Not to.be cited without prior reference to the author
C.M. 1986/B:17
Fish Capture Committee
International Council for the
Exploration of the Sea
Theme session U
A COMPARISON OF TWO METHODS FOR MEASURING THE
EQUIVALENT BEAM ANGLES OF HULL MOUNTED TRANSDUCERS
(preliminary)
by
PalI Reynisson
Marine Research Institute
Reykjavik, Iceland
ABSTRACT
A first
directivity
attempt
to
measure
the
of split beam transducers,
conventional
using
the
information available
on the
parallel interface of
the ES400 split-beam echo sounder, is described.
Measurements on two transducers are
presented and
compared to those obtained by a method described in an
earlier paper (Reynisson, 1985) , were the displacement
of a
standard target in the sound beam is calculated
from the geometrie configuration of the set-up.
The
difference
in
equivalent
beam angles as estimated
by these two methods are 2 dB.
Measurements of the
compensated sensitivity throughout the beam are presented.
Variations over a 9 dB range were observed.
sons are discussed.
possible rea-
- 2 INTRODUCTION
A method for measuring the equivalent beam angles
of hull mounted transducers has been developed at the
Marine Research Institute in Iceland.
This method
relies on the possibility of estimating the displacement
of a reference target, suspended on three lines below
the transducer, by geometrical considerations when the
length of one ,of the lines is changed by a certain amount.
. . It was stated that a m~re direct way of measuring the
position of the sphere
was needed in order to test
further the accuracy of this method, and that the use of
a split
beam echo sounder could provide the means for
this (Reynisson, 1985).
The principle of the ES400 split-beam echo-sounder
and
some of it features havebeen described by Foote
et.al.
(1984).
The information available on the parallel
computer interface of this instrument makes it possible to
sampIe miscellanous da ta for every transmission, such as
the
p~sition
of
the target
in the beam and target
strength, compensated for the transducers directivity. In
this way the directivity of the conventional beam as
weIl as the sensitivity
changes
of
the compensated
signal can
be measured in considerable detail, fairly
quickly.
Measurements of this kind were undertaken this year,
as .ES400 echo sounders are now installed on·two Icelandic
research vessels.
The directivites of the split beam transducers as
measured' by the two methods mentioned are presented, as
weIl as the sensitivity of the compensated beams.
MATERIALS AND METHODS
The
equipment
and
set-up
for' measuring the
directivities of the transducers is the same as described
by Reynisson (1985) whichis very similar as is used in a
standard target calibration. of ordinary echo sounders
(Foote et.al. 1981).
The parallel interface of the
ES400
was connected
to a Hewlett Packard 9816 personalcomputer.
For every ten centimeters of the depth column, a read
pulse is sent from this interface, and by giving a three
,
- 3 hit control code, different information is
data lines. ' Further'details
the eight
available on
are given in
the manufactures instruction manual (SIMRAD ES400, P2092E,
1985).
To handle this amount of data,' either a very fast
computer is needed or some preselection is neccessary.' In
this case
for
the data sampling was limited
every
transmission.
This
was
the
computer
at
one
done
presettable counter with comparator to
to
to
by' using
give a
the desired depth.
reading
a
read pulse
The counter was
then reset by the next trigger from the echo sounder.
scematic diagram
of this ,:is sh~wn, i~~figure 1.
A
The
reflected pulse and read pulse were monitored on a digital
oscilloscope and made to coincide in time as in figure 2.
The angle information was sampled in the
transmissions and
the compensated target
first two
strength on
the third by sending the neccessary control codes to the
ES400 interface.
The oscilloscope which was also connected to the computer, sampled and stored the
peak
vol-
tage of
the reflected
pulse in every transmission.
The average peak voltage was then stored with
the
corresponding angle - and target strength information for
later analysis.
