6
Drift er s for Tomography B. M. Howe, R. W. Gill, C. W. May, J. A. Mercer, and J. H. Morison Applied Physics Laboratory, College of Ocean and Fishery Sciences University of Washington, Seattle, WA 98105 Abstract - Ocean acoustic tomography needs many instruments to take advantage of the n2 growth of information as the number of sources and receivers increases. We discuss the proposed de- sign of inexpensive, drifting acoustic receivers to fulfill this need. The proposed drifters have a GPS receiver for navigation and timing and a cellular phone link for real-time data transfer. The latter is in anticipation of the global satellite cellular system, Iridium, being built by Motorola. 1. INTRODUCTION During the next 5-10 years we expect to see the follow- ing developments in thc field of ocean acoustic tomogra- phy: (1) process-oriented experiments with up to 20 transceivers, (2) a global system to monitor ocean climate change, and (3) real-time monitoring for nowcast and fore- cast purposes. The number of transceivers in a process-ori- ented experiment is now limited by the logistic capabilities of the groups deploying the instruments and, naturally, by cost. In such an experiment with nf = 15 transceivers, there are at (nt - 1)/2 = 105 paths. Adding nd = 15 drift- ing receivers would result in nt nd = 225 paths for a total of 330 paths, an increase of 200% in the amount of data. For an experiment covering an area 1000 km by 1000 km, this makes the difference between an experiment that resolves the mesoscale and one that does not. If the cost of a drifting receiver is significantly less than that of a mooring, it will be cost effective to deploy drifting re- ceivers to complement moored transceivers. Although a global system is barely on the drawing boards, it will un- doubtedly consist of a number of cabled acoustic sources. Drifting receivers would inexpensively augment fixed and moored receivers for the purposes of climate study and, at the same time, provide a large amount of high quality, in situ, subsurface data for nowcast and forecast systems. A two-way, real-time telemetry link would enablc the drifter's program to be modified to reflect changes such as the installation of a new acoustic source. The concept of a tomography drifter is not new. Cor- nuelle [ 11 wrote about purcly subsurface driftcrs in 1085. There has been talk of modifying ALACE or RAFOS floats to receive tomographic signals, and people have been discussing moorings with telemetry and doing away with the anchor. The rccent Moving Ship Tomography (MST) experimcnt created a synthetic receiving apcrturc by lowering a hydrophone array many times around a 1000-kin diameter circle with six transmitters in the intc- rior [2]. Drifting receivers can replace the ship-tethered array to form a synthetic aperture receiver. The RELAYS (Real-time Link and Acquisition Yare System) drifting buoy [3],[4] was built to fulfill many of the purposes we address here. It consisted of a small, boat- shaped float, a buoyant tether line, a subsurface float, and a vertical array of instruments, including hydrophones. Only limited experience was obtained with this system, primar- ily because of problems with the durability of the tether where it attached to the surface-following buoy. The primary advantage of drifting receivers is a subslan- tial increase in the quantity of data for a small cost. Sec- ondary advantages include real-time data, a two-way link for adjusting to changing conditions, expendable (inexpensive) instrumentation, recording of data ashore, and rapid data distribution [51. The only fundamental dis- advantage is that there is no direct control over where the drifter goes. This can be minimized indirectly by dcploy- ing the drifters in areas with weak mean currents and using their tomographic capability to sample across regions with high velocitics. The main functions of the proposed tomography drifter arc to receive acoustic signals in the occan sound channel, process them, and telemeter the data to shore. The major requirements for the drifter, in addition to performing its primary functions, are accurate positioning and timing, a stable platform with a life of 2 years or longer, and ease of deployment. The key design feature that is a dcparture from traditional occanographic practice is the use of a tuned spar buoy. This provides an exceptionally stable platform with a high probability of long life. Such a plat- form minimizes possible fatigue-related failures, the neme- sis of most surface-coupled buoys. We expect that the drifter-with a powerful processor, accurate navigation and timing, and telemetry-would serve as a general-pur- pose platform, with additional sensors to measurc air tem- perature and pressure, sea surface temperature, ambient noise for wind and rain, barotropic velocity (using electric field sensors), and tcrnpcrature and conductivity as a func- tion of depth, to name just a few. Section 2 discusses specific design considerations, in particular, the major design decision of whether to use a spar buoy or a surface-following buoy. Section 3 de- scribes the major components, and Section 4 summarizes the major considerations. 736 0-7803-0838-7192 $3.00 1992 IEEE

