III1__I__Y__~IIIIPI_1__LII~~_I~-_II.Y
AN UNDERWATER SOUND SYSTEM USED
FOR ACCURATE POSITIONING
by
S.B.,
MICHAEL LEO HOWARD
University of North Carolina
at Chapel Hill
(1964)
SUBMITTED IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR
THE DEGREE OF
MASTER OF SCIENCE
in
OCEANOGRAPHY
p~Ti..
at the
MASSACHUSETTS INSTITUTE OF
TECHNOLOGY
July, 1966
Signature of Author.......
.. ..........
... ........
..
Department of Geology and Geophysics,
July 29, 1966
Certified. by..
Accepted by........
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The s J.
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Chairman, Departmental Committee
on Graduate Students
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AN UNDERWATER SOUND SYSTEM USED FOR ACCURATE POSITIONING
Michael L. Howard
Submitted to the Department of Geology and Geophysics on
July 29, 1966 in partial fulfillment of the requirement
for the degree of Master of Science in Oceanography.
ABSTRACT
A small battery-driven transponder was developed from
an Edgerton pinger, a hydrophone,
and an amplifier.
transponder was used in the Charles River.
The
It was demon-
strated that distances from the transponder could be determined to within 2.7 feet.
The maximum distance re-
corded was 1200 feet.
As further application of the transponder, it was decided to attempt to run a search pattern of a series of
concentric circles with the transponder at the center.
Due to difficulties in equipment other than the transponder, it proved difficult to cruise in such a circle.
With the transducer looking at the bottom, the transponder
ceased to run in sync at distances greater than 400 feet.
Thesis Supervisor:
Harold E. Edgerton
Professor of Electrical Engineering
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CONTENTS
Abstract...... ..................
..... . . ..... .........
Table of Contents ....................................
2
3
List of Illustrations................................4
Statement of the Problem.............................
Special Terms...
...................
........
.......
6
History of Navigation with Transponders..............7
Theory of Navigation with a Transponder.............1O
Experimental Procedure and Treatment of Data........14
The Transponder.....................................
19
The Seismic Recorder and Transducer.................23
Summary and Results...............................23
Recommendations for Further Investigation ..........
Acknowledgements..................................33
Bibliography........................................34
31
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LIST OF ILLUSTRATIONS
Figure #1, Data..............oeeeeeeoe....... . . . . . .20
Figure #2, Data................................ S. . . .
.21
Figure #3, Data................................ S. . . .
.22
Schematic #1, The Battery-Powered Pinger....... ......24
Schematic #2, The Transponder Circuitry........ • ..***25
• . .25
Figure #4, The Transponder Circuitry........... 0...
26
Figure #5,
27
The Transponder in Case............. . ....
Figure #6, The Transponder in Case with Bridle. *.. . . 28
Figure #7, EG&G Seismic Recorder............... . •.29
Figure #8,
EG&G Model 221 Transducer........... •••..30
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STATEMENT OF THE PROBLEM
One of the most chronically disturbing problems in
oceanography is the ever increasing need for more accurate
navigation.
One aspect of this vast problem is the need
for small research vessels in inshore coastal waters to be
able to determine their positions very accurately.
Accurate
positioning can be realized through the use of a single
pinger-transponder and an accurate compass.
Such a trans-
ponder has been used and a position has been obtained with
a radial accuracy of 2.7 feet at a range of 400 feet from
The accuracy of the position in the angu-
the transponder.
lar, i.e.
9, direction is a more complicated matter and is
explained fully in
the section on theory.
The transponder has been used as an aid in searching
for underwater artifacts.
In this instance, returning to
a particular spot is the only desired result.
Therefore,
an absolute position is unnecessary, but a position that
is merely determined relative to the pinger-transponder is
sufficient.
In this case, certain parameters need no longer
be considered.
For example, the exact position of the
transponder need not be known; the exact speed of sound in
the medium need not be known.
The only parameters that
need be considered is the constancy of the speed of sound
in the medium and the accuracy of our compass.
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Ranging from a pinger-transponder has some interesting theoretical and practical problems which will be fully
described.