When measuring the directivity by the. "geometric"
method,
the
changed
by 10
ES400
data
sphere.
lines
such
length
of one of the
centimeters at a
obtained
for
At other timesthe
were
that
changed
suspension lines
time~
each
lengths
continuously,
and
new
of
ten
position
the
but,
sets
was
of
of the
'suspension
very
slowly,
more detailed information could be gained.
RESULTS
The equivalent beam angles were calculated for each
transect, as weIl as for the whole beam according to
both methods, and are,given in table I . T h e
average
deviation
of the
compensated target strength measured
throughout ,each
strength measured
transect
relative to.,
the
targe
on the acoustic axis are also given
in this table. The conventional two way directivities of
,the transducers in the seperate planes and the resulting
energy contours are shown in figures 3-6.
The sensitivity changes .ofthe compensated signal are shown in fig-
r - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- -
...
- 4 ure 7 and 8. The transeets made by the sphere
ured by the ES400 are shown in figure 9.
as
meas-
DISCUSSION
When looking at table 1. there are several striking
differenees in the equivalent beam angles a~d the direetivites in the seperate transeets as measured by the two
methods:
1)
Por both transdueers the split beam measurements give
2 dB higher equivalent beam angles than the
geometrie methode When eomparing the seperate transeets
these differenees range from 0.3 to 3.8 dB.
2)
On transeets where the geometrie method gives
the smallest beam with, the split-beam measurements give
the widest. It should also be noted that aeeording to
the ES400 measurements, the beams are more deformed from
the eireular than might be expeeted.
Aeeording to the
geometrie
method the
beams are· fairly eireular, and
although some bias in the estimated angles
is possible,
it is
very unlikely that this bias would differ mueh
from one transeet to the other, unless very strong tidal
eurrents were present.
3)
When monitoring the movements of
the sphere
through
the beam,
the
angle information on
the
port/stb transects did not change although the sphere was
moving.
This can
clearly be seen on the direetivity
diagrams in figures 3
and 5,
as weIl
as on the
transeet-diagrams in figure 9. These "gaps" are on the
starboard and port side of transdueer I and 11 respeetively.
possible explanations for these differences on the
seperate transeets
as weIl as the jump of the observed
angles are that either some misalignmentwas present in
the
phase
relationship
of the echo sounders four
receivers, or that the mounting arrangements
of the
transdueers have ehanged the beam pattern or influeneed
in some way
the phase
relationship of the echoes.
Wether
it has any bearing on the matter or not, it
is interesting to note that transdueer I is mounted on
the port side of R/V Arni Fri5riksson and transdueer 11
on the starboard side of R/V Bjarni S~mundsson.
----------
,
- 5 Anomalies in the
phase
relationship
will
of
course
affect the compensation of the received signal.
Infigures 7 and 8 where these compensation errors 'are
shown, the units of the abscissas are chosen in such a
way that the areas 1imited by the measured points are
representative
of the average error,
as the
area
weighted intergral of
the
compensation
must
be
estimated.
This has been further adressed by MacLennan
et.al.
(1986). The diagrams in figure 7 show that the
compensation for transducer I is fairly good except in
the aft/stb direction.
For this
transducer a
new
memory microcircuit (PROM) had been installed, containing
a new set of lobe correction factors. For transducer 11
the original PROM was used. In this case the average
compensation error is about 1 dB, which
if taken by
itse1f is acceptable.
But the diagrams in figure 8
show that the errors are much more severe in this case.
Estimating roughly the extremes, shows that variations in
measured target strength of -4 to +5 dB
are to
be
expected
from a uniform target, depending on its position in the beam.
When measuring the target
strength
of live fish this might not affect the mean more than
the stated average deviation of 1
dB, but
it would
deform the
true distribution of target strength. This
will also in effect shorten the usable dynamic range of
the scale chosen.
REFERENCES
Foote, K.G., H.P. Knudsen, G.
Vestnes, R.
Brede and
R.L.
Nielsen,
1981.
" Improved
ca1ibration
of
hydroacoustic equipment with copper spheres." ICES, C.M.
