Understanding the FM Slide Chirp Advantages in Hydroacoustics for Fisheries Assessment

Improving Signal-to-Noise Performancein Hydroacoustic Monitoring Systems

through the use of FM Slide (chirp) Signals

© 2012 Hydroacoustic Technology, Inc.

Presented by Patrick Nealson [email protected]

SE Acoustics Consortium Workshop, Presented March 14, 2012

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.

Presentation Overview

SEAC Workshop March 13-15, 2012

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)

Reverberation level (scattering from surface, bottom, or objects in the water) is proportional to spatial resolution, DR. Therefore reverberation is minimized by minimizing DR.

Rc

2

Spatial (Range) Resolution in Acoustic Systems

t


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 .

Noise in Acoustic Systems


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).

Balancing Data Needs with CW Pulse Systems

B. Long Burst = (wide pulse width)

Reduced Detection Range

Improved Spatial Resolution

Reduced Spatial Resolution

A. Short Burst = (narrow pulse width)

Improved Detection Range

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.

Reduced SNR results in diminished ability to quantify targets at greater range


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.

FM Slide - Brief Background

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.


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).


PreamplifierTVG

AmplifierBandpass

FilterEnvelopeDetector

T

t

SNR= A2T/2No

A

No is the acoustic

noise spectral density

TwidthpulseOutput

CW Echo Sounder Receiver


MatchedFilter

T

t

QuadratureDemodulator

SNR= A2T/2No

A

BandwidthSweep

widthpulseOutput1

FM Slide Signal Receiver

PreamplifierTVG

AmplifierEnvelopeDetector


T

f2f1

DtD-T

freqtime

Am

plitu

de

Del

ayt2t1

f2

f1

t = 1/(f2 - f1)

time

Fre

quen

cy

Am

plitu

detime

Received Signal Envelope Filter Delay vs Frequency

Received Signal Frequency vs Time Matched Filter Output

Pulse Compression for FM Slide


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

Performance Gain Provided by FM Slide Signal


Effect of Windowing on FM Slide Output


FM Slide Data in Echoview


Available Chirp Options in HTI Systems

0.18 ms output PW

0.36 ms output PW

0.72 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


Echo Sounder Measurements of Effects of Noise(1 of 2)


Echo Sounder Measurements of Effects of Noise(2 of 2)


Tone BurstPulse Signal

FM Slide Signal

Effect of FM Slide Signal on Chart Recording


Comparison of CW vs. FM Slide Signal on Color Echogram

HTI Hydroacoustic Short Course 2011 Section 1: A Brief Introduction

Multiple frequency chirp acoustics, cameras, and physical sensors combined on one vehicle towed at various depths (“tow-yo”)

Woods Hole Deep Tow - Plankton & Fish


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.

Conclusions


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?”

Remaining Challenges/Questions

Thank you.

© 2012 Hydroacoustic Technology, Inc.

Presented by Patrick Nealson [email protected]

SE Acoustics Consortium Workshop, Presented March 14, 2012

Improving Signal-to-Noise Performancein hydroacoustic monitoring systems through the use of FM slide (chirp) signals

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