SPECIAL TERMS
Transponder.
The transponder is the apparatus that
has been built for the investigation.
It has the ability
to hear an interrogative signal and to answer such a signal.
It is a battery-operated pinger and is wholely submersible.
Pinger.
The pinger is the electronic circuitry that
produces the electrical pulses which the transducer converts into the acoustic energy which propagates through
the water.
Hydrophone.
A hydrophone is a microphone that can be
immersed in the aqueous medium.
Free-running.
The term --
free-running --
refers to
the state of the transponder in which the pinger emits a
pulse at a constant, slow rate when there are no triggering impulses.
In synchronization.
or simply --
in sync --
The term --
in synchronization--
refers to state of the transponder
in which the pinger is being triggered at a constant rate
by incoming signals.
The transponder when running in
sync
must be running at a faster rate than its free-running rate.
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Threshold signal. The threshold signal is the weakest interrogative signal that will allow the transponder
to run in sync.
Discharge capacitor.
The discharge capacitor is the
capacitor that is discharged through the transducer in
order that a ping can be emitted.
HISTORY OF NAVIGATION WITH TRANSPONDERS
Since the ocean is opaque to all electromagnetic radiation with the possible exception of blue-green light and
the radio spectrum below 20 Kc/ sec,1 it has become necessary to turn to acoustical radiation which does propagate
through the sea.
The body of knowledge built up as a
result of the study of this propagation of sound waves
through the water is known as underwater acoustics, and
that field of applied acoustics that is concerned with
2
Sound
water as the propagating medium is known as sonar.
is, unfortunately, a poor substitute for light, and sonar
is a study that is fraught with difficulties.1
Historically, the need for more and more accurate navigation is one of the most pressing problems of oceanography.
This need has caused oceanographers to turn to sonar.
In early 1964, the Raytheon Corporation unveiled a
sonar system which if given an initial position would continuously plot the ship's position on a chart.
The system
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employed an array of four transducers and worked on the
Doppler Shift principle. 3
Sonic beacons have also been proposed to fulfill
certain needs,
General Dynamics has developed a nuclear
power beacon that emits a squeal that can be heard for 20
miles.
The beacon with a life of 10 years is expected
to be used as a warning of hazards to navigation.
Vacuum
Reflex Limited has proposed setting out a string of either
pingers or transponders along the middle of narrow, dangerous channels.
The system would be constructed such that
the range of each of the pingers would overlap considerably,
i.e. maximum range would be up to 6 miles with spacing between pingers only 1/4 to 1/2 mile.
A vessel moving down
the channel would simply steer for maximum signal strength
in order to be directly over the string of pingers.
Today a great deal of work is being done on transponder
navigation systems.
These systems vary to a large extent
from one transponder being interrogated by a ship with
three hydrophones to a ship with a single hydrophone interrogating several transponders. 6
The triple hydrophone
array yields its range from the transponder by measuring
the time-delay between interrogation and reply and its
bearing from the transponder by measuring the time differences among arrivals of the reply at each of the three
hydrophones. 6
Systems that employ several transucers 7
deduce range to each of several transponders by measur-
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ing time-delay between interrogation and response; determine circles whose centers correspond to the positions.
of each of the transponders and whose radii correspond to
the appropriate range from the transponder, and then determine position by locating the common point of the intersection of all of the circles.
Transponders today are expensive and have limited
range,
e.g. $5000 for a medium range -- 5 to 10 miles --
and long-life transponder. 7
However, in the present case
where small range in shallow depth is desired, it has been
shown that a very inexpensive transponder can be used.
References
1.
Hubert M. Hoover, "An Experience Report on Acoustic
Command", Geo-Marine Technology, 1966, Vol. 2 #5,
Pp 16-19.
2.
J. Warren Horton, Fundamentals of Sonar, Annapolis,
U.S. Naval Institute, 1959, P 5.
3.
"Automatic Nautical Navigation", J. Franklin Inst.,
1964, Vol. 277, Pp 178-179,
4.
