The Southeast Regional Acoustics Consortium (SEAC) met in March 2012 at Florida International University bringing together academic institutions, federal and regional fisheries and environmental management agencies, and private industry that conduct active acoustics research in the coastal environments of the US from North Carolina to Texas and the US Caribbean. Informal presentations and discussions highlighted the latest tools for fisheries research, organized around high-priority research objectives and management drivers (e.g., stock assessment improvements, integrated ecosystem assessments) and HTI’s Pat Nealson conducted a presentation to help demystify FM Slide/Chirp signals in hydroacoustics for fisheries assessments.

1. Improving Signal-to-Noise Performance
in Hydroacoustic Monitoring Systems
through the use of FM Slide (chirp) Signals
Presented by Patrick Nealson pnealson@HTIsonar.com
SE Acoustics Consortium Workshop, Presented March 14, 2012
© 2012 Hydroacoustic Technology, Inc.

2. Presentation Overview
Most acoustic echo sounders used for fisheries studies employ
CW, or “tone-burst” pulse signals. With this type of signal the
user must trade off spatial target resolution with output signal-
to-noise, which determines the maximum useful range.
Alternatively, one can use a wide band signal such as an FM
slide to simultaneously maximize spatial resolution and range.
This presentation shows how the FM slide signal achieves this
improved performance and presents laboratory measurements
and field results that illustrate its advantages.

3. Spatial (Range) Resolution in Acoustic Systems
 Spatial resolution is proportional to the echo
sounder output pulse width,
 Single echo isolation requires good spatial
resolution (single echo isolation required for in-
situ target strength estimation and echo
counting/fish tracking)
c
 Reverberation level (scattering from surface,
R bottom, or objects in the water) is proportional
2
to spatial resolution, R. Therefore
reverberation is minimized by minimizing R.
SEAC Workshop March 13-15, 2012

4. Noise in Acoustic Systems
Effects of noise on acoustic assessment systems:
 Source of bias and variance for in situ target strength measurement.
 Adversely affects single echo isolation.
 Introduces bias and variance errors in the abundance estimates
obtained using echo integration.
 The useful operating range of any practical fisheries or plankton
acoustic assessment system is limited by the available signal to noise
ratio (SNR).
Methods for improving SNR:
 Decrease noise sources (better electronics, reduce flow noise,
increase transducer directivity, etc.).
 Increase energy in transmitted signal, Es= PsT
 Decrease the output noise by decreasing the bandwidth .
SEAC Workshop March 13-15, 2012

5. Resolution or Range,
Requires a Compromise with CW Systems
Depending on the application, it can be difficult to achieve both a
sufficient signal-to-noise (SNR) ratio and high single-target detection
(STD) resolution using a CW pulse echo sounding system.
In CW systems, optimizing STD resolution requires the use of short
duration transmitted pulses. Signal energy in a transmitted pulse
and in the received echoes is proportional to the pulse duration.
Shorter pulses have less energy, resulting in reduced SNR and a
diminished maximum sampling range. The use of longer pulse
durations increases SNR and thus range, but diminish the ability to
resolve single scatterer echoes (reduced spatial resolution).
SEAC Workshop March 13-15, 2012

6. Balancing Data Needs with CW Pulse Systems
A. Short Burst =
(narrow pulse width)
 Reduced Detection Range
 Improved Spatial Resolution
Reduced SNR results in diminished ability to quantify targets at greater range
B. Long Burst =
(wide pulse width)
 Improved Detection Range
 Reduced Spatial Resolution
Maximizes SNR, but reduces the ability to resolve single-target scatterers (estimate TS) and
gain precise range estimates to the bottom and other acoustic targets.

8. FM Slide - Brief Background
FM Slide signals are widely used in radar, military and
bathymetric mapping hydroacoustic applications, to provide
precise ranging and to optimize SNR.
The technique can be employed for the same purposes in narrow
band fisheries research echo sounders.
Data collection improvements are only recognized in the
presence of non-reverberant noise, but there isn’t a “down side”
with respect to employing FM slide signals. The method is
transparent to the user and echo amplitudes, TS and integration
results are equivalent to those derived from CW signals of the
same output pulse duration.
SEAC Workshop March 13-15, 2012

9. Tried, True & Certainly Not New
Near the end of World War II, Navy radar system
engineers developed the FM Slide to improve
performance. The Navy still uses technique today to
improve range and resolution performance.

