The Minimode float tracking system

Deep-SeaResearch, 1974,Vol.21, pp. 573 to 595.PergamonPress.Printedin GreatBritain.

INSTRUMENTS

The Minlmode

AND

METHODS

float tracking s y s t e m *

J. C. SWALLOW,~ B. S. MCCARTNEY~'a n d N. W. MILLARD~

(Received 12 October 1973; in revtaedform 13 February 1974; accepted 13 February 1974) Abslract--A system has been developed which enables up to 18 neutrally buoyant floats at various depths to be tracked simultaneously from a ship, compatible with hydrographic observations being made at the same time. It was used in the Mid-Ocean Dynamics Experiment during April and May 1973. Approximately one fix per day was obtained on each of the floats, which were at depths betwec~t 500 and 4000 m. Acoustic ranges were dependent on ray propagation, but sometimes reached 70 km, and range errors were generally less than =k0.5 km, INTRODUCTION

Tins PAPERdescribes an improved method of tracking neutrally buoyant floats from a ship. The traditional methods of taking bearings or homing on individual floats are expensive in ship time, and tracking even a small number (about six) can occupy a ship completely. Moreover, in many situations there is a need for a more detailed description of the currents than can be got from six floats, and simultaneous hydrographic observations are needed as well. Possible ways of improving float-tracking capability were being considered when the first proposals for a mid-ocean dynamics experiment (MODE-I) were made. Consequently the design was aimed at contributing to that particular project, although of course intended for more general use. The aim was to sample in more detail the low frequency (period ~ 50-100 days) mesoscale (wavelength ~ 300-400 kin) energetic (velocities ~10 cm/sec) motions noticed during the Aries observations (CREASE, 1962; SWALLOW, 1971). Float tracking on the appropriate scale was accomplished during MODE-1 by the use of SOFAR floats (RossBY and W,~B, 1970, 1971) at 1500 m depth. In our case, since the ship's endurance would limit the duration of individual float trajectories to about 3 weeks, and acoustic propagation in the sea would limit ranges from floats at most depths to a few tens of kilometres (see below), it was clearly not feasible for the shipborne system properly to match the scale of the expected fluctuations. The most useful work that could be done would be to attempt the detailed mapping of relatively small areas. For that reason the system came to be known as "Minimode'. It was designed to meet the following requirements: (1) Eighteen floats to be tracked simultaneously, (2) The tracking method to be compatible with making STD or CTD observations to 3000 m depth, *MODE Contribution Number 11. tInstitute of Oceanographic Sciences, Wormley, Godalming, Surrey, U.IC (formerly National Institute of Oceanography).

573

574

J.C. SWALLOW,B. S. MCCARTNEYand N. W. MILLARD

(3) Floats to be recoverable, so that they could be rearranged as necessary, (4) Floats to be at any depth in the range 500 to 4000 m, (5) The tracking method to yield one fix per day per float with an accuracy of 1 kin, over as large an area as possible, (6) Floats to have a useful life of at least 3 weeks, (7) If possible, to avoid interference between the float tracking system and the echo sounder. In the event, during the MODE-1 cruise of the R.R.S. Discovery in April and May 1973 all these requirements were met or improved upon for most of the time. For a short period 22 floats were tracked, with four channels each containing signals from two widely spaced floats working simultaneously. Besides tracking the floats, the ship occupied 143 CTD stations, and 25 deep casts of water bottles were taken. Bathymetry was recorded almost continuously. For most of the floats, three ranges were measured on most days, not including the short range 'running fixes', which averaged one per float every two days. For most of the time, the accuracy of fixing was better than I km, thanks mainly to the very stable reception of Loran C signals. As a result, inaccuracies due to timing, variations in sound speed, and horizontal separation of the acoustic array and the Loran C antenna began to contribute significantly to the fixing error. The summary shipboard plot of the 24 float trajectories collected during April 1973 (Fig. 1) shows that despite the small horizontal scale (pairs of floats at the same depth were not more than 65 km apart) there were appreciable differences of velocity within the group of floats at each depth. Out of 52 floats launched during the whole experiment, 41 were recovered, and with a better release mechanism that would have been greatly improved. The effectiveness of the new float tracking system can be illustrated best in terms of the total number of float-days accumulated, i.e. the sum of the durations in days of all the trajectories obtained. For this Discovery MODE-1 cruise the total was 714, which may be compared with the 867 days accumulated in all the float tracking done by the National Institute of Oceanography in the preceding 17 years, including the Aries work.

DESIGN

Basic acoustic considerations Acoustic propagation in the deep ocean is governed by the vertical profile of the speed of sound and, to a first approximation, the horizontal gradients of sound speed over ranges of the order of ten times the water depth may be ignored. Variable nearsurface conditions cause unpredictable horizontal propagation from shallow source to shallow receiver, but for the duration of a month or two the temporal variations in the sound speed profile at one location may be ignored for propagation throughout most of the water depth. Ray path diagrams based upon horizontal stratification may therefore be expected to give a reasonable guide to the propagation. Ray diagrams have been computed using a nine layer model of the vertical distribution of sound velocity with uniform gradients in each layer (Fig. 2), for sources located at depths of 10 m, 500 m, 800 m, 1500 m, 3000 m or 4000 m. Two of these are shown in Fig. 3. These diagrams show that, ignoring surface and bottom reflections, there is no single depth to which a receiver could be lowered to guarantee hearing the sound sources from floats at depths between 500 m and 4500 m, at ranges beyond 20 kin, in water

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5.3 km deep in the area of MODE-I. There is therefore a need for multiple depth receivers, or a variable depth receiver, which would be difficult to implement with moored receiving arrays, but can be incorporated in a shipboard installation where other deep sampling instruments, such as the STD and CTD sensors, routinely use armoured electrical cables down to 3000 m or more. It is possible that by working at longer ranges and at sufficiently low frequencies this depth restriction could be avoided, but we have no first-hand experience in that field and there was no efficient lowfrequency ( < 1 kHz) transducer readily available to us. The choice of float signal frequency and power was a compromise between the cost and weight of a lower frequency transducer or the higher power and hence battery weight needed for a higher frequency design. The final choice of the 5 kHz to 6.5 kHz band is well below echo-sounder frequencies and matches the available ITC 2003