"Nuclear Navigation Beacon", Mech. Eng., 1964, Vol.
86 #2, P 57.
5.
"Sonar Beacons-Low Cost Aids to Navigation in Shallow
Waters", British Communications and Electronics, 1964,
Vol. 11 #10, P 722.
6.
ogy
Joseph A. Cestone, "Navigation Systems", Oceanol
1966, Vol. 1 #1, Pp 63-64.
7.
D. M. Sherwood, "Acoustic Navigation Systems", Nay,
Eng. J., Ye6., $964, Vol. 76, Pp 937-940.
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THEORY OF NAVIGATION WITH A TRANSPONDER
The function of the transponder is to answer any acoustic pulse that it receives.
If it is assumed that a pulse
travels directly from the interrogating vessel to the
transponder and the answer along the inverse path back to
the interrogating vessl, and that the speed of sound through
sea water is known, then a measurement of the travel-time
from the ship to the transponder and back can be used to
determine the range.
The theory rests upon the assumption that the speed
of sound is constant.
For if the speed were not constant
in time, then the travel-times would change for a given distance.
If the speed were not constant in space, then the
ray-paths would not be straight.
In point of fact, the speed of sound is not constant,
but is a function of pressure, temperature, and salinity.
However, Wayne D. Wilson's1 formula for the speed of sound
in sea water shows that a variation in termperature of 10 C
shows a change in speed of approximately 2%.
The expression
for calculating the change in the speed of sound is dominated by a linear term.
Thus the speed of sound in sea water
is approximately 0.2%/deg C or about 3m/sec-deg C.
Since
the transponder was used at a range of no more than 120
meters, it is instructive to convert this change in sound
speed to a distance at the maximum range.
This change is
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approximately 1 foot/deg C at a range of 360 feet.
This
change of 1/360/deg C is not quite negligible since the
experimental error is of the order of +1.1 feet.
Therefore,
if ranges from the transponder are to be taken over a period
of time in which the water temperature could change by
several degrees, it would be advisable to keep a temperature record and alter the value of the sound speed in the
calculations of ranges.
If the temperature of the water changes with respect
to space instead of time, then the sound wave would not
travel in a straight line.
This refraction would ultimate-
ly appear as a shadow zone in which no signal could be
heard.
Under normal conditions, refraction would cause the
wave to curve such that the ray path would be concave
downward. 2
Typically, the limiting ray that is emitted at
a depth of 10 feet becomes tangent to the surface at a distance of 500 feet. 2
Thus it can be readily seen that the
actual path length would be negligibly longer than a
straight line path.
Other complicating effects are the Lloyd's Mirror
Effect and bottom effects. 1
The only bottom effect that
could possibly be of importance to the experiment would be
the occurrence of a "fast" bottom, i.e. a bottom through
which the speed of sound is faster than through the water.
In this case, it is possible for the first arrival of the
I
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12
pulse of the recorder to be something other than the direct
wave.
For this experiment, the Charles River Basin has a
very "slow" bottom.
The Lloyd's Mirror Effect is the constructive and destructive interference effects that occur between the
direct acoustic wave and one reflected from the surface of
the water, i.e. the top surface of the medium.
This inter-
ference effect is reduced to negligibility since the ping
length is only 10 msec. and the frequency is 2 pings/sec.
Therefore, the pulse length is only 1/50 the time between
pings.
Since the interference is due to the difference in
path length and therefore travel-time, the probability
that the pulses will overlap is very small.
There remains one other complicating factor that needs
discussion.
The transponder works in a three dimensional
medium as its depth must be considered.
The depth can
easily be determined by "ranging" directly over the transponder.
With this information, the true horizontal dis-
tance from the transponder to the interrogating vessel can
be calculated using the known depth, the recorded range,
and the Pythagorean Theorem.
This calculation is un-
necessary if the depth of the transponder is constant,
i.e. if there are no tides, and if it is not necessary to
know the exact position but just to be able to return to
the position.
Establishing the horizontal distance from the interrogating vessel to a single transponder merely establishes a
circle with the transponder at the center and the interrogating vessel somewhere on the circle.