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Page 1: [IEEE OCEANS 92 Mastering the Oceans Through Technology - Newport, RI (October 26-29,1992)] OCEANS 92 Proceedings@m_Mastering the Oceans Through Technology - Drifters For Tomography

Drift er s for Tomography

B. M. Howe, R. W. Gill, C. W. May, J. A. Mercer, and J. H. Morison Applied Physics Laboratory, College of Ocean and Fishery Sciences

University of Washington, Seattle, WA 98105

Abstract - Ocean acoustic tomography needs many instruments to take advantage of the n2 growth of information as the number of sources and receivers increases. We discuss the proposed de- sign of inexpensive, drifting acoustic receivers to fulfill this need. The proposed drifters have a GPS receiver for navigation and timing and a cellular phone link for real-time data transfer. The latter is in anticipation of the global satellite cellular system, Iridium, being built by Motorola.

1. INTRODUCTION

During the next 5-10 years we expect to see the follow- ing developments in thc field of ocean acoustic tomogra- phy: (1) process-oriented experiments with up to 20 transceivers, (2) a global system to monitor ocean climate change, and (3) real-time monitoring for nowcast and fore- cast purposes. The number of transceivers in a process-ori- ented experiment is now limited by the logistic capabilities of the groups deploying the instruments and, naturally, by cost. In such an experiment with nf = 15 transceivers, there are at (nt - 1)/2 = 105 paths. Adding nd = 15 drift- ing receivers would result in nt nd = 225 paths for a total of 330 paths, an increase of 200% in the amount of data. For an experiment covering an area 1000 km by 1000 km, this makes the difference between an experiment that resolves the mesoscale and one that does not. If the cost of a drifting receiver is significantly less than that of a mooring, it will be cost effective to deploy drifting re- ceivers to complement moored transceivers. Although a global system is barely on the drawing boards, i t will un- doubtedly consist of a number of cabled acoustic sources. Drifting receivers would inexpensively augment fixed and moored receivers for the purposes of climate study and, at the same time, provide a large amount of high quality, in situ, subsurface data for nowcast and forecast systems. A two-way, real-time telemetry link would enablc the drifter's program to be modified to reflect changes such as the installation of a new acoustic source.

The concept of a tomography drifter is not new. Cor- nuelle [ 11 wrote about purcly subsurface driftcrs in 1085. There has been talk of modifying ALACE or RAFOS floats to receive tomographic signals, and people have been discussing moorings with telemetry and doing away with the anchor. The rccent Moving Ship Tomography (MST) experimcnt created a synthetic receiving apcrturc by lowering a hydrophone array many times around a 1000-kin diameter circle with six transmitters in the intc-

rior [2] . Drifting receivers can replace the ship-tethered array to form a synthetic aperture receiver.

The RELAYS (Real-time Link and Acquisition Yare System) drifting buoy [3],[4] was built to fulfill many of the purposes we address here. It consisted of a small, boat- shaped float, a buoyant tether line, a subsurface float, and a vertical array of instruments, including hydrophones. Only limited experience was obtained with this system, primar- ily because of problems with the durability of the tether where it attached to the surface-following buoy.

The primary advantage of drifting receivers is a subslan- tial increase in the quantity of data for a small cost. Sec- ondary advantages include real-time data, a two-way link for adjusting to changing conditions, expendable (inexpensive) instrumentation, recording of data ashore, and rapid data distribution [51. The only fundamental dis- advantage is that there is no direct control over where the drifter goes. This can be minimized indirectly by dcploy- ing the drifters in areas with weak mean currents and using their tomographic capability to sample across regions with high velocitics.

