Ocean Engineering 43 (2012) 56–63

Contents lists available at SciVerse ScienceDirect

Ocean Engineering

0029-80

doi:10.1

n Corr

E-m

michele

gmann@

a.worby

journal homepage: www.elsevier.com/locate/oceaneng

An acoustic signal propagation experiment beneath sea ice

Ron S. Lewis a,n, Mich�ele Drogou b, Peter King a, George Mann a, Neil Bose c, Anthony Worby d

a Marine Environmental Research Lab for Intelligent Vehicles (MERLIN), Faculty of Engineering and Applied Science, Memorial University of Newfoundland, St. John’s,

Newfoundland and Labrador, Canada A1B 3X5b Institut franc-ais de recherche pour l’exploitation de la mer (Ifremer), B.P. 330, 83507, La Seyne sur Mer, Francec National Centre for Maritime Engineering and Hydrodynamics, Australian Maritime College—University of Tasmania, Locked Bag 1395, Launceston, Tasmania 7250, Australiad CSIRO Division of Marine and Atmospheric Research, Castray Esplanade, Hobart, Tasmania 7000, Australia

a r t i c l e i n f o

Article history:

Received 10 January 2011

Accepted 2 January 2012

Editor in Chief: A.I. Incecikfor AUV missions. Deployments can be coordinated via vessels or on-ice camps. The science and

operational specifications on missions preclude shepherding of the AUV and result in a reliance on

Available online 4 February 2012

Keywords:

Autonomous underwater vehicle

Polar missions

Sea ice

Acoustic performance

Range estimation

18/$ - see front matter & 2012 Elsevier Ltd. A

016/j.oceaneng.2012.01.018

esponding author. Tel.: þ1 709 864 8814; fax

ail addresses: ron@mun.ca (R.S. Lewis),

.drogou@ifremer.fr (M. Drogou), peter.king@m

mun.ca (G. Mann), nbose@amc.edu.au (N. Bo

@utas.edu.au (A. Worby).

a b s t r a c t

A polar environment presents unique operational challenges for Autonomous Underwater Vehicles

(AUVs). Each ice environment whether it be sea, fast or shelf poses risks with significant consequences

longer range acoustic signal transfer for communications and localization due to the typically lateral

nature of the mission.

In November 2009, a 10 kHz acoustic beacon system was tested for ranging capability and

suitability of use for an emergency AUV location and monitoring system in the Antarctic sea ice

environment. The system was deployed from the RV Aurora Australis north of the Australian Camp

Davis. This work includes discussion on test plan formulation, prediction using simulation and field

performance results of the acoustic system. Actual noise data and acoustic signal detection measure-

ments are presented and compared with the simulation. Conclusions were drawn on deployment

configuration and the testing setup. Proposed modifications will be evaluated in future experimenta-

tion planned for late 2010.

& 2012 Elsevier Ltd. All rights reserved.

1. Introduction

The untethered and uninhabited characteristics of Autono-mous Underwater Vehicle (AUV) technology are particularlyattractive for high risk under ice scientific applications. Thetechnology has been utilized to varying degrees of success insuch similar harsh environments for several decades now. Theearliest reported under ice AUV work was undertaken with theUnmanned Arctic Research Vehicle or UARS AUV in 1972(Francois and Nodland, 1972). Since that time, records have beenestablished with respect to depth: 4062 m (Kunz et al., 2008) andendurance: 330 km (Kaminski et al., 2010). Various entities suchas National Oceanography Centre, International Submarine Engi-neering Ltd., and Woods Hole Oceanographic Institute haveestablished track records for under ice AUV operations and madevaluable contributions in the area of practical considerations andoperational challenges for under ice AUV deployments. Doble

ll rights reserved.

: þ1 709 864 6193.

un.ca (P. King),

se),

et al. (2009) and McPhail et al. (2009) provide a good history ofunder-ice AUV operations.