576

J . c . SWALLOW, B. S. MCCARTNEYand N. W. MILLARD

velocity (m/s) 1450

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transducers; these short, open cylinders of lead zirconate titanate have a wide frequency range due to the coupling of cavity and ring modes of radiation, with also the advantage of a favourable directivity pattern, that is omnidirectional in the horizontal plane, and principally horizontal in the vertical plane, when the cylinder axis is vertical. However, it became clear that their cost would add appreciably to the total float cost, so it was desirable, even apart from operational advantages, to make the float recoverable, by dropping a weight on command. Various systems by which the float positions could be fixed were considered. The determination of float position by taking bearings from the ship was too inaccurate at long range, so that free-running pingers were rejected. It would be possible to obtain ranges from precisely timed pingers but the clock stability was marginally practicable over the period of a month, and either the float would have to transmit pulses regularly when only a few would be heard, thereby wasting power, or work in a predetermined schedule of transmission periods which would conserve power, but would be rather restrictive on other ship operations. The system adopted was to make each float a transponder, so that a range could be obtained directly from an interrogator at the ship. Float fixes were then obtained by combining ranges from different ship positions at different times, typically 3 per day. Whenever the ship passed within about 10 km of a float, a running fix could be obtained by measuring the range from several positions

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in quick succession. The quiescent power taken from the float batteries during its listening periods was calculated to be small compared with the energy drained during an expected five to six interrogation periods per day. The interrogation pulse was 30 msec long at 5.1 kHz and it was not certain how secure this would be in ambient noise and therefore how many false triggers the transponder would make per day. However, based on a simple calculation using band-limited Gaussian noise, a trigger threshold 14 db above the r.m.s, background noise should restrict the transponder to less than 10 false trigger pulses per day, an acceptable figure compared with an expected number of approximately 200 interrogation pulses per day. A slight increase

578

J.C. SWALLOW, B. S. MCCARTNEY and N. W. MILLARD

in threshold ratio could reduce the number of theoretical false triggers a great deal without affecting the range of interrogation significantly, but further analysis along these lines is not justified in view of the inadequate statistics on, for example, animal noises at the float depths. Some preliminary experiments in the Bay of Biscay in December 1971 and May 1972 showed that whilst propagation loss out to ranges exceeding 30 km could occasionally be as much as 6 db less than the value calculated for spherical spreading plus absorption, due to sound convergence, more frequently there was a loss of up to 15 db due to divergence. It was concluded that an interrogator source level of ÷ 9 0 (--6, ÷ 15) db re 1/~b at 1 m was required to trigger from 30 km a float having a bandwidth of 75 Hz and threshold 14 db, in ambient noise equivalent to sea state 6. The main interrogator array had an axial source level of ÷ 105 db re 1 tLb at 1 m, a beamwidth of approximately 20 ° between 3 db down points and consisted of six ITC 2003 transducers spaced half a wavelength apart in an oil-filled, polypropylene tube. The pulse power amplifier delivering 650 W peak and pre-amplifier lowered down with the array were contained in a separate pressure case. In the quietest ambient noise conditions a float should be triggered by the above source, if a ray path exists, out to 70 km. In addition to the above array, which was attached and lowered on station with the CTD sensor package on an armoured cable to 3000 m depths, (Figs. 5 and 6), another interrogator consisted of two ITC 2003 transducers half a wavelength apart inside a towed body; this had a horizontal source level of + 9 4 db re 1/~b at I m and was used for the short range running fixes with the ship underway at speeds up to 11 knots in good conditions. Since it was intended to use a number of floats simultaneously, some method of coding was required. Multiple pulse codes were rejected because of the possibility of multiple paths by refraction or reflection, so a simple frequency coding was adopted, each float transponding at a unique frequency in the band 5.56 to 6.60 kHz, with a pulse 100 msec long. The derivation of these frequencies is described later. The possibility of interrogating floats at different frequencies, all floats to reply at the same frequency, was rejected because each float would need to be interrogated in turn at each station and this would require too much ship time, though it would have allowed a simpler receiving system than that discussed below. The horizontal source level of each float was of the order of +91 db re 1 t~b at I m, which could be detected adequately from long ranges by the deep interrogator array due to its directivity against ambient noise and ship noise. However, the small array towed at 10 to 15 meter depth on the port quarter was limited by ship (probably propeller) noise and flow noises so that ranges in excess of 20 km were rare. Unlike the float receivers, which require a 14 db signal-to-noise excess on each pulse, the shipborne receiver system could display float replies at a much lower signal-to-noise ratio by virtue of the trace-to-trace coherence on the recorder. In practice, if a propagation path was present at all, the deep interrogator and receiver gave good clean signal returns, whilst the towed interrogator was normally used only when floats were known to be at relatively close range.

Shipboard installation (Fig. 7) The shipboard system consists basically of five units shown schematically in Fig. 4; they are a transmitter unit, a receiver, a wet paper facsimile (Mufax) display, an