The main functions of the proposed tomography drifter arc to receive acoustic signals in the occan sound channel, process them, and telemeter the data to shore. The major requirements for the drifter, in addition to performing its primary functions, are accurate positioning and timing, a stable platform with a life of 2 years or longer, and ease of deployment. The key design feature that is a dcparture from traditional occanographic practice is the use of a tuned spar buoy. This provides an exceptionally stable platform with a high probability of long life. Such a plat- form minimizes possible fatigue-related failures, the neme- sis of most surface-coupled buoys. We expect that the drifter-with a powerful processor, accurate navigation and timing, and telemetry-would serve as a general-pur- pose platform, with additional sensors to measurc air tem- perature and pressure, sea surface temperature, ambient noise for wind and rain, barotropic velocity (using electric field sensors), and tcrnpcrature and conductivity as a func- tion of depth, to name just a few.

Section 2 discusses specific design considerations, in particular, the major design decision of whether to use a spar buoy or a surface-following buoy. Section 3 de- scribes the major components, and Section 4 summarizes the major considerations.

736 0-7803-0838-7192 $3.00 1992 IEEE

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2. GENEKAL DESIGN CONSIDEKP,’TlONS

Figure 1 illuslrates the general concept of a tomography drifter. The drifter consists of two parLs: a buoy hull with GPS and telemetry antennas at the top and electronics and batteries at the bottom, and an array with receiving hy- drophones, a ti!tmetcr/compass package, and a pressure sensor. These major components as they rellate lo our pro- posed tomography drifter will be discussed in detail in Section 3. Here, we focus on the criteria for accurate positioning and liong life.

The absolute positioning accuracy required of the hy- drophones is a function of the internal-wave travel-time variance. The variance grows linearly with range from the acoustic source ;and is approximately (10 r n , ~ ) ~ at 1000 km and (20 ms)2 at 3000 km (Dushaw, personal communica- tioin, 1992, andl [86]). For a sound speed of 1500 m/s, this is equivalent to an rms range error of 15 m and 30 m, re- spectively. The stand-alone GPS data collected by the buoys have an crror of anywhere from 35 In to 100 m, largely a functio’n of “selective availability.” With diffcr- entia1 GPS (DGPS) data collected on shore, position errors range from 5 ni at 1000 km to 20 m at 3000 Ikm.

Hydrophone TilUcornpass

Pressure

Acoustic source

Fig. 1. Schrrnal.ic of the proposed tomography d.rifLer. The key elemen~s are the hydrophones, the GPS recciver, and the data

telemetry link. The latter would initially be ARGOS, and alter 1996 wmould be the Iridium cellular system.

The Liltmeter/compass and pressure data are used to ex- tend the GPS position data at the surface to the subsurface elements. To minimize array tilt, the array’s drag should be minimized and the tension should be high. The array cable should be the minimum diameter possible, and all in- strumentation on the array should be as small and stream- lined as possible. Increasing the array tension requires a weight at the bottom of the array. The detrimental effect is that the array cable must be rated to carry the extra tension. The weight is chosen so the tilt will be no more than 1” (10 m in 600 m) when there is a constant velocity shear of 20 cm s-l over the depth of the array.

Timing data from GPS are accurate to 0.1 ps, much better than the 1 ms required.

The most difficult requirement is long life, with failure due to fatigue being the likeliest cause of a short life. Sur- face waves expose the drifter to 1 to 30 million cycles per year, some of which will be extreme events associated with storms. Table I shows wave height and period for several wind speeds and sea states.

TABLE I

Significant Period of Wind Wave Spectral

Sca Speed Height Maximum State (knots) (m) ( s )

3 15 1.2 6 6 2.5 4 10 7 38 12 15 9 SO 24 20

The weakest link in all surface buoys with subsurface arrays subjected to such cyclic stresses is the buoy/cable junction. Failures typically occur where there is a step transition in bending stress. A solution that has met with some success is to use a bending strain relief boot, or “carrot” (J. Irish, personal communication, 1991, and @I). Another solution is to eliminate the cause of the problem, namely, the motion at the junction. One way to do this is to use a spar buoy tuned so its resonant frequency is lower than the frequency of any expected surface wave. Such spar buoys have been built here at APL and deployed with success. Olson 191 constructed a spar 32 m in length with a resonant frequency of 31 s. In waves 3 to 5 m high with periods of 10 to 12 s, the spar had a vertical motion of 0.25 In peak to peak. More recently, a 20-m spar with a resonant period of 16 s was deployed in a sea state 4 and showed a vertical motion of 0.5 m peak to peak (J. Osse and P. Dahl, personal communication, 1992). The design of the proposed tuned spar is discussed in the next section.