In order to estab-
lish the position of the vessel, it is necessary to use
the ship's compass.
If it is assumed that the direction
of motion of the vessel is always perpendicular to the
radius of the circle, then any error in this assumption is
translated into an error in position along an are of the
circle.
This error in absolute terms is a function of the
radius as well as the error in angle, i.e. the error is
rdQ.
Thus the error is larger at larger radii.
If it can
be assumed that the vessel's compass has been accurately
compensated or that the deviation and variation have been
considered and that the helmsman is skilled and can steer
a curved course as well as he can steer a straight one,
then the compass error will be of the order of + 2 deg.
The error at a range r will be 0.0349r.
References
1.
Wayne D. Wilson, "Equation for the Speed of Sound in
Sea Water", J. Acoust. Soc. Amer. 1960, Vol. 32 #10,
P 1357.
2.
L. Spitzer, Jr., "Deep Water Transmission", Physics of
Sound in the Sea Part : Transmission Research Analysis
Group Comm. on Undersea Warfare, National Research
Council, 1945, P 118.
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EXPERIMENTAL PROCEDURE AND TREATMENT OF DATA
During the early phases of the investigation various
modifications of the battery operated pinger, different
transducers, and different hydrophones were tested in order
to determine which configuration of components would make
the best transponder.
A free-running pinger was hung over the edge of the
dock of the M.I.T.
Boat House.
Then the experimenter in a
boat would proceed down the rowing course.
At each of the
marks, e.g. 1/2 mile, the experimenter would stop the
vessel, shut down the engine in order to minimize noise,
hang a hydrophone over the side into the water and measure
the signal that appeared in a portable oscilloscope.
The
signal from the hydrophone was multiplied 100 times by a
Hewlett-Packard Model 266A AC amplifier and then into a
Tetronix Model 321A Portable Oscilloscope.
The first experiments entailed the use of an Edgerton
battery-operated pinger-driver, Schematic #1, with B+ voltage of 135v, and a discharge capacitor of l~fd, a Harris
Model LM-12 transducer, and an Electrotech hydrophone.
It became quickly obvious that this configuration
would never do in that it was impossible to hear the
pinger at any sizable range.
than 1/8 mile.
The maximum range was less
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The next attempt at an improved transmitting unit
consisted of virtually the same pinger-driver -- although
this driver had an 110v AC power supply for convenience -and an EG&G Model 221 Transducer.
definite improvement.
The transmitter was a
By raising the discharge capacitance
to 1*fd, the transmitter became the most powerful one which
was used.
This transmitter would be heard to a range of
3/4 mile.
During these tests, a Chesapeake Instrument Corporation hydrophone, commonly known as an 8-ball, was tried and
found to be superior in that it was almost as sensitive
but had a far superior signal-to-noise ratio.
As the investigation evolved, the AC driven pinger with
an EG&G transducer was set aside as being too heavy and inconvenient.
In returning to the concept of a battery-
driven pinger-transponder, the investigation centered upon
use of the Bendix toroidal, 6" diameter, transducer.
Making the assumption that the Bendix-transducer would
be used, the investigation turned toward the pinger circuitry.
Various "cut and try" experiments yielded the
result that a B+ voltage raised to 225v would give substantially larger acoustic outputs while increasing the
value of the discharge capacitance did virtually nothing.
The next task involved making the transponder.
It was
discovered that by inserting an inductance as shown in
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Schematic #2 we could simply connect the hydrophone
through the amplifier and into the pinger-driver circuit.
With the laboratory tests completed and successful,
the transponder was temporarily mounted in a wooden box
and taken to the M.I.T. Boat House.
were performed.
There two experiments
First the transponder was put aboard the
test vessel and left to free run at a frequency of 1 ping/
sec
A. standard pinger-driver with an EG&G transducer
was mounted at the end of the dock.
at a frequency of 2 pings/sec.
The pinger was running
Thus by taking the transpond-
er down the rowing range, stopping at certain points along
it, and lowering the hydrophone into the river, it became
immediately
obvious when the transponder was either in
sync or free runuing.
range of 3/4 mile.
