t). ψ(t) is the analytic
signal of real waveform u(t), and the Fourier transform of ψ(t) (i.e.,
Ψ(f)) contains no negative frequencies.
|U(f)|
ENVELOPE
Q(f)
-f +f
+1
- fo2 + fo2 -f +f
|Ψ(f)|
FT
ENVELOPE u(t) ← → U ( f )
Ψ(f) = 2 Q(f) U(f)
FT
ψ (t ) ← → Ψ( f )
-f +f
55
+ fo2
57. ANALYTIC SIGNAL OF A REAL WAVEFORM (2 of 2)
ο If U(f) is the Fourier transform of waveform u(t), U(f) can be folded
over as show below to obtain Fourier transform Ψ(f).
ο The inverse Fourier transform of Ψ(f), ψ(t), is a complex waveform
in the time domain and provides a convenient way to describe the
envelope and phase of the real waveform u(t). ψ(t) is the analytic
signal of real waveform u(t).
ο If u(t) is a narrowband waveform, the frequency content of ψ (t),
(i.e., Ψ(f),) is negligible near zero frequency as well as null for all f < 0.
|U (f)|
Q(f)
+1
-f +f -f +f
- fo + fo
FT
ENVELOPES |Ψ (f)| u(t) ← → U ( f )
Ψ(f) = 2 Q(f) U(f)
-f FT
+f ψ (t ) ← → Ψ( f )
56
+ fo
58. ANALYTIC SIGNAL WITHOUT APPROXIMATION (1 of 2)
|U(f)| Q(f)
ENVELOPE
+1
-f +f
-f +f
- fo2 + fo2 FT
u(t) ← → U ( f )
|Ψ(f)| Ψ(f) = 2 Q(f) U(f)
ENVELOPE
FT
ψ (t ) ← → Ψ( f )
+∞
u(τ ) dτ
-f +f
ψ (t) = u(t) + j
∫
− ∞ π (t −τ )
+ fo2 u(t) = Re (ψ (t))
+∞ is the Hilbert transform of u(t)
u(τ ) dτ
∫
− ∞ π (t −τ ) ˆ
and is often denoted by u(t).
57
59. ANALYTIC SIGNAL WITHOUT APPROXIMATION (2 of 2)
ο ψ (t) is the analytic signal of real waveform u(t), and is always given by
+∞
u(τ ) dτ
ψ (t) = u(t) + j u(t) = u(t) + j
ˆ
∫
− ∞ π (t −τ )
even if u(t) is not narrowband! However, it may be difficult to find ˆ
u(t).
1.5 u(t) = - sin(2πfot)
u (t ) -0.5 < t < +0.5, Seconds
Instantaneous Amplitude
1
ˆ
u (t ) u(t) = 0; t > |0.5| Seconds
0.5 Envelope
fo = 3 Hertz; T = 1 Second
0
The envelope of u(t)
-0.5 is given by:
u 2 (t) + u 2 (t)
ˆ
-1
-1.5
-1.5 -1 -0.5 0 0.5 1 1.5 2
Time, seconds 58
60. APPROXIMATING THE ANALYTICAL SIGNAL WITH A
NARROWBAND COMPLEX WAVEFORM (1 of 6)
|U (f)|
Q(f)
ENVELOPE
+1
-f +f -f +f
- fo + fo
FT
|Ψ (f)| u(t) ← → U ( f )
ENVELOPE
Ψ(f) = 2 Q(f) U(f)
-f FT
+f ψ (t ) ← → Ψ( f )
+ fo
+∞
u(τ ) dτ
The analytic signal is always given by ψ (t) = u(t) + j
∫
− ∞ π (t −τ )
If u(t) is narrowband, ψ(t) can be approximated by
ψ (t) ≅ ψ (t) = [a(t) + j b(t)] exp[ 2π j fo t ]
ˆ
where a(t) and b(t) are real, +fo is a central frequency of U(f),
and a(t) and b(t) vary slowly compared to 2π fot.