To maximize drifter reliability and lifetime, and to min- imizc hydrophone motion, wc propose to combinc both of these ideas, the tuned spar buoy and the bending strain re- lief boot. Our general philosophy is to minimize all possi- ble motions, since it is relative movement of components

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that leads to fatigue failures. We are hoping for a 2-year life; beyond 2 years, corrosion and biofouling become im- portant. A tuned spar is somewhat more difficult to deploy than a simple surface follower, but the reward is substan- tially better reliability.

In summary, the requirements for the proposed drifter

= acquire, process, and telemeter acoustic tomography

locate hydrophones to order 20 m and keep time to

have a life of 2 years or longer.

are

data

1 ms

3. DESCRIPTION OF MAJOR COMPONENTS

A Tuned Spar Buoy

The resonant period of a spar buoy is approximately T,. = 2nl/m/k, where m is the total mass of thc buoy and k is the spring constant; k is given by pgA, where p is the water density, g is thc acccleration due to gravity, and A is the spar cross-sectional area at the watcr wrface The resonant period can bc increased by increasing m or dc-

array. We expect that the array will produce a significant amount of drag in the vertical which will further reduce the resonant frequency and the heave response.

Given the above discussion, the ideal spar for this pur- pose is a long, slender stick with a single, large mass added at the bottom, far removed from the effects of the wave orbital velocities and pressure. A disadvantage of a slen- der stick is that there is little reserve buoyancy above the nominal waterline. Thus a compromise must be made: the spar's diameter must be increased to provide reserve buoy- ancy above the waterline, at the cost of reducing the reso- nant period and increasing the heave response. Also, the spar itself must be sufficiently stiff that it does not bend significantly and become subject to fatigue failure.

Fig. 2 shows the proposed buoy design, and Table I1 gives the important design parameters. The predicted heave frequency response (the transfer function) is shown in Figurc 3. The response is 0.4% at a wave period of 10 s and 5 % at a wave period of 15 s. Drag caused by the array cable or by the effective discs at the bottom of the spar has not been accounted for here. Shown also in Figure 3 is the heave response to a sea state 7 wave spectra (a Bret- schneider spectrum with a 40 knot wind speed; see [ll]). Wave spectrum statistics (based on a Rayleigh probability distribution of wave amDlitudes) are shown in Table I11

crcasing k . An effective way of increasing m without and the heave responsc spectrum statistics in Table IV. adding in-air weight is to add a lightweight container that floods whcn the buoy is deployed. The minimum diameter of the spar where it pcnctrates the surface is determined by both the buoyancy and strength requirements. Olson's buoy had a total mass (including the captured watcr) of 1060 kg and a surface diameter of 0.064 m; the in-air weight was 585 kg.

The heave response as a function of frequency can be modeled as thc forced response of an oscillator (sce, for example, [7], [lo], and [ll]). The forcing function is the product of the wave pressure (exponcntially attenuated with depth) and any horizontal projection of area, intc- gratcd over the immersed length of the spar. For a simple spar with a constant diameter, the only vertical force is the wave pressure acting on the bottom surface area (neglecting friction). Minimizing this force provides the motivation for making the spar long. If the cross-sectional area changes with depth, say in a step fashion, the heave response will be affected by wave pressures acting on the arcas of the steps. To minimize the heave response, any steps should be as deep as possible. Because wave pressure fluctuations decay with depth, it is possible to have the downward component of force on a step exactly cancel the upward force on the bottom of thc spar, thus producing a notch in the response curve [ 121 ,[ 131. For the buoys wc consider hcrc, this is difficult to achieve in

practice. Linear friction is easy to include in response cal- culations, but its effects arc relatively small and hard to es- timatc; howcvcr, Olson empirically found the exponential friction decay time to be about 35 s for thc spar without an

H-- - GPS and telemetry antennas * Reserve buoyancy

10 m x 75 mrn C$

t B rn x 1 50 rnrn @

5 rn x250 rnrn Q

1300 rn rnax

m = 2000 kg k = 43 N/m T = 4 4 ~

- AVATAR electronics

Surlyn foam buoyancy Entrained mass (1500 kg) - Depth release ballast - Bending strain relief boot

EM cable

TilVcompass

Hydrophones

CTD

Fig. 2. Details of the tomography drifter.