The transponder ran in sync up to a
The second test involved free running
the transponder at the boat house dock and running down
the range to see how far the pinger with the Bendix transducer could be heard.
Unfortunately, this range was only
1/2 mile.
The transponder was then mounted permanently in a
water proof case and prepared for tests in the Charles
River; see photographs in "transponder" section.
The first procedure was to determine the range at
which the final configuration could be used, i.e. using
the EG&G Seismic Recorder with an 8-ball hydrophone and the
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17
EG&G transducer as the equipment with which the transponder
was called and heard.
The transponder was placed on the
bottom near the west end of the M.I.T.
Sailing Pavilion
dock and the seismic recorder and related equipment placed
aboard a Boston Whaler.
The transducer was mounted such
that the direction of sound propagation was approximately
30 deg. from the vertical and therefore partially pointed
toward the transponder.
As had been found previously, the
transponder ran in sync to as great a range as before, yet
its answering pings were lost at a range of 1200 feet since
the noise from the engine and the gasoline gernerator tended to obliterate any indications of a signal if the gain
of the recorder were turned up beyond 10,000.
1.
See Figure
This gain is approximately 1/10 of the full gain of the
recorder.
Next, the transducer was placed on the bottom of the
Charles River in a channel at a depth of 40 feet.
It was
quickly found that if the walls of the channel intercepted
the straight line between the boat and the transponder, then
there was no answering signal heard.
The accuracy of the range from the transponder -- in
terms of travel times -- was simp y determined by judging
the amount of variation of travel-time between two points.
Assuming a speed of sound in water of 4800 feet/sec, that
variation in time was converted into a variation in feet.
The speed of sound in sea water does vary but not more
18
than 1 ft/deg C with the pressure and salinity variations
negligible in this case, and so our estimates of the
variation -- in feet -- vary to a nearly negligible amount.
The accuracy of the position was + 1.4 feet with temperature variations considered.
The final phase of the investigation involved running
search patterns with the aid of the transponder.
The
theory is that by keeping the travel time constant, a
circle around the transponder would be run.
Then by in-
creasing the range from the transponder in increments, a
pattern of concentric circles could be run.
In this case,
when close to the transponder, i.e. at ranges of the order
of 100 feet, maintaining a constant travel time was very
difficult.
The difficulties involved were lack of pilot
skill; finite time delay between receipt of a signal, its
display on the recorder paper, and movement of the paper
until the record of the signal can be seen by the pilot;
and obliteration of the signal by noise or by other displayed returns, or by glare from the sun.
See Figure #2;
a constant travel time would show Line P as being straight.
At the far range it was found that the accuracy was
just slightly reduced.
Running a search pattern remained
equally difficult due to the time lag between receipt of
signal and display on the recorder.
It is strongly felt
that these searches can be run using a read-out instrument
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that is similar to a non-recording fathometer.
See Figure
#3; note variation in Line P again.
During the search, we were looking at the bottom and
at the range from the pinger at the same time, i.e. the
transducer was pointed directly down.
In this configura-
tion, synchronization with the transponder was lost at a
range of 400 feet.
It was also noted that in addition to
the sharp direct arrival of answering signals from the
pinger, there were recorded smeared out arrivals that
had undergone two reflections, i.e. one from the surface
and one from the bottom.
Higher multiples were also noted
occasionally.. See Figure #2.
It was possible to look at both the bottom and the
range from the pinger by gating the seismic recorder such
that the transducer was fired every fifth sweep of the
recorder.
Thus, the transducer could be fired every 1/2
second while using the full 11 inch scale to cover any 100
msec. interval in the 1/2 second period.
Note Figure #3.
THE TRANSPONDER
The transponder is made from the Edgerton free-running
battery-operated pinger.
is
The firing voltage of the pinger
attained periodically as a function of the time constant
of the series RC circuit.
However, during the charging
cycle of the RC circuit, if any additional voltage is
supplied by the output of the listening circuit which is
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the hydrophone connected to a broadband amplifier with
a gain of about 5000.