The presence of an ice layer results in several ramificationsthat must be considered for under ice operations (King et al.,2009). The potential inability to access the surface affects both themission critical components such as fault response and can resultin longer horizontal ranging and navigation requirements in theabsence of shepherding (Bose et al., 2010). A fundamentalconsequence of the ice boundary layer is the significant effecton radio frequency and satellite surface communications chan-nels. There is a reliance upon longer range, low frequencyacoustics for safe AUV operations and risk mitigation. Quantifica-tion of a minimum monitoring distance in terms of calculatingprobability of an AUV loss based on its fault history is now animportant tool for AUV missions (Brito et al., 2010). The workdescribed in this paper seeks to establish the dimensions of theregion in which an operator can reasonably expect acousticmonitoring of a vehicle in the sea ice environment using aspecified frequency. There are few reported acoustic under iceexperiments in the literature. Mikhalevsky et al. (1999) andGavrilov (2002) describe the Transarctic Acoustic PropagationExperiment and the Arctic Climate Observations using Under-water Sound experiment, respectively. Both experiments utilize amuch lower acoustic frequency to record signal transmissions on

R.S. Lewis et al. / Ocean Engineering 43 (2012) 56–63 57

the order of hundreds and thousands of kilometers in order toidentify small changes in sound propagation time as an indicatorof thermal changes in the ocean, i.e. acoustic thermometry.

Jakuba et al. (2009) explain a robust approach using long-baseline acoustics for a deepwater AUV seabed survey of arelatively small operating area. The authors discuss someapproaches to this technological challenge of localization andcommunication. The motivation for the work described in thispaper differs in the prescribed missions that are over the horizonor beyond line of sight nature as described below.

The Australian Climate and Ecosystem Cooperative ResearchCenter is planning an AUV sea ice mission to collect data on seaice and the sea ice environment (King et al., 2010). In advance ofthis mission, a small team of AUV engineers participated in anAustralian Antarctic Division operational cruise. The purpose ofthis participation was twofold. One goal was to evaluate thecapacity and capability of the RV Aurora Australis as a supportvessel for a sea ice deployment with a large AUV. The second goalwas to engage in preliminary mission planning by determiningsafe bounds on AUV mission operations and to evaluate riskmitigation technology. The central theme to both goals wasfundamentally a better understanding of acoustic performanceusing the RV Aurora Australis in a sea ice environment.

A basic acoustic system was assembled to act as an emergencyAUV localization and monitoring system. This system usedcommercially available components and was based on theapproach used by the National Oceanography Centre in its workwith Autosub AUV series (McPhail et al., 2009). Several completecommercial systems were evaluated to provide this capabilityincluding transponders and modems. The factors used to assem-ble a basic system included: form factor, horizontal operations asopposed to deep water operations, and cost. The resulting systemconsists of a 10 kHz acoustic beacon and a ship based receivingsystem.

This system was tested in the following field locations:Holyrood, Newfoundland and Labrador at 47123034.700N,53107057.700W; and approximately 540 kms north northeast ofDavis Station, Antarctica at 64110017.0400S, 82159029.1600E. Theemphasis during this testing was to put limits on unidirectionalacoustic signal range capability; this is reflected in the descriptionof this work. Prominence within this work is given to the sea icetrials as the equipment is intended for work in the Antarctic.

Three geometric parameters were explored as part of theacoustic trials to enable conclusions on the following AUVmission specifications: overall range between the AUV anddeployment ship, the expected depth that a hydrophone mustbe lowered beneath the RV Aurora Australis’ keel, and the suitabledepth that the acoustic beacon should be deployed from astranded AUV beneath the ice to ensure signal propagation.

The paper is organized as follows. The hardware descriptionused for the open water and the sea ice trails is described in

Fig. 1. Acoustic beacon being deployed through ice (left), hydrophones

Section 2. Functional testing of the acoustic system is described inSection 3 including preliminary horizontal ranging results fromthe Holyrood open water trials. Section 4 describes the prescribedtests for the Antarctic sea ice trials. The sea ice sound velocityprofile is given in Section 5 as well as simulation and rangeprediction results based on this profile. Noise measurements arediscussed in Section 6. The test results are contained in Section 7and a conclusion and recommendations are provided in Section 8.

2. Hardware and software

Motivated by physical size, reliability and energy cost, anindependently powered low frequency omnidirectional acousticbeacon system was chosen to use as an AUV emergency locationsystem. The core system, composed of two free running acousticpingers and a pinger receiver system, is based on an underwatersource location system made by RJE International which relies onsignal intensity (audio feedback) and hydrophone directionalityfor coarse bearing estimation. The system in this form is notuseable for remote position estimation.

Supplemental hardware and acoustic equipment wasemployed for sea ice environment testing. Sound velocity mea-surements were derived using a combination of different sensorsand depth. The sensors used to generate the profile included aFalmouth Scientific Inc. MCTD conductivity temperature depth(CTD) sensor and an expendable bathythermograph (XBT) sensor.A wider bandwidth acoustic receiving system was deployed inparallel with the pinger system for spectral analysis outside of theband of interest and for signal analysis and detection.