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TABLE II TUNED SPAR BUOY PARAMETEIRS - -

Buoy weight in air 400 kg Added mass 1500 kg

Total mass 2000 kg Diameter at surface 0.075 m

Array weight 100 kg

Resonant frequency 4 4 s Length 2 buoy in water 23 m

Period (s) 40 713 5 2 1

0.02 0.05 0.1 0.2 0.5 1 Frequency (Hz)

Fig. 3. Heave reslpnse for the tuned spar buoy. The three curves show (a) the normalized heave response (the transfer function,

nondimensional), (b) a Bretschneider wave spcctrum (wind speed 40 knots), and (c) the buoy heave response to the wave spcctrurn.

(b) and (c) have units of meters2kerl.r.

TARLE HI WAVE SPECTRUM STATISTICS

Fraction of Average Largest Wave

Amplitudes Amplitude - Considered (m)

0.010 7.620 0.100 5.815 0.333 4.574 0.500 4.057

L .ooo 2.862

Expectcd Wave

Maximum Number Amplitude

of Waves (m)

50 6.848 100 7.365 500 8.43 1

1000 8.980 10000 10.1 11

1 00000 11.209 - - I

r m s of wave amplitusde spectrum = 3.23 m Probable amplitude of wave = 2.28 m

One way to surnmarize these results is to say that the energy associated with thc motion of the tuncd spar buoy is onlly 4% of what a surface-following buoy would experi- ence. The strain cnergics in the tuned spar system should

be a factor of 25 less than those for a surface follower. It is not known whether this is enough improvement to as- sure a 2-year life, but it is significant.

TARLE IV HEAVE RESPONSE SPECTRUM STATISTICS

rms of response spectrum = 0.66 m. Probable amplitude of heave responsc = 0.46 m.

The detailed dcsign of the tuned spar buoy will be based on the experience of Olson [91 and on a more recent spar (Osse, personal communication, 1992). The spar will be assemblcd from the following components (from top to bottom):

1.

2.

3.

4.

5.

6. 7.

upper module containing the tclemetry and GPS antennas

14 m of 75-mm (3-inch nominal) diameter 6061- T6 aluminum pipe (4 m above water) 8 rn of 150-mm (6-inch nominal) diameter 6061- T6 aluminum pipc

5 rn of 250-mm (IO-inch nominal) diameter 6061- T6 aluminum pipe, for elcctronics and batteries, covered with Surlyn buoyancy material to 350- mm (14-inch nominal) diameter

4 m of 750-hm (30-inch nominal) diameter sheet aluminum “barrel” surrounding the lower portion of the 250-mm section to hold the captured water

couplings and reducers for the pipe sections bending strain relief boot.

The ovcrall length is 27 m. The 6061-T6 aluminum rep- resents a compromise bctween strength, machinability, availability (cost), and corrosion resistance. The strength of the couplings is especially important during deployment when peak bending stresses can occur. For redundancy, both the couplings and each pipe section will be water- tight. An open cable tube will run down the center so that (waterproof) cables can connect the instrument case with the antcnnas a t the top. For debugging purposes, there will be a connector on top of the spar as well as on the bottom to facilitatc communication with the electronics in the in- strument casc. The clcctrical pigtail from the array is con-

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Page 5: [IEEE OCEANS 92 Mastering the Oceans Through Technology - Newport, RI (October 26-29,1992)] OCEANS 92 Proceedings@m_Mastering the Oceans Through Technology - Drifters For Tomography

nected directly into the bottom of the instrument case. P.11 exposed aluminum will have a fusion-bonded cpoxy coat- ing and a copper-based antifouling coating. Sacrificial aluminum-zinc anodes will be attached to the couplings.

