Deep-Sea Research, 1954, Vol. 2, pp. 145 to 151. Pergamon Press Ltd., London.

Underwater telemetering

A telemetering depth meter

WILLARD DOW

Summary--This report describes an inexpensive telemetering depth meter capable of determining depth of towed gear and other information, and of transmitting the data acoustically through the water to the surface. The device is somewhat unusual in that it uses the ocean rather than the air as the metering medium. Built into a torpedo-shaped fish for towing purposes, the instrument is self-contained and may be quickly attached to any trawl wire.

INTRODUCTION

A TELEMETERING depth meter which can be made to determine depth of gear, current flow, or other information and then transmit the data acoustically through the water has been developed at the Woods Hole Oceanographic Institution. The acoustic signal is detected at the surface by a hydrophone connected to a sonar or low frequency radio receiver in the ship. The depth meter unit is self-powered and may be clamped on to any trawl wire in a few moments.

The device is somewhat novel in that it uses the ocean itself as the telemetering medium. That sound travels great distances through water with little attenuation has been known for many years, and highly efficient echo ranging and echo sounding equipment has been developed which takes advantage of this phenomenon. A sound "carr ier " signal can be modulated or coded at the transmitter in accordance with the information desired by employing the Same principles and coding techniques already developed for sonobuoys and radiosonde. The carrier is " t u n ed i n " at the ship much as a broadcast is tuned in on a radio, and then demodulated or decoded to recover the original information just as music or speed is detected and reproduced by a radio set. In fact, a long wave radio receiver can be used without alteration for detecting the acoustic signal by replacing the radio antenna with an underwater an tenna - i.e., a receiving hydrophone.

REVIEW OF CURRENT METHODS

Equipment developed to date for oceanographic research may be roughly divided into classes. The first consists of instruments which, when lowered over the side of the ship, produce a reading or a record which can be inspected when the device is recovered. The Bathythermograph (SPILHAUS 1937), bucket thermometer, and Nansen Bottle are examples of this type of instrument and they have a number of special advantages. They are simple, rugged, and compact. Being self-contained, they require no electrical cable to the ship. They have the disadvantage of providing data which may be valid only for a very small area or during a short space of time, and of making such information available only upon recovery of the instrument. In those parts of the world where conditions in the sea are reasonably homogeneous these factors are of minor importance, but in re~ions where conditions may vary

145

146 WrLLAPa~ DOW

significantly within a relatively small area, or are subject to rapid changes, instruments which can supply continuous information without constant attendance are much to be preferred, and may be essential in some instances.

The second class of instruments, exemplified by various depth (VON ARX 1950) and current meters (UFFORD 1946) do provide continuous monitoring, but have the disadvantage of requiring electrical cables to transmit the information to the ship. Anyone who must operate such equipment at sea soon discovers that cables are a nuisance to handle on deck, that they readily become tangled with trawl wires, nets, or other gear being towed at the same time and even on occasion become fouled in the screws. Often they are subjected to sufficient strain and wear as to eventually leak or break. Replacement can be a time consuming and expensive process.

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Fig. 1. Schematic diagram of telemetering depth meter.

Telemetering equipment can be designed to incorporate the best features of both classes of instruments without incurring their disadvantages. It can be made rugged, compact, self-contained and yet capable of supplying continuous information to the ship. Oncesubmerged, the gear operates without attention for long periods of time. In fact, if data is desired from a point somewhat remote from the ship, the telemetering unit can be attached to a floating buoy by an-ordinary wire or rope and suspended at any desired depth below the surface. A remote fiydrophonecould be effected by equipping such a buoy with a receiving hydrophone and a/iiplifier- modulator. The information received by the hydrophiJne would be amplified and transmitted through the water to the ship as modulation 0f:the carrier signal in the

Underwater telemetering 147

manner described above. Alternatively the same instrument might be used as a deep hydrophone by suspending it from the ship's hydrographic wire, thereby elimina- ting the necessity for a special electrical cable capable of withstanding the severe strain.

These are only a few of the many possible applications. Telemetering units of this nature can do much to satisfy the long existent need for compact self-contained instruments capable of supplying continuous information about the ocean to a ship at sea.

GENERAL DESCRIPTION OF THE TELEMETERING SYSTEM

The telemetering depth meter makes use of the principles described above. The need for this equipment became particularly pressing when it was required to tow plankton nets and fish trawls in scattering layers. This operation requires that the depth of the plankton nets and fish trawls be known and ~aonitored continuously.

/TRANS.*SSION ~ .

~ - - ""- ~,~. ~ G NET

PLAN OF OPERATIONS

Fig. 2. Plan of operations.