579

The Minimode float tracking system

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580

J.C. SWALLOW,B. S. MCCARTNEYand N. W. MILLARD

instrumentation tape recorder with a loop-bin adaptor, and a control system. The tape loop is required because the display device, a modified Mufax recorder, is capable of recording only one signal at a time, whilst it is necessary to save ship time by interrogating all the floats simultaneously. The record and replay sequences employ different interconnections as shown in the figure. During the record sequence the system timing is derived from the 100 kHz output of a high precision digital clock permanently installed in the ship. This timing signal is divided down to provide the drive signal for the synchronous motor in the Mufax recorder. At the start of its sweep a trigger pulse is generated; this initiates transmission of the 5.1 kHz, 30 msec interrogation pulse. At the same time it resets the timing lines at 0.1 sec and 0.5 sec intervals for the display, these timing lines also being derived from the digital clock. The Mufax synchronization frequency and the trigger pulse are recorded on separate channels of the loop tape recorder. The float reply signals pass into the receiver where they are separated into 18 parallel signal channels, which with a synchronization channel and one spare, are then multiplexed in time division sequences on to four tape tracks; for example, signal channels 2, 6, 10, 14 and 18 are recorded sequentially on one tape track. During the recording sequence any signal channel may be selected manually for display on the Mufax. When the complete loop has been recorded the sequence is terminated automatically, a phototransistor sensing a gap in the tape oxide, and the loop recorder stops. During the replay sequence all timing signals are derived from the tape recorder tracks. Thus the Mufax synchronization and timing lines are derived from the synchronization track and the Mufax dutch is operated by the first pulse on the trigger track, so that the transmission reference time is on the left-hand edge of the recorder. The signal channel which is replayed on to the Mufax via a time division decoder can be selected manually or automatically. In the latter case the photoelectric 'end-of-loop' sensor controls channel selection, switching on to the next channel after the loop has replayed once completely. The replay sequence is stopped manually when all channels have been replayed satisfactorily. The data remain on the tape for further replays if necessary until the next interrogation and recording sequence. Here, an explanation will be given why, when the acoustic data in the water are multiplexed in frequency channels, it is necessary to use time division multiplex, a sequential sampling in time of the channels, on to the tape recorder. The maximum tape length which may be accommodated in the loop bin adaptor is 75 It, which at 3~ in/sec tape speed gives a recording period of 4 minutes. A shorter period could not be tolerated because it would give insuftieient pulses, especially for long range floats. On replay it is necessary to speed up the tape recorder and Mufax by a factor of 2, 4, 8 or 16, since interrogations may be required at 500 m depth, 1500 m and 3000 m depth as the array and CTD are lowered or raised, and all 18 channels need to be replayed between these stops. In practice a speed up factor of 4 has been used, interrogations being made usually at 500 m on lowering and at the final depth of 3000 m, only occasionally at 1500 m on the ascent. Each channel takes one minute to replay, so all channels can be replayed within 20 minutes, and generally all data plotted before the next interrogation. A typical record in which 14 channels have been replayed is shown in Fig. 8. With the possibility of various replay speed-up factors it was decided to use time division multiplex, which automatically decodes correctly whatever the replay speed, since the decoder sequence is locked to the synchronization frequency on the

Fig. 5. A B C D -

The instrument package about to be recovered.

conductivity-temperature-depth (CTD) probe; Rosette multisampler; tone-operated switch for connecting either the interrogator array or the CTD and Rosette to the conducting cable; interrogator circuits. [./king

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Fig. 7. Shipboard equipment. control, transceiver and filter circuits for interrogators: tape loop recorder; wet paper (Mufax) recorder.

Fig. 8. A typical record. Only 14 floats were in use at the time. The transmission pulse is at the left-hand side. The time marks are 0.1 set apart, intensified at every 0.5 sec. At the top, the signals arriving in one channel (No. 15) were displayed during the interrogation. The reply from that float can be seen first at a delay of 3.9 sec. When the Mufax sweep period changed from 8 set to 8.4 set, the recorded delay decreased at each successive sweep until all the pulses spaced 8 set apart had been received, finally settling at I .I sec. That total shift in recorded delay indicated a travel time of seven whole phases plus the initially recorded fraction of a phase, or 7 I’ 8-C 3.9% 59.9 sec. Below, the signals in channel 15 and in each of the other occupied channels were replayed in turn, after the interrogation. A switching transient obscured about 1 set of the last sweep in each replayed channel, but caused no serious inconvenience.

The Minimodefloattrackingsystem

581

tape track for the Mufax motor. Without time division multiplex, 18 filters would be required for each replay speed. After amplification and filtering of the broad band 5.4 kHz to 6.8 kHz, the received signal is passed to 18 mixers. Each mixer has a different local oscillator frequency, but all are derived by division from a 486 kHz crystal oscillator. Eighteen circuits provide dividers from 81 to 98, and following each mixer the 600 Hz upper sideband is filtered with a bandwidth of 14 Hz, using a two-pole, Butterworth, active RC filter. Any one of the 18 a.c. outputs may be selected for display on the Mufax and loudspeaker during the recording sequence, before feeding rectificers, smoothing and then the tape encoder. It is the time of arrival of each float signal which is needed, not the signal level (whose dynamic range at the transducer exceeds 120 db), which is just as well since the dynamic range of the display does not exceed 20 db. It is inevitable that the receiver should overload during the transmission pulse and for short range float returns, but the circuits are designed to accept such overloads without paralysis, recovering immediately. Occasionally a signal return may be lost as it arrives during a subsequent transmission pulse; more frequently a large signal overloads the amplifier, so that energy is spread over adjacent bands and erosstalk occurs. This is apparent on audio output by its tone difference and can also be recognized on the Mufax record by the symmetry of the breakthrough on either side of the true channel, and by the tendency for breakthrough to be transient at the beginning and end of the pulse. During the first half of each interrogation the Mufax recorder sweep period was 8 seconds and, since signals were returned as long as 100 seconds later from floats 75 km away, it was necessary to determine on which phase the return was displayed. This was achieved by changing to an 8.4-seconds sweep in the second half of each interrogation. The total travel time to the float and back was equal to an integral number of phases plus the different fractions of a sweep displayed at the two speeds; thus the phase could be determined by measuring the displayed difference between the fractions of a sweep. The change in Mufax synchronization speed was obtained by selecting a division oftbe 100 kHz clock frequency by 100 or 105. The receiver and Mufax display could be used one channel at a time without the recording and replaying sequences; this method was useful for close range running fixes and for the float recovery period. Most of the detailed electronic circuitry employs standard well-known techniques, but two aspects may be worth comment. First, during the replay mode the sychronization signal is vital for driving the Mufax, generating timing lines and driving the signal decoder, so that in the event of a tape drop-out it is advantageous to use a phase-locked loop with a long time constant to act as a flywheel for synchronization. Secondly, the electronic unit lowered with the interrogator contains a pulse power amplifier for transmission and a low-noise pre-amplifier with line driver for reception. Since the signal input for the former shares the same cable terminals as the latter and the power output transformer has the receiver input winding on it also, the possibility of feedback and oscillation around this loop exists. Oscillation is prevented by diode limiting at the line drive output and by the use of hybrid line transformers at both ends of the armoured cable. D.c. power is also supplied down the same co-axial cable and filtered from the signal. One minute is required to charge the storage capacitors in this unit, extending the total interrogation period to five minutes.