59
61. ANALYTIC SIGNAL AND COMPLEX ENVELOPE OF REAL WAVEFORM u(t,fo)
ψ(t,fo) IS THE INVERSE FOURIER
U(f,fo) IS FOURIER TRANSFORM OF u(t.fo)
TRANSFORM OF Ψ (f,fo)
|U ( f , fo ) | | Ψ( f , fo ) | FT
ψ (t, fo ) ←→ Ψ( f , fo )
ENVELOPE
ENVELOPE
OF |U( f , fo )|
OF |Ψ ( f , fo )|
-f +f
-fo 0 fo -f 0 fo +f
DOWNSHIFT BY fo
ψ(t,fo) is the analytic signal υ(t) IS THE INVERSE FOURIER
~
of real waveform u(t.fo). TRANSFORM OF Ψ(f)
~
| Ψ( f ) | FT ~
υ (t ) ←→ Ψ( f )
υ(t) is the complex envelope
ENVELOPE
of real waveform u(t,fo). ~
OF |Ψ ( f )|
60
-f 0 +f
62. FACTORS OF THE COMPLEX NARROWBAND WAVEFORM ψ (t)
ˆ
Im Im
exp(+2π j fo t) |ψ (t)| =
ˆ
1/2
[a(t)2 + b(t)2 ]
b(t)
Re Re
a(t)
unit exp(-2π j fo t)
circle absent
a(t) and b(t) are real and
ψ (t) = [a(t) + j b(t)] exp(+2π j fo t)
ˆ vary slowly with respect
u(t) = Re (ψ (t) )
ˆ to 2π fot.
[a(t) + j b(t)]is the
complex envelope of ψ (t ).
ˆ 61
64. DEFINITION OF INSTANTANEOUS FREQUENCY
j[ Φ(t) ]
ψ (t) = A(t) e
ˆ is a particularly convenient form of the
analytic signal because Φ(t) is the argument of ψ (t ) and the
ˆ
instantaneous phase of u(t) in radians.
The instantaneous frequency of u(t) in Hertz is
finstantaneous(t ) ≡ fi (t ) ≡ 1 d [Φ(t)]
2 π dt
Given fi(t), the argument of ψ (t) and instantaneous phase of
ˆ
u(t) is given by
Φ(t) = 2π
∫ fi (t) dt + θ o
And we can write ψ (t ) as
ˆ
ψ (t) = A(t) e
ˆ [ ∫ ]
j 2π fi (t) dt + θ o
63
65. AMPLITUDE, FREQUENCY, AND PHASE MODULATION
ο In the expression ψ (t )
ˆ = A(t ) e [ ∫
j 2π fi (t) dt + θ o]
Variations in A(t) are amplitude modulation or AM
( A(t) = a(t )2 + b(t )2 ) ,
Variations in fi(t) are frequency modulation or FM.
ο Most active sonar waveforms are either frequency or amplitude
modulated; frequency modulation is more common.
ο Phase modulation or PM is used in underwater communications;
for example, 90o or 180o phase changes are embedded in θ (t)
and the analytic signal is expressed as
ψ (t) = A(t) [ exp ( j (2π fo t + θ (t) ) )]
ˆ
i.e., θ(t) ‘switches’ or ‘flips’ the phase of the carrier frequency.
64
66. IMPORTANCE OF THE COMPLEX ENVELOPE
ο In the expressions
ψ(t) = [a(t) + j b(t)] exp(+2π j fo t) , and
‹
u(t) = Re(ψ(t) ) = a(t)cos(2π fo t) – b(t)sin(2π fo t)
‹
both the amplitude and frequency modulation of u(t) and ψ(t)
‹
are embedded in the complex envelope [a(t) + j b(t)] :
Amplitude modulation is a(t)2 + b(t)2 = A(t ) , and
Frequency modulation is 1 d [θ (t ) ] where θ (t) = Arg ( a(t) + j b(t))
2π dt
ο All the properties of the waveform (favorable and unfavorable!)
independent of fo are embedded in the complex envelope.
ο This means analyses of most waveform properties (e.g., Doppler
tolerance, range resolution) can be carried out by considering
only the complex envelope without regard to a ‘carrier’ frequency.
65
67. OBTAINING THE ANALYTIC SIGNAL AND REAL WAVEFORM
o General form of the analytic signal is ψ (t ) = A(t ) e j[Φ(t)]
d
Start with fi(t) for the
finstantaneous(t ) ≡ fi (t ) ≡ 1 [Φ(t)]
waveform you want. 2 π dt
∫
Φ(t) ≡ 2π fi (t) dt
Next integrate to Need not be
find the phase. narrowband.