The instrument determines this depth and continuously transmits the information acoustically through the water to the ship. The submerged fish contains a stable heterodyne oscillator (Fig. 1), the frequency of which is varied between 16 and 26 kc. by a variable capacitor which in turn is driven by a bourdon tube sensitive to pressure and having access to the sea. As the instrument descends, the increasing water pressure causes the bourdon tube to rotate the capacitor and the frequency of the oscillator increases. The oscillator output is amplified by a power vacuum tube and applied to a transducer in the tail of the fish. The sound radiated by the trans- ducer is picked up by a receiving hydr0phone, which is towed behind the ship at the

148 WILLARD DOW

surface (Fig. 2). The hydrophone is connected to a tuned receiver in the vessel. As the tuning dial of this receiver is calibrated in terms of frequency, reference to a frequency vs. depth curve gives the depth of the instrument, or alternatively, the dial may be calibrated directly in terms of depth. Current or temperature information can be applied as a modulating signal on the depth "carrier signal" in accordance with the principles outlined above.

STRUCTURE

The instrument is self-contained in a torpedo shaped housing, or fish, about 53 inches long and 3½ inches diameter (Fig. 3). The housing is ~divided into three compartments separated by watertight bulkheads (Fig. 4). The forward compartment contains the bourdon tube and the variable capacitor fastened to it as well as the relay which acts as a power switch. The middle compartment contains the electronic chassis and battery stack. The aft compartment houses the forward end of the trans- ducer and its cable and is removable from the rest of the housing as a unit. Con- nections between compartments are made by sealed feed-through connectors. The watertight bulkheads prevent damage to the electronics if either the bourdon tube or transducer should become damaged or leak.

MOUNTING

The depth meter fish is mounted on the trawl wire by a triangular bracket (Fig. 3). The fish is free to rotate in a vertical plane in the bracket so that it can always swim parallel to the surface. The bracket is free to swivel about the wire but cannot move vertically because of the stops. The swivels and stops are of a new quick-disconnect design which permits the instrument to be mounted or removed in a few seconds time (Fig. 5). The trawl wire slips into a slot on the side of the clamp. An L-shaped piece of brass is then inserted from the top into a slot a t right angles to the first slot. The handle on the apposite side is then turned clockwise. This operates a screw which in turn forces a vertical grooved bar against the wire. The wire is thereby effectively clamped between this bar and the L-shaped insert. To disconnect, the procedure is simply reversed. The swivels are of similar construction except that the clamping feature is omitted. A weight or depressor is hung on the lower end of the wire to obtain a reasonable angle of tow.

OPERATING PROC~_J)URE

Hydrophone Placement. Experiments at sea indicate that the main obstacle to good reception of the depth meter signal is screw noise from the towing ship. For example, the n0n-directional hydrophone (Fig. 6), must be towed 1,000-1,500 feet behind the vessel before this background ceases to limit the sensitivity of the system. However the performance of the directional unit (Fig. 7) appears far superior in this regard, as might be expected, and tests are under way to determine if revised towing arrangements and orientation will permit it to be used close to the ship.

Depth Meter Placement. When the hydrophone is in the water the depth meter is lowered just below the surface. This automatically starts the eleotronic oscillators by comptetiag a relay circuit through a salt water switch which is mounted on the nose of the fish. (The switch also insures that the device cannot accidentally he left operating when not in use and thus prolongs battery life.) The ~strument

Fig. 5. Wire clamp.

Fig. 6. Non-directional towed hydrophone.

Fig. 7. Directional receiving hydrophone.

Fig. 3. Depth meter mounted on trawl wire in towing position.

Fig. 4. Telemetering depth meter interior chassis and transducer.

Underwater telemetering 149

should remain in this position a few moments until a stable zero depth reading on the receiver indicates that the electronics have reached operating temperature, the unit may then be lowered to the desired depth.

Operating Time. Battery life, using mercury batteries, is approximately four hours. Since the average net tow does not usually exceed this time period, and since battery pack replacement is quick and easy, it was deemed inadvisable to increase the bulk and weight of the instrument to provide longer battery life.

Receiver Tuning. To receive the depth meter signal, the beat frequency oscillator is turned on and the main receiver tuning dial adjusted until an audible note indicates the carrier has been picked up. This note is then adjusted for zero beat and the frequency read from the receiver dial. A glance at the frequency versus depth calibration curve will then give the depth, or alternatively the dial may be calibrated directly in terms of depth. If the signal becomes weak some readjustment of hydro- phone position may be to advantage.