582

J.C. SWALLOW,B. S. MCCARTNEYand N. W. MILLARD

Float electronics (Fig. 9) The interrogation signal received from the ship by the float transducer is transformer-coupled into a two-stage, wide-band amplifier. Before passing into a fairly high Q tuned stage, with a bandwidth of 70 Hzcentred on the interrogation frequency of 5.I kHz, the signal goes through a clipping circuit. The clipping level of this circuit is set so that, when combined with the attenuation characteristic of the tuned stage, it is impossible for nearby floats to trigger the transponder circuit, even though they may produce signal levels 100 db greater than and only a fraction of an octave away from the interrogation signal. It also ensures that a float does not trigger offits own bottom or surface reflection.

Fig. 9. Schematic diagram of float circuit.

A diode bridge detects the signal from the tuned stage and also provides the automatic gain control voltage, derived from the ambient noise level, which is fed back to the wide-band amplifier with a time constant of I see. The threshold level, which has to distinguish between false fluctuations due to sea noise and true signal is provided by a zener diode suitably biased to make triggering possible only when the signal level is 12 db or more above the mean d.c. voltage, which is equivalent to an a.c. input signal-to-rms noise level greater than 14 db. When this threshold is reached a monostable pulse generator is fired which in turn triggers a second monostable. The first remains in its switched position for between 4 and 5 sec, thus providing a 'dead time' during which subsequent signals are rejected. The second circuit provides a 100 msec gating pulse which removes the reset level on an 8-stage binary divider for that time, thus producing a 100 msec envelope of a chosen frequency from a crystal oscillator running at 256 times that frequency. This signal is buffered and then drives a class C pulse power amplifier designed to provide 100 W of electrical power into the series tuned transducer via a transformer on which a tertiary winding is used for the receiver. Protection of the receiver from the power amplifier is

The Minimod¢float tracking system

583

achieved by a series resistor and a pair of back-to-back diodes, other methods being unsuitable owing to the change of signal frequency between receive and transmit. The acoustic release receiver is basically a standard NIO acoustic command receiver (HAggis, 1969, and subsequent improvements), modified to match the transducer used with the float. It is a FM system using a 10 kHz carrier modulated by frequencies between 300 and 510 Hz. The same transducer is used to receive this signal, which undergoes a single stage of wide band amplification before the tuned stages. The output from the discriminator is then fed into a release channel selection filter which provides enough gain to operate a relay latching circuit. This latching relay first discharges a large capacitor charged via a resistor from the main battery pack into the pyro release thus causing the external weight to be dropped. Secondly, it operates a non-latching relay which changes components in the two monostables previously mentioned so that the 'dead time' is reduced to less than 4 see and the pulse length is reduced to 25 msec. This gives two indications that the signal has been received by the float and that the weight should have been released. The float circuits are assembled on to a light aluminium framework which when wrapped with a layer of polyethylene sheet fits nicely into a 4-in. I.D. tube. The ceramic ring transducer, enclosed in moulded polyurethane rubber, has the following electrical and acoustic properties: receiving response +45 db re 1 t~V/tzb at 5 kHz, ÷37 db re 1 ~V/t~b at 10 kHz, transmission efficiency about 60 % over the range 5 kHz to 7 kHz, clamped capacitance 28,000 pf. From the expected number of interrogations and false triggers during a maximum operating life of one month, it was estimated that a battery pack providing 200 W hr would be needed for the float circuits. Ledanch6, manganese alkaline and mercury cells were considered, and are compared in Table 1. Table 1. Leelanehd

Priee/W hr W hr/kg W hr/cm3 Flotation cost/W hr Total ¢ost/W hr

1"8p 50 04)5 5.7p 7.5p

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3"3 75 0-2 3.8 7.1

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A pack of Leclanch6 cells would need appreciably more flotation material which, besides increasing the cost, would make the float more difficult to handle. Mercury cells would be marginally better than manganese alkaline ones on the same grounds, but the latter were chosen because of the great saving in cost. Each pack was made up from 24 Mn 1300 cells, each 1.5 volt 10 ampere hour capacity, connected in series. The 36 volt output supplies the power amplifier, the largest power drain, and a 12 volt tapping drives the rest of the circuitry. In addition to this main pack, a much smaller stack of mercury cells provides a 34 volt isolated supply used only during the release operation to energize the latching relay circuit. Battery volts were measured on recovery of float and an indication of how future packs will behave can be gained from Fig. I0. The end of battery life was calculated on a basis of 1 volt per cell. This occurs after a period of about 30 days.

584

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Mechanical design offloats (Fig. 11) The float body consists of an RR 77 aluminium alloy tube 13 ft (3.96 m) long, with nominal outside diameter 4] in. (121 mm) and wall thickness ½in. (9.5 mm). The end caps are of the same alloy and have O rings sealing against the ends of the tube. They are held on with plastic clips. The transducer is mounted externally at one end, protected by an open cage made of polyethylene. In the water, the tube floats vertically with the transducer downwards. The pyro release, containing an electrically ignited combustible alloy link, is suspended approximately 60 cm below the transducer, with the release weight (approx. 1"5 kg) below that. At the other end of the tube are a hoop, a flag and 5 m of stray line with a grapnel on the end, all made of polyethylene, to help in recovering the float. Each float was weighed in water of known density and temperature with the transducer in place. The other external components were weighed in water separately later. The buoyancy available for internal loads varied from 6030 g to 6390 g, for a float at 500 m depth in the MODE area. Of that, typically 3180 g was taken up by the batteries, 2580 g by the circuit and wiring, and 60 g by spacers. The rest was added in the form of pre-weighed metal strip. For the compressibility, the wall thickness and outside diameter were measured

Fig. 11.

A float after recovery.

A - aluminium alloy tube; B - transducer: C - remains of pyro release; D - recovery hoop; E - plastic grapnel and line; F - plastic flag. The lower view shows the transducer and end cap assembly in more detail.