Finally, insert Φ(t) in the Φ(t) = 2π fo t + θ (t) + θo
general form for ψ(t). If narrowband.
where d[θ(t)]/dt << 2π fo
Variations in A(t) are amplitude modulation or AM
( A(t) = a(t )2 + b(t )2 ) ,
Variations in fi(t) are frequency modulation or FM.
o A(t) is often explicitly stated, i.e., rect(t/T).
o Re[ψ(t)] gives the real waveform, u(t). 66
68. OBTAINING THE ANALYTIC SIGNAL AND REAL WAVEFORM
o General form of the analytic signal is ψ (t ) = A(t ) e j[Φ(t)]
d
Start with fi(t) for the
finstantaneous(t ) ≡ fi (t ) ≡ 1 [Φ(t)]
waveform you want. 2 π dt
∫
Φ(t) ≡ 2π fi (t) dt
Then integrate to An indefinite integral with
find the phase. a constant of integration.
Finally, insert Φ(t) in the Φ(t) = 2π fo t + θ (t) + θo
general form for ψ(t). If narrowband.
where d[θ(t)]/dt << 2π fo
Variations in A(t) are amplitude modulation or AM
( A(t) = a(t )2 + b(t )2 ) ,
Variations in fi(t) are frequency modulation or FM.
o A(t) is often explicitly stated, i.e., rect(t/T).
o Re[ψ(t)] gives the real waveform, u(t). 67
69. THREE COMMON WAVEFORMS
The following slides give complex representations,
i.e., ψ (t ) for three common waveforms:
o CW (continuous wave)
o LFM (linear frequency modulation)
o HFM (hyperbolic frequency modulation)
68
70. FT sin ( π f T )
COMPLEX NARROWBAND u (t) = rect (t /T ) ← →
T = U( f )
CW WAVEFORMS OF πfT
FT
UNIT AMPLITUDE u (t) exp ( 2π j fc t) ← →
U ( f − fc )
ψ (t) = 1T
rect (t/T ) exp [ j 2 π ( fc t + θo )] 0.8T Shift to
2π frequency fc
0.6T
where T fc >> 1 cycle
0.4T to get U(f - fc )
U(f)
1 if |t/T| < 1/2
rect (t/T ) = 0.2T
0 if |t/T| > 1/2
0T
θο radians is a constant
that determines the phase -0.2T
of the CW waveform.
-0.4T-10 -8
∫
-6 -4 -2 0 2 4 6 8 10
(Φ(t) ≡ 2π fc dt = 2π fc t + θo ) T T T T T T T T T T T
FREQUENCY
Cycles /Second 69
71. THE LINEAR FREQUENCY MODULATED (LFM)
WAVEFORM OF UNIT AMPLITUDE
Instantaneous frequency
Bandwidth, W Carrier
fc frequency >> W
-t +t
-T/2 +T/2
Instantaneous phase (radians) = Φ(t) = 2 π fc t + θ (t) , t < |T/2| and TW >> 1
and instantaneous frequency (Hertz) = f (t ) ≡
1 d[ Φ(t) ]
i 2π dt
= 1 d [ 2 π fc t + θ (t) ] = fc + 1 d [θ (t) ]
2π dt 2π dt
Linear frequency modulation means
1 d [θ (t) ] = k t so θ (t) = 2 π k t 2 + θ
o
2π dt 2
i.e., the instantaneous frequency is a linear function of time, and k = W/T from the figure.
Therefore the LFM’s instantaneous phase = 2 π ( fc t + k t 2 + θo ) (radians)
2 2π 70
72. THE LINEAR FREQUENCY MODULATED (LFM)
WAVEFORM OF UNIT AMPLITUDE
Instantaneous frequency
W fc >> W
-t +t
-T/2 +T/2
j( fc t + k t 2 + θo )
t
ψ (t) = rect [] [T
exp 2 π
2 2π ]
j[ Φ(t) ] t
Given: ψ (t) = A(t) e and A(t) = rect [] T
Step 1 fi (t) = fc + k t
Step 2
Φ(t) = 2π
∫ fi (t) dt
Φ(t) = 2 π ( fc t + k t 2 + θo )
2 2π 71
73. HYPERBOLIC FREQUECY MODULATED (HFM) WAVEFORMS,
THEIR TIME-PERIOD AND TIME-FREQUENCY DIAGRAMS
Instantaneous period (linear) = τ(t)
τ1
∆τ το
τ2
-t +t
-T/2 0 +T/2
Instantaneous frequency (hyperbolic) = Finst(t) = 1/τ(t)
F2 = 1/τ2
W
F1 = 1/τ1
-t +t
-T/2 0 +T/2
HFM waveforms and linear period modulated
72
(LPM) waveforms are the same.