SEA TRIALS

"Blue Dolphin " Cruise. A preliminary model of the device was given a sea trial during a 10-day fishing cruise aboard the schooner Blue Dolphin in August 1953, where it was used to determine the depth of fishing nets during trawling operations. It was found that the signal could be heard reliably at depths up to 1,500 feet by means of a non-directional hydrophone towed behind the ship (see below), but was inter- mittently lost in screw noise at greater depths. However, the meter could readily be heard to 3,200 feet when the ship was not under way. Calibration of the instru- ment was checked during the towing operations by echo sounding on the unit from a following vessel. Comparisons of the echo sounder and depth meter records for these runs yielded differences ranging from 4.8 % to 8.3 % of the depth (as determined by the echo sounder). However, it was found that the depth meter oscillators had been somewhat over compensated for temperature changes. A rough determination of the ~rror from this source was made before the final run and applied as a correction to the telemetered signal. Comparison of records for this run indicated a deviation of only 6 feet in a 343-foot determination. This would correspond to an error of 1.8% for the depth meter reading, but since the echo sounder readings may also have been in error by this amount, this value is only an approximation. In the course of these runs a complete depth versus wire out curve was established for the net and depressor unit, and the curve was used in tests on a second expedition as described below.

"Atlantis" Cruise. Following the Blue Dolphin cruise, the depth meter was stabilized for temperature changes in the laboratory and then given additional tests on Atlantis cruise 196 to Puerto Rico. During this cruise the readings of the instru- ment were checked in two ways.

First a net-depressor-depth meter assembly practically identical to the one used on the Blue Dolphin cruise was towed at various depths and depth meter readings were taken to depths of 1,080 feet. The length of wire out and the wire angle were recorded, and the depth meter readings then compared with values taken from the depth vs. wire out curve described above (Fig. 8). While there was undoubtedly some error in applying this curve to the Atlantis operation, nevertheless the mean

150 WrLLAaD Dow

difference between values taken from the curve and the depth meter readings was only 13.0 feet which represents 0.9% of full scale for the depth meter (1,530 feet).

The second check consisted of a vertical lowering with the ship hove to. During this test the depth meter frequencies were plotted against length of wire out as read by the Atlantis meter wheel for 18 different depths. The resulting curve was then compared with the original laboratory calibration curve (Fig. 9).

There are thought to be two principal sources of error in these experiments. First, the laboratory calibration follows a smooth curve, but the meter wheel calibration curve is erratic particularly at shallow depths (Fig. 9). This may be due to slippage

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- - DEPTH METER READINGS - - METER WHEEL READINGS

DIAL READINGS

Fig. 8. meter.

Depth vs. frequency curve for depth (Depth readings from depth vs. wire out curve described in text).

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4 0 0

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DIAL READINGS Kc

Fig. 9. Depth vs. frequency ca~bration curves for telemetering depth meter.

between the wheel and trawl wire when the tension is light. Secondly, the receiver dial was calibrated only to 0.1 kc. which with this depth range corresponds to 16 feet. Since the mean difference between the meter wheel and laboratory curves is approximately 13 feet, a considerable portion of this discrepancy could be due to reading error, A more finely divided receiver dial is desirable for future work.

It should be pointed out that if greater accuracy is desired at shallow depths, an alternative " nose" for the fish containing a low pressure bourdon tube can be plugged in. One of these units containing a 500-foot bourdon was taken on the Atlantis cruise, but was not used because the scattering layer investigations were

concentrated around greater depths.

NEW DEVELOPMENTS

A new and more efficient model of the instrument having higher output power and lower bat tery drain has recently been developed at the Woods Hole Oceano-

Underwater telemetering 151

graphic Institution. The new instrument has provision for applying temperature information as modulation of the cartier signal. The carrier frequency simultaneously provides depth information as in the older unit. If the instrument proves successful it will be made the subject of a future report.

ACKNOWLEDGMENTS

The author wishes to thank the following people for substantial contribution to this project:

Dr. J. B. H~sEy made suggestions concerning types and placement of transducers involved in the acoustic link as well as recommendations concerning the towed fish. Mr. S. T. KNoa~r designed and helped fabricate the structure of the depth meter, mounting bracket, towed hydrophones, etc. and helped test the equipment both in the laboratory and at sea. He also contributed substantially to other phases of the work. Mr. W. E. WITZELL assisted Mr. KNOTT in the work outlined above. Dr. BENJAMIN B. LEAwTr assisted in the overall design of the towing assembly and he and Dr. R. H. BXCKUS helped test the equipment at sea. Mr. M. E. EDWARDS constructed the electronic chassis for Mod, 1 and 2 of the equipment. Mr. H. A. CAIN constructed the special battery packs designed for the instrument.

Contribution No. 729 of the Woods Hole Oceanographic Institution. This work was supported under Contract NObsr-43270 with the Bureau of Ships.

REFEREN~ SPILHAUS, A. F. (1938), A bathythermograph. J. Mar. Res., 1 (2): 95-100. UrrotD, C. W. (1946), An electronic bathythermograph. Unpublished Manuscript. VoN ARx, W. S. (1950), Some current meters designed for suspension from an anchored

ship. 3". Mar. Res., 9 (2), 93-99.