The Minimode float tracking system

585

across 4 diameters at each end of each tube, and mean values taken. The ratio of mean wall thickness to mean outside diameter varied from 0.0791 to 0.0805, corresponding to a range in compressibility of only 4-1 ~o, so that a mean value could be used for the whole batch with negligible error. The end caps with four lead-through plugs (two for the transducer, two for the release) were thought to be the weakest part of the float. They were all tested in a pressure tank to the equivalent of 5000 m depth. METHOD OF OPERATION

Testing before use Besides the tests made when the circuits were being assembled and aligned, each of the 36 float circuits and transducers was lowered over the side to at least I000 m depth to make sure that it would transpond, did not have an excessive rate of false triggering, and that the release circuit would fire. Some circuits needed minor adjustments, and about one-third of the transducers had poorly bonded rubber seals that would have caused trouble if not found soon enough. These overside tests were made concurrently with a transatlantic STD section along 32°N and required very little extra ship time.

Float assembly With a stock of 36 floats, and as many as 9 being prepared for launch at one time, it was most convenient to be able to devote one small laboratory to float assembling. The tubes were stowed in a rack in 9 layers. The inboard 9 floats could be assembled in advance and kept ready, but in fact there was no need for any floats to be made up more than a few hours before use. Assembling and checking a float was about 15 rain work for two people. A loading sheet was written out for each float, which served as a check list for the items needed.

Launching Floats were launched over the stern with the ship making slight headway. They were lowered in two bights of line horizontally into the water, and then the lines were slipped. The stray line and grapnel were held until the float was known to be transponding; it was then released and observed to sink. Floats could be launched at intervals of 1 or 2 minutes.

Tracking The basic method was to measure ranges to as many floats as possible by recording interrogations, and responses during the 4-minute periods, mainly at 500 m and 3000 m, whilst waiting for thermometers to equilibrate during CTD dips. This was supplemented by running fixes using the interrogator fish, usually at 9 knots, when passing within about 10 km of a float. The proportion of fixes obtained using the array lowered with the CTD, to those obtained with the fish, varied according to the position and depth of the float, but in most cases was about 70 %: 30 ~o. As described in the section on the shipboard installation, the range information is displayed on a Mufax recorder. For the interrogation using the array lowered with the CTD, it was usual to play through all 18 channels even though some floats might be

586

J . c . SWALLOW,B. S. McCARTNEVand N. W. MILLARD

known to be out of range. A typical record can be seen in Fig. 8. For the first half of the record for each channel, the width of the paper represents 8 seconds of travel time. The arrival time of the reply pulse from the float, as read from the Mufax record, has to have some multiple of 8 seconds added to it to give the total travel time. The number of 8-second sweeps to be added is determined from the second half of each record, when the Mufax recorder speed is decreased by 4.76 %. This change in recorder speed caused the recorded float signal to shift by an amount equivalent to the same percentage of the total travel time. A convenient table related the observed shift in the float signal to the number of 8-second phases to be added. From the observed two-way travel time between the interrogator and the float, the horizontal range was obtained from a simple table using the nominal depths of float and interrogator and assuming straight ray paths with a sound velocity of 1.5 km/sec. The error incurred by making these assumptions will be discussed later. Multiple arrivals were often observed, and at the shorter ranges these could be used for estimating float depths. Both surface and bottom reflections and more complicated paths could be present. In general, arrivals from ranges exceeding 10 km (twice the water depth) were not sufficiently sensitive to float depth to be useful for that purpose. Recordings were sometimes made using the interrogator fish whilst stopped, with the fish at 15 m depth. Ranges were limited to about 15 km or less, but three such recordings in quick succession was an effective way of fixing a large number of closely spaced floats. More usually, the interrogator fish was employed for getting ranges on one float at a time, whilst underway between stations, or for deliberately fixing a float after launching or before recovery, or because it had not been reached by other interrogations. Such 'running fixes' were more accurate than those done from CTD stations, being done at shorter range, and spread over a shorter time, often based on many more range measurements. Whenever possible, course changes were made during these running fixes so that they could be used for depth determination.

Recover)' After fixing the float, the ship was stopped at a horizontal distance away comparable to the float depth, and the release signal sent, whilst watching the float signal on the 4-second sweep on the Mufax. Switching of the release circuit in the float was seen by the shortening of its pulse length, and reduction in dead time allowing replies every 4 seconds, instead of to alternate sweeps. Often, a change in slope of the record of the float signals indicated that the weight had been dropped. The time taken for a float to reach the surface varied from 12 to 15 minutes from 500 m, to about 2 hr from 4000 m. Homing on the rising or surfaced float could be done acoustically to within 0.2 km or less, and by then it had usually been seen. The average time between sighting and recovery was 11 minutes. PERFORMANCE

Interrogation ranges During the second period of Minimode float tracking, in May 1973, a record was kept of the successful and unsuccessful interrogations in range intervals of 5 km for the floats at nominal depths of 500 m, 1500 m, 3000 m and 4000 m from the most frequent

The Minimode float tracking system

587

interrogator depths of 500 m and 3000 m. The percentage of successful interrogations as a function of range is given in Fig. 12. The percentage quoted on the right-hand side is an overall figure for the corresponding float and interrogator depths; it is not an average of the range interval percentages, since each interval in general contains a different number of samples. The number of samples in any range interval is affected by the track of the ship and its programme of other activities, as well as the initial disposition and subsequent dispersal of the floats. Table 2 gives, for all float and interrogator depth combinations, the number of samples in each range interval, the percentage of success in that interval and the cumulative percentage of success out to that range. The final overall figure for all data is seen to be 66 ~o, but it is worth noting that 43 ~o of all unsuccessful interrogations were for ranges exceeding 70 km ; if the data beyond 70 km were ignored the success rate would be 75%. The data include 79 samples obtained with an interrogator depth of 1500 m, but these were considered inadequate for subdivision as in Fig. 12.