1981/B:20
Foote, K.G., F.H. Kristensen and H. Solli, 1984. "Trials
of a new split-beam echo sounder." ICES, C.M~ 1984/B:2l.
MacLennan, D.N. and I. Svellingen, 1986.
tion of a sp1it-beam echo sounder."
"Simple calibra-
ICES, C.M. 1986/B:8.
Reynisson, P., 1985. ·""A method for
measuring
the
equivalent beam angles of hull mounted transducers."
ICES, C.M. 1985/B:4.
Table 1.
Equivalent beam angles as calculated for each transect as weIl as the whole beam.
Also given is the average deviation of the compensated signal relative to the onaxis sensitivity.
Units are in decibels.
Average deviation
Transect
Equivalent
'!geometric 11
I
Aft/port-fore/Stb
-21.2
-17.5
-0.5
11
Fore/Port-Aft/Stb
-20.5
-18.9
-1.2
11
Port/Stb
':'20.3
-18.8
-0.2
11
Whole beam
-20.6
-18.5
·-0.7
II
Aft/Port-Fore/Stb
-20.6
-19.6
-0.3
11
Fore/Port-Aft/Stb
-20.9
-17.1
2.1
11
Port/Stb
-19.5
-19.2
0.8
11
Whole beam
-20.5
-18.5
1.0
Transducer
Be am angles
Split beam
of on-axis sensitivity
0'\
I
- 7 -
(k4GD
[] "10
0]
g IP"
0
@
n
"C:~D
IIr
--,
WI
Il I=--
I ~ I' ,.
I.•
D
I'
'r,,"llIllt',.,
I
l.--
Calibrated out nut
~
~ I U
.1.'
L
I, ".
"' . . . I • •
'J I.,
,
.-
I ,I,
.t •• YI ".
""~.
Preeettable
counter
Read pulees I
DiP: 1tal
oecl11oBcope
Computer
I
I
Read pulse
ror parallel
datalollging
t
Figure 1.
A scematic set-up of the electronic equipment
used for the measurements(adapted from SIMRAD
ES400 instruction manual, P2092E).
VOL.TS
z
+D..
.......:'--
...L-.:L.-
-========={
0.0000
o.oa_
cao
SECONOS
Figure 2.
The envelope detected echo signal and the
read pulse from the presettable counter.
A'nl r,./June '86
Sp 1I t
be.m
Spl ft beom
-lil
-lil
-2il
-2il
Port.
-3il
-3il
( 4.96 Xl
b_leA 3_4.6153 ( 3.27 =l
b_le A 3 __4.6525 ( 4.43 =l
b*leA 3--2.3848 I 2.97 =l
c*Ie"'3-.065S C 19.35 X)
.*leA 3-.e6e3 I 12.83 =l
c*la"'3--.%k:J3" ( 11.35 ):)
._leA 3 __ .BIS8 I IS.88 =l
b*12"'3·-".5S~9
Degre ••
10
5
6
2
2
4
3
4
5
Degreea
5
6
lil
10
8
6
5
3
a
2
3
4
5
5
6
10
I
OJ
I
Arni F'r./June
Spl iot beom
Arni
Spl
-10
-2e
101ogCf'i)=-18.47 dB
b=-3.7S85, c=-.0605
-30
b.~aA3-4.291B ~
2.:37 x)
c*I0-"3--.2"774 ( 6.; X,
Frid~iksson
beam, June '86
270 --H-'++---}:~-f+f-I-I.--90
AfVStb
rcre/Fcr't
it
4 dE between energy levels
b*10"""--Z.5946 ( 3.94 X)
c.le.. .3--.03;e
( 14.21
x)
down to
-2e
dB
Degrees
10
5
8
6
4
Figure 3.
3
2
2
3
4
5
6
5
10
Two way directivity diagrams for transducer
and the resultllf energy contours.
