Rapp. P.-v. Réun. Cons. int. Explor. Mer, 189: 394-397. 1990

A prototype wideband sonar for fisheries in lakes and rivers

M. E. Zakharia

Zakharia, M. E. 1990. A prototype wideband sonar for fisheries in lakes and rivers. - Rapp. R-v. Réun. Cons. int. Explor. Mer, 189: 394-397.

We have developed, in close collaboration with the Ichthyology Laboratory of the Catholic University of Lyon, a prototype fishing sonar, mainly designed for river and lake applications. This sonar is a fixed-position, wideband system with a frequency range of 50 to 75 kHz. The transmitted signal is a linear period modulated chirp (“Doppler tolerant”). The signal duration is limited to 1-28 ms by processing com­plexity. Coherent processing (or matched filtering) is used to separate the signal duration from the time (range) resolution. Angular scanning is provided, using a pair of receivers and correlation methods. The sonar performs a real-time (electronic) scanning in an angular sector of ±45° with 33 simultaneous observation directions. The angular resolution is about 6°, and the range resolution can be as good as a few centimetres.

M. E. Zakharia: I.C.P.I. Lyon, LASSSO, Laboratoire de Systèmes et Signaux SOnar, CNRS, URA 346; 25, rue du Plat, 69288 Lyon Cedex 02, France.

Notation

T Signal duration.B Signal bandwidth.

te Time sampling rate,TVG Time-varied-gain,r Target range,0 Target angular location,c Sound speed,Ar Distance sampling rate,ör Distance resolution,A0 Angle sampling rate,00 Angle resolution.X Wavelength,X Delay,C.C.D. Charge Coupled Device,D.A.C. Digital-to-Analogue Converter

Introduction

The acoustical limitations encountered while operating a sonar device in lakes or rivers are very particular to the specific situation. Propagation problems and fish behaviour can be quite different from those found in the open sea. We often have to operate in shallow water; minimum depths range from a few metres to tens of metres, and maximum observation distances range from tens of metres to hundreds of metres. Such conditions can severely limit the lower bound of the operating frequency.

The high-frequency limit is determined by two main factors. The first one is the absorption: although it has not been studied in detail, propagation absorption in­creases with frequency in fresh water. The second is the target-strength dependence on the fish aspect in the higher frequency domain (Dahl and Mathisen, 1981; Zakharia, 1982; Zahkaria, 1983).

Fish behaviour in rivers and (small) lakes is very different from that in the open sea. The schools are much smaller, and for many applications (fish counting, fish-ladder efficiency evaluation, etc.) fish have to be considered individually (and not only as members of a school).

A ll th e se spec ia l c o n s id e ra t io n s led us to th e co n cep t

o f a so n a r p ro to ty p e d es ig n ed fo r in lan d w a te r app lica ­

t ions.

Receiver architecture

The receiver architecture is described in Figure 1. It is built around a transmitting transducer and two pairs of receiving arrays. Each pair can be used for angle scan­ning in a given direction (horizontal or vertical). For each channel, the processing can be summarized as: preamplifying, filtering, time gating, time-varied-gain (TVG), frequency shifting (heterodyne), matched fil­tering, and envelope extracting.

The signal is shifted in frequency before matched filtering in order to reduce the filter complexity (since we need at least 2 B T samples to define the response of

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REAL-TIME

POLARDISPLAY

Y < r

DISPLAY

VERTICAL

HORIZONTAL

SELECTION

REAL­TIME

CROSS­CORRELATOR

3 3 p o in t s

F ILTERINS TIME GATING

TVGFREOUENCY

SHIFTING

MATCHEDFILTERING

and

ENVELOPEEXTRACTION

MATCHEDFILTERING

and

ENVELOPEEXTRACTION

THRESHOLDING COUNTER AND DISPLAY

FILTERING

TIME GATING

TVGFREQUENCY

SHIFTING

Figure 1. Receiver architecture.

the filter matched to a signal with bandwidth B and time duration T). The matched filters were designed with charge-coupled-device (C.C.D.) tapped delay lines (Zakharia, 1982; Zakharia and Pey, 1983). The number of taps was limited to 96.

Figure 2 shows the pulse compression effect on a test signal and on a real echo (from a pike (Esox lucius), side aspect).

The TVG is digitally controlled, with 10-centimetre steps. It only takes into account the geometrical spread­ing and neglects the absorption as we did not have sufficient data for this frequency range in fresh water.

Sector scanning

The sector scanning is done by computing the correla­tion function between two receiving arrays. The real­time correlation scheme is shown in Figure 3. It com­putes the product of signal 2 with deldyed copies of' signal 1 and then integrates th is(product. The beam is steered by increasing the delay, i Thirty-three observa­tion directions are computed, and the computation has to be done in parallel to maintain the real-time require­

ments. A constant delay of 16 taps has been added to signal 2. to scan positive and negative angles.

The display can then be done on an oscilloscope by very fast multiplexing of the 33 directions. A polar display block diagram is presented in Figure 4. The coordinates of the observation points also have to be computed and sent to the x and y channels of the oscillo­scope while the correlation result is sent to the z channel (light modulation). The scanning is done at 1.2 MHz. Two markers (angle and distance) have been added to the display.