Table 2. Range (kin) Interval (%) Cumulative (%) Interval sample

0

5

10

15

20

25

30

35

40

45

to

to

to

to

to

to

to

to

to

to

5 95

10 85

15 92

20 93

25 95

30 83

35 40 45 78 78 62

50 67

50 55 60 65 70+ to to to to 55 60 65 70 58 52 56 27 13

95

87

90

91

93

90

88

82

80

19

60 122 121 207 254 176 148 144 116 114 106 115

86

84

79

77

75

66

55 301

It is of interest to compare the performance data in Fig. 12 with the theoretical ray paths computed from a sound speed profile taken at the time (Fig. 3A and B) for interrogator depths of 500 m and 3000 m, respectively. The initial angles of projection of the rays were chosen for equal acoustic flux between adjacent rays, so that the density of rays gives some indication of relative intensity when allowance is made for horizontal spreading and absorption. The sea-bed depth for the area of this experiment was mainly around 5300 m but with abyssal hills peaking to 4500 m, so that ray paths passing below 4500 m may not have been always available. Surface and bottom reflections were not included in Fig. 3 for clarity, but were present at times and could trigger floats in shadow zones; in the case of short ranges these multiple path transponder replies would have large obvious errors when plotted, and were regarded as unsuccessful interrogations, whilst for the long range cases the errors, though significant, were not so obvious and introduced noise on the float track plots. The shadow zones were not quite so complete as the figures suggest, due to the limited number of rays which have been plotted; this is particularly apparent for example from the 500 m interrogator, where the 2.9 ° ray passes deeper than 500 m at 17 km range, but a 2 ° ray on another plot (not shown) crosses the 500 m line at 31 km. This is due to the 500 m array being at the deeper limit of a layer having nearly constant sound velocity caused by the relatively thick 18°C Water. The histograms (Fig. 12) for 500 m and 1500 m both show dips in the mid ranges consistent with the shadow zone of Fig. 3A, with a gentle fall and a more rapid recovery at 50 km and 40 km, respectively, where the deep refracted rays bunch up at the start of the convergence zone. From the same interrogator depth the data for the 3000 m and

588

J.C. SWALLOW, B. S. McCARTNEY and N. W. MILLARD

DEPTHS A R

F L

R A

O A

0

5

10

15

20

25

30

30

40

"45

I

,

~o% ,oo % ~°l .o o

50

55

60

65

70

O V E R A L 1,.

+

~

1II ....

I

5°°~°° ~5ol I I 0 L

. i

|

o,:1 I 4°°°ol 3.o00 5oo

°/o 50

FT-11 ½ , ,

l*l

,

62

1,

NO

67

0 IDATA

3,000 3.000

,ooi,,i

j i .......

'°°1'"j

I

O~o~O I O"

,ooj 3.o00 4.ooo

,_

IFI 1 lh

78

°/o so I

65

0 ~ 0

5

10

15

20

25

30

HORIZONTAL

Fig. 12.

35

40

RANGE

45

50

INTERVALS

55

60

65

7 0 4-

(Kin)

Percentage success of interrogations as a function of range and depths,

The Minimodefloat trackingsystem

589

4000 m floats show generally high percentages out to 50 km; there a deep shadow zone begins, which for the 4000 m floats prevents any interrogations at all for the last four range intervals. The 500 m floats interrogated from 3000 m replied less reliably beyond 55 km consistent with a shallow shadow zone of Fig. 3B. The 1500 m floats interrogated from 3000 m are weakest between 50 and 55 kin, at which range the rays are bunching to a focus at a depth of 3000 m where the floats were interrogated with 100% success. Dips at 40-45 km and 65-70 km for the 4000 m floats are again in accord with shadow zones depicted by the ray traces. Some of the shorter range low percentages for the deep floats are due to weak directivity of both interrogator array and float transducer rather than propagation limitations. The general agreement of the data with what the ray traces predict leads to the following conclusions: (i) The vertical sound speed profile was reasonably represented by 9 constant gradient layers, (ii) The temporal changes during May 1973 and the horizontal gradients of sound speed are not significant out to 75 kin, and (iii) The system performance in any other ocean area where the sound speed profile is known should be predictable within reasonable limits. The lack of sensitivity to temporal and spatial scales of one month and 75 km is not too surprising since the sound rays cover most of the water depth and spend relatively little time in the more variable surface waters. Ranging errors

The two basic sources of error in the determination of the horizontal range of a float from the array are timing errors and errors in assumed sound speed. The timing errors arise from the unavoidable and finite rise times of the circuitry and transducers in the transmitter, interrogator array, transponder transducer, receiver and power stage, the interrogator array again as receptor, the receiver circuits and the playback circuits. All timing is referenced to the digital clock which has an absolute accuracy of the order of 1 part in l0 s. Travel time is determined from the Mufax record and can be resolved to 0.01 sec, which for a float at maximum range amounts to 1 part in 104, so that clock errors are insignificant. From initiation pulse to acoustic pulse in the water the interrogation system has a measured delay time range of 3.0 to 3-2 msec, the measured transponder delay time is between 6.4 and 25.0 mscc and the receiver system delay time is 3.6 to 19 mscc, extended on playback to 34 to 50 msec. The maximum overall rise time is therefore (3.2+25.0+50.0) msec, amounting to 0-08 sec when read off the Mufax. The observer reads the record at the leading edge of the marked pulse, and the transponder itself triggers after a time dependent on the signalto-noise ratio, so that the above overall delay time is an absolute maximum error for the weakest return, and generally the timing error is nearer a minimum value of 0.04 sec. These timing errors cause the range to be overestimated by between 30 m and 60 m, with the longest range floats tending to have the largest errors. For short-range fixing with the towed array, when tape loop recording is not used, the timing error could be between 13 and 47 reset, but generally will be less than 0.02 sec, equivalent to 15 m range error. The sound speed profile shown in Fig. 2 was computed from temperature, salinity