I, as measured by the ES400
Acnl F"c./June '86
Ren t F"c ./June '86
Spllt beom
Sp 1 i t
beam
-113
*
]
-213
Portl2711'
5tb(90)
-313
bOle-3--1.7S:S : 1.4S
~,
'.IZ.... 3·-.:129 ( B.41 ::)
b*10-3--I.S514 ( 1.75
~,
.*10-3--.0121 ( 13.07
~l
b<:e-3--2.6031 ( 4.1l
c.:e.. . 3-.023'
~,
( 21.1 Xl.
b.:Z"'3-~.931S
( 4.75 X)
c.:2.... 3-.2077 ( 59.3& X)
Degl"ees
113
9
7
6
5
2
13
2
3
5
6
7
Degres.
9
113
6
113
5
4
3
2
3
5
6
8
9
Acnl F"r./June '86
Spl tt beam
Arni Fridriksson
Spl it beam, June '86
-111
-213
'oce/PortlJ33)
270 ---JH-t-t--t--r-H+++-- 90
Rft/Stb l 1531
·10Iog(fi)~-20.62
b=-2.1215, c=-.0143
dB between energy levels
-311
1.49 ::,
.<10-3-.0176 : 9. 4
~,
b*le-3--I.B276 l 3.36
~,
.*le-3-.01l2 l 22.74
~)
dB
down to -20 dB
OegI"'8 ••
113
9
7
6
Figure 4.
5
4
3
2
2
3
5
6
8
111
Two way direetivity diagrams for transdueer I
method, and the resulting energy eontours.
as measured by the
geometrie
111
Bjarni Szm./July
Ejarnl Szm./July
Splttbeam
Spl it b.om
-20
-20
Ai'1,/Pcrt
F"ore/Stb
Port.
-30
Stb
-30
b.1B.... 3-2.51 .. 7 ( 8.32 ,,)
"10-3--'.22S6 ( E.I Xl
"IBA 3-.1!3 ( S7.04 Xl
b*:~"'3·-3.S=14
c*:e....3--.eesl ( 199. J1 ;;)
( 4.53 %)
":2-3--.e']2 ( :a.66 Xl
Degreea
10
S
8
6
5
4
o
2
6
8
Deeree.
S
10
10
7
5
3
o
2
3
5
6
S
10
~
o
I
S~mundsson
BJarni
..
Spl i t beam, July '86
'.'
317
• 8·
279
-20
99
to!"e/Port
FHt/E1.b
1~log(fi
137
-30
b.10"'3--~.a7es
c.~e"'3·-.eS78
4 oB
b*le 3--... 1SBI ( 3.3 %)
( 5.,48 %J
c.10 3--.ez71 ( 31.54
( 21.13 xJ
Yo)
oB
)=-:8.51
b=-3.3017, :;=-.21638
cet~een
do~n
ene~gy
to -20
levels
oB
Degrees
10
6
Figure 5.
o
2
3
S
10
Two way directivity diagrams for transducer II, as measured by the ES400
and the resultiij energy contours.
tt
(
Bjarnt S.~und=son
TC3-EK38. 8.7.36
EJarnt Samundssen
TC3-EK38. 8.7.8E
b~2:.Bl
-10
I
I \
I
-20 .
/
38
218
-30 '
b.;2 3--j.9B4 ( 1.73
c.:~ 3.-.~47
-30
b*t21A,3--2.5423 t 3.73 ::0
7.)
b'lB~J·-2.I4S<
c.10.... 3--.027 ( 26.51 %,
( 31.B4 7.)