10. A Pulse Compression Technique
FM Slide/Chirp is a method which simultaneously provides the
high energy of a long pulse width with the high resolution of a
short pulse width.
Extended range is realized through the use of a wide transmit
pulse where the frequency is varied linearly over time (i.e.
“swept”) within the pulse.
Higher spatial resolution is achieved by compressing the echo
returns to narrow output pulses using cross correlation
techniques (use of a matched filter to compress the pulse in time
and increase it in amplitude).
SEAC Workshop March 13-15, 2012

12. CW Echo Sounder Receiver
TVG Bandpass Envelope
Preamplifier SNR= A2T/2No
Amplifier Filter Detector
No is the acoustic
noise spectral density
A
T
Output pulse width T
SEAC Workshop March 13-15, 2012

13. FM Slide Signal Receiver
SNR= A2T/2No
TVG Quadrature Matched Envelope
Preamplifier
Amplifier Demodulator Filter Detector
A
T
1
Output pulse width
Sweep Bandwidth
SEAC Workshop March 13-15, 2012

14. Pulse Compression for FM Slide
Received Signal Envelope Filter Delay vs Frequency
T
Amplitud
Delay
t
-T
e
time f1 f2 freq
Received Signal Frequency vs Time Matched Filter Output
Frequency
Amplitude
f2 = 1/(f2 - f1)
f1
t1 t2 time time
SEAC Workshop March 13-15, 2012

15. Performance Gain Provided by FM Slide Signal
The potential performance gain provided by the FM slide
(relative to a CW pulse) is
G = (T) (BW)
where T = length of the input pulse
BW = frequency range of the FM slide
Example:
T = 5 msec
BW = 10 kHz
G = (0.005) (10,000) = factor of 50, or 17 dB
SEAC Workshop March 13-15, 2012

16. Effect of Windowing on FM Slide Output
SEAC Workshop March 13-15, 2012

17. FM Slide Data in Echoview
SEAC Workshop March 13-15, 2012

18. Available Chirp Options in HTI Systems
0.72 ms output PW
0.36 ms output PW
0.18 ms output PW
Note: Output PW’s are > 1/bandwidth
because:
1) the effective swept bandwidth is less
than specified due to transducer “roll off”,
and
2) Windowing effects in the matched filter
SEAC Workshop March 13-15, 2012

19. Echo Sounder Measurements of Effects of Noise
(1 of 2)
SEAC Workshop March 13-15, 2012

20. Echo Sounder Measurements of Effects of Noise
(2 of 2)
SEAC Workshop March 13-15, 2012

21. Effect of FM Slide Signal on Chart Recording
Tone Burst FM Slide
Pulse Signal Signal
SEAC Workshop March 13-15, 2012

22. Comparison
of CW vs.
FM Slide
Signal
on Color
Echogram
SEAC Workshop March 13-15, 2012

23. Woods Hole Deep Tow - Plankton & Fish
Multiple frequency chirp
acoustics, cameras,
and physical sensors
combined on one
vehicle towed at various
depths (“tow-yo”)
HTI Hydroacoustic Short Course 2011 Section 1: A Brief Introduction

24. Conclusions
The FM slide signal provides a method to obtain both good
spatial resolution and high noise immunity in acoustic
assessment systems
The advantages this provides to acoustic assessment are:
 Better single echo isolation for echo counting and in situ TS
estimation
 Lower bias and variance in both in-situ target strength and echo
integration density estimates
 Extended operating range for acoustic systems sampling in noise-
limited environments
Up to 17 dB of additional processing gain can be realized, extending
useful range by a factor of up to 2.6 times that of a CW pulse.
SEAC Workshop March 13-15, 2012

25. Remaining Challenges/Questions
 Adding additional FM slide output PW values and exploring
refinements to pulse windowing and filtering methods to extend the
utility of the method.
 Document and extend information describing FM Slide/Chirp
implementation to facilitate use of the technique in other instruments.
 Publish experimental data validating equivalent output metrics (Sv,
TS) with CW systems under high SNR conditions.
 Workshop Relevance: Stock Assessment Improvements
 Questions? “ Does the varying frequency in the FM slide signal affect transducer performance?
“ Can detection of large acoustic targets in FM Slide range lobes cause problems?”
“What if targets have frequency-dependent scattering within the FM slide bandwidth?”
SEAC Workshop March 13-15, 2012

26. Improving Signal-to-Noise Performance
in hydroacoustic monitoring systems
through the use of FM slide (chirp) signals
Thank you.
Presented by Patrick Nealson pnealson@HTIsonar.com
SE Acoustics Consortium Workshop, Presented March 14, 2012
© 2012 Hydroacoustic Technology, Inc.