590

J.C. SWALLOW, B. S. McCARTNEY and N. W. MILLARD

and depth determined by the CTD probe down to 3000 m and by deep water bottle and thermometer samples for the depth range 3000 m to 5300 m. The main features of the profile are a negative gradient from 1530 m/sec near the surface to 1522 m/sec at 250 m, after which there is a nearly constant speed layer to 500 m due to the 18°C Water, followed by a strong negative gradient down to 1492 m/sec at 1100 m due to the main thermoeline; around the sound speed minimum at 1250 m the gradient is very low until the positive gradient starts at 1400 m, giving a speed of 1548 m/sec near the sea bed. The ray diagrams show that the sound travels over indirect paths at a nonuniform speed, though naturally always turning towards lower speeds. Calibration of the time base to obtain float ranges from travel times is therefore not a straightforward matter. For practical convenience, however, a constant speed of 1500 m/see was chosen initially to allow float tracks to be plotted without too much delay. For short ranges, less than about 10 km, a correction based upon a straight path and 1500 m/see speed was made using the known approximate differences in float and interrogator depth to convert slant range to horizontal range. In order to estimate the range errors to which these procedures give rise the horizontal ranges, X, and travel times, t, along certain plotted rays on an enlarged diagram were read off, the travel times were converted to estimated range, X', using 1500 m/see and geometrical corrections as above and the difference ~X----X'--X plotted for each interrogator and float combination in Fig. 13. In these plots a positive error implies that the average sound speed to that range was less than 1500 m/sec assumed speed and vice versa. For ranges measured from the 500 m interrogation depth, errors are generally negative, except for 500 m and 1500 m floats beyond 50 km. This is because the rays either remain in the high speed surface waters, or pass quickly through the low speed layers to the higher deep speeds for the first 50 km; after 50 km the rays pass into the lower speed waters again, bringing their mean speed below 1500 m/see. The errors from an interrogator depth of 3000 m show practically the reverse characteristics, being mostly positive errors, except for ranges less than 40 km for floats at the two deeper levels. In this latter case the rays start at a speed higher than 1500 m/see and go to higher speeds, whereas for the other floats the rays from 3000 m must go through into the low speed of sound regions once or twice. A 3000 m float at 40 km may be reached by either type of ray and both negative and positive errors are shown in this region. In practice the fastest ray path, if strong enough, will trigger the float and using the first arrival at the receiver usually leads to a negative error, and an underestimated range. Owing to the errors of reading off ray diagram ranges and times the magnitudes of these errors are not reliable to better than -t-0.1 kin. Maximum errors can be as high as +0"5 kin at medium to long ranges. In a plot of all the position circles obtained in a 2½-dayperiod on an almost stationary float (Fig. 14) when three short range running fixes were taken, there are discrepancies of as much as 0"3 km in some of the long range position lines, in the sense to be expected from the ray diagrams. Some slight improvement in accuracy of plotting positions could therefore be gained by using ranges taken from the ray diagrams instead of simply assuming a uniform sound velocity of 1500 m/see; however, as will be seen later, the uncertain position of the array, especially at 3000 m depth, reduces the value of such an elaboration. Errors in navigation

Two independent systems were used for fixing the ship's position during MODE-1.

591

The Minimode float tracking system

DEPTHS A F R L R O A A y T 500 500 m m

e°

km &X 00"2 4 I 20

10

30

40

6O

50

7O

-0'2

500

1,500

0.2 0'4 I

0 0

o

0

0 0

0

O. 0 0

0

u

10

0

-0.2

20

30

40

50

60

0

70

s0

60

5O

6O

7O

-0.4

500

3.000

0"2 I

oo;°•° o v •

-0-2

•

• •

°o•

•

0-2' 50O

4,000

0000 0 0

-0.2

3000

0

0

0

0

0

500 0-2

0o

• I

20

,o

3000

1,500

o.I O. 2

%o°o

I0

0

(9

o 0

4o

S6

0

0

20

0 30

o

0 0

o

0 40

II o

oo

7o0

0 0

0

•e 3,000

3,000

O, 2 I 0'4 30

1o

40

50

60

7O

40

50

60

70

-0-2

3.000

4,000

0.2 T"

o bo

-0.2

Fig. 13.

o

10

20

30 0

0

Errors in estimated ranges as a function of range and depths.

o

0

592

J.c. SWALLOW,B. S. MCCARTNEYand N. W. MILLARD

47-2 l,m 3000m

46"8 km 500 rn

y

I'"

'

I

i

I

o

~"~'~

|'

1

I

1

km

23'3 km

_

3000m

27-7km t 3000m

27.9km A5OOm 27'6 km 500m

14.1 km 3000m 500m

~

23"8km 500m

~

!o ~

SOOm

4"om9 km

Fig. 14. Position fines (arcs of range circles) obtained on a slowly moving float. A, B, C are short-range (<10 km) fixes obtained with the towed interrogator at mean times of 1746/19, 0400/21 and 0228]22 May 1973.

One of these is the normal navigational system used on board the Discovery, a combination of computer-calculated dead reckoning using an NIO two-component electromagnetic log (TUCKER, SMITH, PIERCE and COLLINS, 1970) and gyro repeater, fitted to successive fixes obtained from a Magnavox 702 CA receiver operating on signals from the U.S. Navy navigational statellites. The other system that proved useful in the M O D E area was Loran C. At first, Loran readings were taken by eye and plotted manually on a ½in. = 1 mile plotting sheet with hand drawn Loran position lines on it, linearly interpolated between computed Loran intersections every 100 microseconds on the two lanes used (SS7W and Y). When it became clear that the Loran was comparable in accuracy to the satellite-corrected dead reckoning system, computer sampling and direct calculation of fixes were brought into use. Thereafter Loran fixes based on the mean of 5 samples could be obtained from the computer every 2 minutes when required. Loran C and satellite fixes agreed surprisingly well in the M O D E area. In Fig. 15, I00 satellite fixes received in the period 15-25 May 1973 are compared with simultaneous Loran C fixes. Only those satellite fixes accepted by the computer course correction

593

The Minimode float tracking system

,2O

S

7o

N

'10

I

l -2

'

,

I -1

I 0

~

I 1

,

,

,

I 2 Km

.2O

W

E

-io

|

-2

I --1

f

I

F k~

! 0

! 1.