The external Surlyn foam provides an easy way of ob- taining buoyancy. The first drifters would contain dcpth- released ballast so that, in the event thc spar loses buoy- ancy and starts to sink, a weight would be rclcascd when the bottom of the buoy reached some nominal dcpth. Adding Surlyn foam just beneath the antennas will providc some reserve without increasing water plane area. The bending strain relief “carrot” will be made commercially; it tapers from the 15-mm (0.6-inch) diameter cable to 300- mm (6-inches) where i t attachcs to the lower instrument case.

B . Instrument Array

To collect the maximum amount of acoustic data, i t is necessary to have hydrophones in the sound channel. Thc channel varies in depth betwccn 1300 m (in the tropics) and the surface (in polar regions). Buoys with 600 ni ar- rays would be appropriate for use in the North Pacific or North Atlantic. Longer or shorter arrays could be used, depending on the location.

The array cable will havc a Kevlar strength member with a breaking strength of 45 kN (10,000 ib), a black plastic Hytrel jacket (used on most oceanographic jack- eted-wire rope), and six twisted-pair conductors; thc diani- eter will be 15 mm (0.6 inch). The maximum amount of helix angle will be used in constructing this cable to permit the most elongation. A concern is fish bites and slrum- ming. Experience from the RELAYS drifter (R. Chase, personal communication, 1992) is that struminin& J I $ ’. not a problem and that the plastic jacket is tough enough to re- sist all but determined shark attacks.

The hydrophones will be similar to ones used in sonobuoys. The first arrays will consist of four phones spaced at 3/21 where 1 is the acoustic wavelength of in- terest. (Acoustic frequencies planned or in usc arc between 70 Hz and 400 Hz, corresponding to h between 2 1.5 m and 3.75 m.) A fluxgate compass combined with inexpcnsivc inclinometers will give east and north tilt components ac- curate to 1”. At the bottom of the array will be a Seacat CTD unit measuring conductivity, temperature, and pres- sure. Both the tiltmctcr/compass and the Scacat will bc digital devices and will be interrogated by the main pro- cessor when data are required; each will require one twisted pair. Together with the prcssurc sensor at thc bot- tom and a model for the array shape, Lherc is enough in- formation to determine hydrophone location rclativc to the buoy to the desired accuracy. Tests using the APL floating long-baseline acoustic tracking range would be used to confirm thc positioning accuracy [14].

C. Processor

Thc proccssor will be based on the AVATAR eleclron- ics used to acquire and process tomography data on moor- ings [15]. Its architccture is structured around a powerful central microprocessor (Harris 80C86) controlling a num- ber of single chip computers (SCC, Motorola MC1468705G2) ovcr a low-speed interprocessor bus. Both these chips arc widcly used and are easily upgraded if necessary. The SCCs are used for dedicated functions such as controlling the clock, A/D system, GPS receiver, telemctry link, and power monitoring, and €or acquiring cnginecring data (tilt/compass, CTD). Acoustic data processing will consist of Doppler processing, replica cross corrclation (via the Hadamard transform), and beamforming. In typical scenarios with 15 sources, all the sourccs would be programmed to turn on at nearly Lhe same time. Because the transmissions take a finite amount of time to propagatc across the entire array (including the drifters), the rcccivcrs will have to capture the entire time window (about 20 minutes) and then do the processing, requiring on the order of 5 Mbytes of solid state memory. Thc timing signal from thc GPS receiver will be used to synchronize the inexpcnsive oscillator that controls the real-time clock. A 486 PC will be used to program the proccssor and to sct it up for deployment. With this processor architccturc, i t will be very easy to add sensors as dcsircd.

D . GPS Receiver

Many GPS rcccivcrs are now on the market. We require CIA codc carrier phase, six channels, continuous tracking, a fast lock on time, small size, low power, and low cost ($1000). To prevent any possibility of interference be- tween the communications antenna and the GPS antenna, the two will not be operatcd simultaneously. The processor will pcricdically activate the GPS receiver to see where the driftcr is and then calculate when to begin a reception. GPS data (and all other nonacoustic data) would be logged Lhroughout a reception period at a low rate.