90
27"
( 5.24 Xl
b*1121"3-3.5429 ( '.82 X)
( 34.3 Xl
c'10~3·-.B4B7
c*19"'3--.2148 ( 36.84 Xl
Degrees
10
9
765
3
o
Z
234
6
B
9
10
10
654
9
3
Z
11
Z
3
4
5
6
9
Bjarnt S.mundssen
TC3-EK38. 8.7.86
Bjarni
315
I
270
38
---t+++-t--*---+--+++1f--
Split
S~mundsson
bea~,
July '86
S0
101ogefi)=-20.47 dE
135
218
b*t2"'3--1.9291 ( 7.01 Xl
b*12"3--2.2t? ( UL2!l X,
e*12'"'3-.3177 ( 77.12
.'10~J • • 001B
%,
b=-2.2420, c=-.0200
4 dB between energy levels
down to -20 dB
( 453.6 Xl
Degrees
10
6
Figure 6.
5
432
o
2
3
5
6
7
B
9
10
Two way directivity diagrams for transducer 11 as measured by the
method, and the resulting energy contours.
geometrie
10
- 12 Spl it beam compensatlon error/Arnl Fr./june '86
3.5
Fore/Stb
Aft/Port
3
2.5
2
1.5
.
-
++ . . +
+
+
~
...
+
+
•
+
+
+
++ + +
+
+++++
.5
Area
--------Spl it beam compensatlon error/Arni
Fr./june '86
3.5
Stb
Port
3
2.5
2
1.5
..
.. . . .... . ..... ..
. ... ..
..
... ..
• 1
-
.5
Area
L
Spl it beam compensation error/Arni
Fr./june '86
3.5
Fore/Port
Aft/Stb
3
2.5
2
1.5
00 0 0
o 00
00
o
0
8
0
8 '8
0
00
o ~3
0
00
o ~~
000
0
:, g
0
0d!,
0,jD 0
.5
0
0
8
~1J''' g, 'bo
80
0
0
(J)
00
o
0
Are a
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _--1
Figure 7.
The normalized compensated sensitivity of transducer I for the seperate transects.
The sens i-
tivity is on a linear scale against an arbitrary
area abscissa, out to 5 degrees.
13 5~m./july
5p! it beam compensatlon error/Bjarni
'86
3.5
Fore/5tb
Aft/Porb
3
2.5
2
+.
+
+
+..
+
+
+
+
+
+ +
++ *+++
++
+
.+:
1.5
+
+ .+t+"" + +* + +t+H+++t+t+
.....++ +++++ **+
'*
+
•
+
+
.....
+++-. . ~ .... ++++
.......
+++t++ .... +++
+
+
++
+
+ +++
+::.:"'..::::..~++ +~
+.++
+
...
+
+ +
+
+
:++++ +..
+.+ ..t
+
.5
++
++
+ +
++
+
+
Area
S~m./july
5plit beam compensation error/Bjarni
'86
3.5
Port
5tb
3
2.5
2
.... -
. -.
I .·5
'N
. . ... .:.. . ... .." -..
:..
•• e
::..-ji
.'
I
.5
..
Mt
••••
"
. . " ..
.• . ..
_.
•
N
l Ht
.
•••
..
Area
Sp~
lt beam compensation e .. ro .. .'Bjarni
S<em./july '86
3.5
Rft/Stb
3
o
000
0
·2.5
m
000
o
0
000
2
000
o
000
o
omo
o
I .5
o
(J)
0
0
01_0
, 0
~..o
QI)
o
.I?"
0
0
0
~000800:
0
o
o
0
00
00
0
o
o
.5
Area
Figure 8.
The normalized compensated sensitivity of trans-
ducer 11 for the sperate transects.
The sens i-
tivity is on a linear scale against an arbitrary
area abscissa, out to 5 degrees.
..
14 -
Spl it beam/Arni
Fr./june
Fa re
'86
_++
"#
,.:1$
"'*- *'"
-
1'"
Stb
...
.,+... "*
+
+
+*
Rft
Spl it beam/Bjarni
S~m./july
'86
Fore
o
Port
.
•• .
......
"'. ......
••• Stb
o
+
o
Aft
Figure 9.
The movements of the reference target through
the beams of transducer I and II, as measured
by the ES400.
A circle is drawn through the
5 degrees limit.
.
...