Understanding the FM Slide Chirp Advantages in Hydroacoustics for Fisheries Assessment


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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.

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  • Questions: 1) Bandwidth of the transducer should have a “flat” frequency response over BW of the slide signal. Possible with appropriate tuning for BW < 10 kHz and acoustic operating frequencies > 30 kHz.2) Large targets can produce smaller adjacent “false targets” shifted in range from FM slide signals if frequency domain windowing is not applied in the matched filter. However, range lobes can be largely eliminated using this technique.3) This would result in some reduction in realized processing gain, although it would be small.4) If acoustic targets are moving in the direction of acoustic propagation (toward or away from the transducer), this will induce some phase variability in the return echo. However, target velocities need to be very high to have a measurable reduction in processing gain (5 m/sec = 0.35 dB reduction in gain).
  • Understanding the FM Slide Chirp Advantages in Hydroacoustics for Fisheries Assessment

    1. 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. 2. Presentation OverviewMost acoustic echo sounders used for fisheries studies employCW, or “tone-burst” pulse signals. With this type of signal theuser 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 FMslide to simultaneously maximize spatial resolution and range.This presentation shows how the FM slide signal achieves thisimproved performance and presents laboratory measurementsand field results that illustrate its advantages.
    3. 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. 4. Noise in Acoustic SystemsEffects 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. 5. Resolution or Range, Requires a Compromise with CW SystemsDepending on the application, it can be difficult to achieve both asufficient 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 shortduration transmitted pulses. Signal energy in a transmitted pulseand in the received echoes is proportional to the pulse duration.Shorter pulses have less energy, resulting in reduced SNR and adiminished maximum sampling range. The use of longer pulsedurations increases SNR and thus range, but diminish the ability toresolve single scatterer echoes (reduced spatial resolution).SEAC Workshop March 13-15, 2012
    6. 6. Balancing Data Needs with CW Pulse SystemsA. Short Burst = (narrow pulse width)  Reduced Detection Range  Improved Spatial ResolutionReduced SNR results in diminished ability to quantify targets at greater rangeB. Long Burst = (wide pulse width)  Improved Detection Range  Reduced Spatial ResolutionMaximizes SNR, but reduces the ability to resolve single-target scatterers (estimate TS) andgain precise range estimates to the bottom and other acoustic targets.
    7. 7. 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
    8. 8. Tried, True & Certainly Not NewNear the end of World War II, Navy radar systemengineers developed the FM Slide to improveperformance. The Navy still uses technique today toimprove range and resolution performance.
    9. 9. 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
    10. 10. 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 TSEAC Workshop March 13-15, 2012
    11. 11. FM Slide Signal Receiver SNR= A2T/2No TVG Quadrature Matched Envelope Preamplifier Amplifier Demodulator Filter Detector A T 1 Output pulse width Sweep BandwidthSEAC Workshop March 13-15, 2012
    12. 12. 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 timeSEAC Workshop March 13-15, 2012
    13. 13. 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 dBSEAC Workshop March 13-15, 2012
    14. 14. Effect of Windowing on FM Slide OutputSEAC Workshop March 13-15, 2012
    15. 15. FM Slide Data in EchoviewSEAC Workshop March 13-15, 2012
    16. 16. 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 filterSEAC Workshop March 13-15, 2012
    17. 17. Echo Sounder Measurements of Effects of Noise (1 of 2)SEAC Workshop March 13-15, 2012
    18. 18. Echo Sounder Measurements of Effects of Noise (2 of 2)SEAC Workshop March 13-15, 2012
    19. 19. Effect of FM Slide Signal on Chart Recording Tone Burst FM Slide Pulse Signal SignalSEAC Workshop March 13-15, 2012
    20. 20. Comparison of CW vs. FM Slide Signal on Color EchogramSEAC Workshop March 13-15, 2012
    21. 21. 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
    22. 22. ConclusionsThe FM slide signal provides a method to obtain both goodspatial resolution and high noise immunity in acousticassessment systemsThe 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 environmentsUp to 17 dB of additional processing gain can be realized, extendinguseful range by a factor of up to 2.6 times that of a CW pulse.SEAC Workshop March 13-15, 2012
    23. 23. Remaining Challenges/Questions Adding additional FM slide output PW values and exploringrefinements to pulse windowing and filtering methods to extend theutility of the method. Document and extend information describing FM Slide/Chirpimplementation 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
    24. 24. 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.