E. Communicalions Link

Iridium, thc global cellular system being built by Mo- torola, is schcdulcd to begin operations the first quarter of 1996. It will consist of 77 low-Hying satellites (the atomic numbcr of ir idium) and the necessary ground stations. Data rates of 2400 baud will be possible. AlLhough the de- tails of the cellular modem are not yet known, it will not rcquirc significantly more power than present hand-held ccllular sets and the cost will be about $3000. The modem link will be two-way so the buoy’s processor can be sent new instructions (such as new acoustic source locations). Evcntually, Iridium will intcgrale GPS so only one antenna and one set of clcctronics will be necessary. Access costs for Iridium would be the same as for any other cellular in- ternational call, say $3/minute.

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Since the Iridium system will not be ready until 1906, the ARGOS system will be used in the interim for one-way communicationis. This will require a signifiicant compres- sion of the dah, not unlike that done in earlly tomography instruments. From the onset, though, we will provide for the cellular link, so when the Iridium system IS operational, the buoys will be ready also. ARGOS costs about $1 Olday.

F . Batteries

The power re~quirement for thc batteries is approxi- mately 1 W. Alkaline battery packs will be used. A total of 25 packs, each with 36 cells rated at 14 A-h, 1.5 V, will be needed for a 2-year deployment. Total battery weight is 112 kg. We expect that solar cells will ble used in the futurc.

4. SUMMAKY

We have proposed an inexpensive, drifting acoustic re- ceiver for use in ocean acoustic tomography. The guiding design philosophy has been to maximize the life of the drifter while recoignizing the need to minimize cost. Im- portant elements include GPS for precise timing and navi- gation and data telemetry using the Motorola Iridium global cellular nietwork.

Our design dcparts from traditional practicc in [.hat i t uti- lizes a tuned spar buoy. In principle, the only additional cost of the spar design compared wilh that for a surface drifter is the cost ‘of h e extra tube sections; otherwise they are nominally the same. Wc expect the cost to start at about $30,000 for a prototype (not including nonrecurring engineering costs,) and then dccline 1.0 about $20,000 in quantity aftcr transition to Industry.

Before a driftcr is actually built, extensive modeling will be nccded to predict the roll, surge, and snap loading rc- sponses in addition to thc heave response. Necessarily, the array and friction will have to bc included in these simula- .

. Full-scale tests will then be made in I(>cal waters to rm the modeling results.

The spar will fit. into a 4 in3 (150 cu ft) shipping box and weigh 500 kg. Each of the small-diameter pipe sections can Ibe carried by one man. Olson and Ossc cxpcricnccd no difficulty in deploying (and recovering) hcir respective spars; Olson worked from USNS Davis while Ossc de- ployed from the Floating Instrument Platform (RP F L I P ) and recovered on USNS Narragansett, a t u g with ii small working deck. The spar hull is subjected 1.0 maximum strcss, particularly at the couplings, during dcploymcnt, so the deploymcnt must bc executed with some skill.

We expect thlc drifter will eventually be powered by solar cells and bc self-sufficient in energy. This, combincd with the data telemetry capability provided by Iridium, is the nnotivation for trying to build a cirilCer with long life. The drifter will bccomc a general purpose platform for other sensors.

REFERENCES

[ l ] B. D. Cornuelle, “Simulations of acoustic tomogra- phy array performance with untracked or drifting sources and receivers,” J. Ceophys. Res., vol. 90, pp.

[2] B. Cornuelle, W. Munk, and P. Worcester, “Ocean acoustic tomography from ships,” J . Ceophys. Res., vol. 94, pp. 623245250, 1989.

[3] R. Chase, oceanographic Observing Systems, An- nual Report, Woods Hole oceanographic Institution, 1983

[4] R. G. Walden and H. 0. Berteaux, “Free drifting RELAYS buoy systems,” Proceedings, 1983 Sym- posium on Buoy Technology, pp. 145, Marine Tech. Soc., 1983

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