I

2 Km

Fig. 15. Distribution of components of displacement between simultaneous satellite and Loran-C fixes (see text).

program have been included. The criterion for rejecting fixes is that the surface current obtained on fitting the D R to the satellite fix in question should not exceed some chosen value, in this case 2.5 knots. Looking first at the differences in the north-south direction, evidently those pairs of fixes with differences beyond + 1.1 km have something wrong with them. Inspection of the derived surface currents and the sequences of Loran fixes before and after makes it clear that these large errors are in the satellite fixes. Rejecting those, the mean north-south difference (satellite minus Loran, north positive) was + 124 m. Satellite fixes computed on board the Discovery are sensitive to horizontal accelerations of the ship, and tend to be most accurate at low speeds. Taking only those fixes obtained when the ship was moving at less than 2 knots through the water reduces the width of the distribution appreciably (shaded area in Fig. 15). In the case of those 56 fixes nearly half the differences are contained within + 100 m. Turning now to the differences in the east-west direction, again the extremes detached from the central distribution can be rejected (in fact they are ones already rejected in the north-south case). The mean difference (satellite minus Loran, west positive) is + 152 m. In this case, however, the distribution of differences is bimodal. Again taking the fixes obtained at less than 2 knots speed (shaded), 55% of the 56 differences are contained in 2 strips ± 1 0 0 m wide, and 96~o are within 2 strips 4-300 m wide. The

594

J.C. SWALLOW,B. S. MCCARTNEYand N. W. MILLARD

broadening of these distributions at higher speeds and the double peak in the east-west difference distribution, are more likely to be due to the satellite fixes than to the Loran C. Even assuming equal contributions to the differences from the two systems, the error of a Loran C fix would be typically no more than :~:200 m, and it may well have been appreciably smaller than that. Of course, when measuring ranges it is the position of the array, not the ship, that must be considered. An upper limit to the horizontal distance between the array and the ship can be got, for each station, from the interrogator depth deduced from the CTD pressure gauge reading, on the unlikely assumption that the wire angle and azimuth were constant at all depths. For interrogations from 500 m nominal depth, the horizontal offset must have been less than 63 m at 87 ~ of the stations. At 3000 m nominal depth, it must have been less than 350 m at 70 700of stations. When the towed interrogator fish was in use, the array was 60 m astern and 15 m to port of the Loran C antenna.

Errors in float positions The errors to be expected in the individual float position lines (arcs of range circles) are a combination of errors of range measurement and errors in estimating the position of the interrogator array. From the above discussion it seems likely that this combined error will not exceed ~k0.4 km in most cases when the interrogator was at 500 m depth, and ± 0 . 5 km when it was at 3000 m. The examples plotted in Fig. 14 are well within these limits. The accuracy of a float position obtained from two or more such position lines depends also on their angle of intersection, and on how well the displacement of the float can be estimated during the interval between taking the position lines. In practice, the mean velocities between successive short range running fixes were used for that purpose. Again, the exampleS in Fig. 14 are fairly typical in giving float positions agreeing to well within 4:1 km of what would be expected by interpolation between the more accurate short range running fixes. The accuracy of the latter is limited only by that of the Loran C, since contributions from range errors, uncertainty of relative position of the interrogator and displacement of the float during the fix, are negligible.

Depth estimation Because of the directivity patterns of the float transducers and the interrogator arrays, it was usually not possible to measure ranges to the floats from directly overhead, which would be the simplest method of determining their depths. Instead, in most cases depths have been obtained by plotting close range running fixes where course changes were made during the fix, using different assumed depths, and choosing the value that gave the best fit. Depths within :kl00 m or better could usually be obtained by this method. Alternatively, delayed signals which could be identified as having travelled by bottom or surface reflected paths have been used. These were satisfactory at short ranges (less than 10 km) yielding depths consistent to within ~-_-50 m when the travel times were marked off on the appropriate paths on a large-scale ray diagram. At longer ranges systematic errors appeared which may be partly due to timing errors on the weaker reflected signals, but are to some extent unexplainable.

The Minimode float tracking system

595

POSSIBLE IMPROVEMENTS In its present form, this float tracking system is most suitable for measuring slow ( < 10 cm/sec) deep ( > 500 m) currents, where displacements over periods longer than a day are of interest. Topographic effects on deep currents, for example the flow near seamounts, or near the continental slope, on horizontal scales of several tens of kilometres, could be studied quite conveniently. Two improvements that could be made immediately are a more reliable release mechanism, and a change in the directivity pattern of at least one transducer (probably the one in the towed fish interrogator) to allow depths to be determined more easily from nearly overhead fixes. A means of remotely adjusting the depth of the floats could be fitted, within their present payload, using a second release signal channel. By developing a facsimile drum recorder with 18 or more simultaneous channels, the use of the tape loop recorder and associated multiplexing could be avoided, which should make the system simpler to operate and maintain. I f more frequent fixes are required, some kind of an array of fixed receivers seems essential; the interrogation pulses could still be sent out from the ship, and the acoustic signals arriving at a number of moored detectors could be relayed back by radio. Such a system was envisaged at first for use in M O D E - I , but was rejected because of the large number of moored receivers that might have been needed, or alternatively the amount of ship time consumed in re-siting them as the float pattern moved. With relative velocities between floats of 10 cm/sec, and tracking with a single ship, it becomes impracticable to fix all the floats every day after about 10 days. An attractive compromise would be to use drifting sono-radio buoys as extra receivers, drogued to stay near groups of floats at certain levels, to enhance the capability of a single ship, but that might be hazardous if fixed moorings were in use in the same area.

Acknowledgements--Many people, too numerous to mention individually, contributed to the design and production of the system described above and shared in learning to operate it at sea. To all of them, the authors are greatly indebted. The loan of CTD equipment by Mr. N. L. BROWNof the Woods Hole Oceanographic Institution is gratefully acknowledged. REFERENCES CREASE J. (1962) Velocity measurements in the deep water of the western North Atlantic. Journal of Geophysical Research, 67, 3173-3176. HARRISM. J. (1969) Acoustic command system. In: OceanologyInternational '69. Proceedings of Conference hem Brighton, Feb. 18-21, 1969: Technical Sessions, day 3, London, BIaS Exhibitions Ltd., 9 pp. ROSSBYT. and D. WEBS (1970) Observing abyssal motions by tracking Swallow floats in the SOFAR channel. Deep-Sea Research, 17, 359-365. ROSSBYT. and D. WEBB (1971) The four month drift of a Swallow float. Deep-Sea Research, 18, 1035-1039. SWALLOW J. C. (1971) The Aries current measurements in the western North Atlantic. Philosophical Transactions of the Royal Society of London, A, 270, 451-460. TUCKER M. J., N. D. SMrrH, F. E. PIERCE and E. P. COLLINS(1970) A two-component electromagnetic ship's log. Journal of the Institute of Navigation, 23, 302-316.