Signal design

The signal used has been chosen for various properties it possesses, mainly for “Doppler tolerance” (Zakharia, 1982; Zakharia and Pey, 1983; Altes, 1976). It is ex­pressed by:

EM ITTE D S IG N A L

H a t c h e d

'm a t c h e d

F IL T E R IN PU T

F IL TE R O U TP U T

i g n a l I n p u t

E c h o o f a P ik e ( 2 8 X 4 c m ) a t 1.5 m.

Figure 2. Pulse compression examples.

395

S(t) = ex p [- ln 2(t/t0)/21ng] c o s [2j i b ln(t/t0)/ln g]

where

g defines the relative bandwidth, t0 defined the signal duration, b defines its bandwidth.

Seeking a compromise for all the perturbations encoun­tered in rivers and lakes (cited below) we chose, for our application, a signal with the following parameters:

g = 1.004472, t„ = 3.15 ms, b = 0.86.

This leads to the signal shown in Figure 2, with the following physical parameters:

frequency bandwidth (defined at —40 dB): 50 kHz to 75 kHz,time duration (defined at —40 dB): 1.28 ms, wavelength (in water): 5 cm < X < 1 cm.

Prototype description

The transducers are built at the CNRS laboratory in Marseille, France (LMA, Labo US) (Zakharia, 1982). The transmitter is a circular assembly of seven elements and the receivers are linear arrays of five elementary elements.

The preamplifiers are located in a waterproof tank.

* ►0

u.

C.C.D Tapped delay line

CD

Figure 3. Real-time correlator.

COSINEDIGITALMEMORY

MULTIPLYINGD.A.C.

LUDISTANCE

RAMPGENERATOR

MU LTIPLYING D A.C.

SINEDIGITALMEMORY

MULTIPLEXER

( 33

CORRELATOR OUTPUTS

Figure 4. CRT polar display block diagram.

The immersed ensemble is connected to the processing unit by a 50-m multiconductor cable. The processing unit can be placed on the riverbank or on a barge.

The main sonar characteristics can be summarized:

Signal characteristics:B = 50 to 75 kHz,T = 1.28 ms,linear period modulation.

Time-varied-gain:1 (none), 10 log(r), 20 log(r), 30 log(r), 40 log(r), ex­

ternal,exact compensation from 1 to 100 metres, approximated compensation from 1 to 500 metres, time gating possible.

Sector scanning:correlation techniques used,beam angle —45° to +45°,angle sampling A0 = 3°,angle resolution (at —3 dB) 00 = 6°,range sampling Ar = 2.5 cm,range resolution ôr = 10 cm,angle and distance markers on the display,horizontal or vertical scanning,images displayed on oscilloscope.

396

. . , . ;is .. ,

DISPLAY

L ; llll

li

SECTOR SCANNING UNIT

MARKERPOSITIONING

COUNTER DISPLAY

PROCESSING UNITREPETITION

RATE

UJ H

HORIZONTAL VERTICAL

= ) ' /

OBSERVATIONZONE

SCANNINGSELECTOR

OU L ÏS TA R T END

none 10 20 30 40ln r 1 In r ln r ln r

TVGSELECTOR

POWER SUPPLY220 V. 50 HZ.

POWER AMPLIFIER50 W.

Figure 5. Sonar presentation.

Counting system: only for individual fish, by thresholding, global or by sector.

Discussion

We describe a specially designed sonar system for vari­ous river and lake applications, where fish density is

rather low and it is often necessary to consider fish as individual targets.

This sonar is currently used, mainly in rivers, for academic observations. We are planning to connect it to a personal computer to permit in situ fish observations for long periods.

The correlation used for scanning is innovative be­cause it reduces the sonar complexity, but it suffers from being a bi-linear representation; this can induce “ghost echoes” due to interacting (cross-correlation) terms, while observing several still targets situated at fixed range and angle positions.

The image display on an oscilloscope suffers from lack of contrast; we are setting up a digital system capable of presenting the display on a standard video colour CRT, using pseudo-colour coding of the data.

A ckn ow led gem ents

The author wishes to thank Dr Goubier, Director of the Ichthyology Laboratory of the Catholic University of Lyon and Director of the Institut Régional de Re­cherches Appliquées en Aquaculture, France, for his help and encouragement.

This work was supported by the Conseil Supérieur de la Pêche, Compagnie Nationale du Rhône, Ministère de l’Environnement, and Conseil Régional Rhône-Alpes.

R eferencesAltes, R. A. 1976. Sonar for a generalized target description

and its similarity to animal écholocation systems. J. acoust. Soc. Am., 59: 97-105.

Dahl, P. H ., andM athisen, O. A. 1981. Measurement of acous­tic backscattering directivity and target strength of salm- onids. University of Washington, College of Fisheries. R e­port No. UW8126. Washington, USA.

Zakharia, M. E. 1982. Contribution à la caractérisation et à l’identification de formes simples par sonar actif: application à un sonar de pêche. Thèse de Docteur-Ingénieur, Uni­versité d ’Aix-Marseille II, France.

Zakharia, M .E . 1983. Problèmes inverses en sonar de pêche: possibilités et limitations. Revue du CETFIEDEC, 76: 79-90 .

Zakharia, M. E ., and Pey, J. M. 1983. Détection et comptage de poissons par sonar haute fréquence. Possibilité de recon­naissance des espèces et d ’estimation de la taille. Neuvième Colloque sur le Traitement du Signal et ses Applications. GRETSI, Nice, France.

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