Physically, the case of the transponder will leak when
it is stressed by the bridle.
The amounts of water that
leaked in were negligible over the course of the experimentation.
THE SEISMIC RECORDER AND TRANSDUCER
The Edgerton Seismic Recorder is shown in Figure #8.
It
contains an Edgerton pinger-driver,
an Alden 11 inch
recorder and various other capacities of which the gating
circuits proved most helpful.
The transducer, an EG&G Model 221, is shown in Figure
#9.
SUMMARY AND RESULTS
It was found that the best sound out-put from the
Bendix Transducer was found to occur with the discharge
capacitance of
qfd and a B+ voltage of 225 volts.
The
8-ball hydrophone proved to be the best since it had an
omnidirectional sensitivity and was not sensitive to
transient noise as the other hydrophones that were tested.
By judging constancy and the sharpness of the first
arrival of the answering pings from the transponder, the
range from the transponder can be determined to a distance
of 1200 feet with a variation of +1.4 feet.
The other coord-
inate's error, rdQ, is + 3.3 feet at a range of 1200 feet.
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In running searches, the same positioning error occurs
as well as errors due to the difficulties arising from
search procedures themselves.
These extra errors are de-
pendant on the skill of the helmsman and so are -certain extent --
irrelevant.
to a
Although these errors can
be eliminated by simple practice on the part of helmsman,
as yet a successful search has not been attained.
Also,
the maximum range was reduced to 400 feet but could be considerably improved with the use of a second transducer
pointed towards the transponder.
RECOMMENDATIONS FOR FURTHER INVESTIGATION
Further work with a single transponder is
in the area of conducting searches.
recommended
Searches can become
feasible and in fact quite reliable if a recorder is
em-
ployed which can display the pinger's answer quickly and
distinctly, e.g. non-recording fathometer adaptation or a
CRT display.
Such a recorder would enable the helmsman to
react much more quickly to changes in range
transponder.
from the
In addition, easy of steering a nearly cir-
cular course will be greatly eased by using large radii.
Radii larger than 400 feet can be attained through the use
of an additional transducer pointed toward the transponder.
In order to eliminate errors due to the dependence on
a compass, a second transponder pinger can be employed.
radii-- one from each transponder --
would establish two
Two
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32
circles which would intercept at two points.
The extran-
eous point of interception could be rejected from a knowledge of the general area in which the true position is
located.
The remaining point of interception is the true
position.
In order to eliminate the extraneous point of interception, a third transponder can be used and the true
position is the point of interception of the three circles.
._1111.
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33
ACKNOWLEDGEMENTS
I would like to thank the Bendix Corporation for the
loan of a Display Model Transducer, EG&G, Inc. for the
loan of a Seismic Recorder, and a Model 221 Transducer,
and the Physical Oceanography Lab at M.I.T.
of various pieces of equipment.
for the loan
I would also like to
thank Mr. V. E. MacRoberts for his help, encouragement and
friendship.
Above all others, I thank Professor H.
E. Edgerton.
I shall never be able to fully express my appreciation to
Doc for everything, for his advice, his help, his enthusiasm,
the loan of his boat, the occaisional loan of his car, and
even his physical labor in moving equipment.
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BIBLIOGRAPHY
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Cestone, Joseph A.
"Navigation Systems", Oceanology I
(June 15, 1966) Pp 63-64.
Hoover, Hubert M. "An Experience Report on Acoustic Command",
Geo-Marine Technology II (May 1966) Pp 16-19.
Horton, J. Warren. Fundamentals of Sonar, Annapolis:
United States Naval Institute, 1959. 4117 pP.
Jackson, D. E. and F. Agresti. "Shipboard Navigation Using
Acoustic Transponder Buoy of Atlantic Missile Range
Shipborne Navigation System", Marine TechnologY
Societ j Buoy Technology
yvQosiieTeholoum -- Transactons
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Mason
Warren P. Physical Acoustics, New York:
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The
Sherwood, D. M. "Acoustic Navigation Systems", Nava
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