2.1. Emergency localization system

The emergency location system consisted of two acousticbeacons and a pinger receiving system. Both pingers (RJE modelno ULB-364/10 and ULB-364/10-PL) were initialized via a waterswitch. One pinger, intended to provide an acoustic heartbeat onthe AUV when deployed, has transmit period of 10 min andactivates immediately in water. The second pinger, with a shorterperiod specified at 40 s, was prescribed as the emergency trans-mitter and would activate via an AUV fault signal. In all otherregards to electrical specifications the beacons were identical.Both operate at 10 kHz, have an omni-directional beam pattern,power output of 183 dB re 1 mPa and 5 ms pulse length. Thebeacons measure 32.20 cm in length and have maximumdiameter 6.35 cm across their transducer face. The ULB-364/10-PL, with the approximate 40 s period, was used exclusively for thesea trials.

The surface pinger receiver system (PRS-300) consisted of aomni-directional hydrophone attached to 100 m of cabling andpre-amplifier. This receiver system is tunable to any frequency in

mounted on depressor weight (right), and 10 kHz beacon (inset).

R.S. Lewis et al. / Ocean Engineering 43 (2012) 56–6358

the 5 kHz to 80 kHz band with a 100 Hz resolution. It is asuperheterodyne receiver using a carrier frequency of 1650 Hz.The receiver bandwidth is 1 kHz and hydrophone sensitivity is�196 dB re 1 mPa. There is system gain and volume control andtwo audio outputs. The hardware deployed for the ice testing isdepicted in Fig. 1, both the emergency localization systemhydrophone and the acoustic support hardware hydrophonesystem described in Section 2.3 are shown.

One audio output was wired directly to a headset for a systemoperator to tune into the signal (adjust gain and volume) tooptimize audibility. The second output was connected to theaudio input on a laptop running Matlab for signal detection andAudacity for recording the PRS-300 output. Audacity is a cross-platform digital audio recording and editing software. The hard-ware setup in the Aurora Australis science lab is shown in Fig. 2.

2.2. Sound velocity profile generation

Characterizing the sound environment is an important aspectfor the study of underwater sound propagation. Generated soundvelocity profiles were input to Chisarlab’s ASCRAY software togenerate simulation results for range prediction used as part ofthe assessment of this hardware. There was no access to a fulldepth CTD for deployment as part of this work. A combination ofCTD casts and XBT cast was used to produce a sound velocityprofile for the environment. The CTD employed was a Falmouth

Fig. 2. Lab setup on the Aurora Australis. Spectra Lab running in foreground and

PRS-300 on far right.

Fig. 3. Hardware configuration for the two acou

Scientific Inc. MCTD3-1000 and the expendable bathythermo-graph was the Sonde XBT 700.

2.3. Acoustic support hardware

The other major component of this work was the acousticdetection and monitoring system that operated independently ofthe emergency pinger system. The hydrophone for the systemwas ITC 8095 connected with 100 m of cable to a PA EGGpreamplifer and Krohn–Hite filter to a PC using a Lynx Two dataacquisition card and running Spectra-Lab software for acousticalspectral analysis. An additional laptop was running a ChisarlabSoftware Technologies software for signal analysis and detection,and was interfaced using a MOTU analog to digital converter.

In Fig. 3, the emergency localization system constitutes theacoustic source and the pinger receiver system. The componentsof the acoustic support hardware are shown in this figure as well.

2.4. Simulation

Simulation to predict acoustic performance was performedusing ASCRAY. The simulation calculates acoustic detection rangeof active, low frequency acoustic targets based on propagationlosses (attenuation and spreading), noise, reverberation, andreceiver characteristics. Propagation losses are determined usingray theory and the water column is treated as a constant stratifiedmedium. The simulation has been developed for open water testswith the required inputs being a sound velocity profile, windconditions, seabed characteristics (choice of 4 models) andambient noise levels. A maximum wind state setting wasemployed in the simulation to replace the lack of an ice boundarylayer setting. Ifremer has utilized this simulation for AUV opera-tions, but its performance has not been reported in the literature.Note though, the focus of this experiment was not a simulationvalidation exercise. Simulation was one of the tools utilizedduring the sea ice experiment planning stage and post processingstages of this work effort. Open water simulation was notemployed.

3. Preliminary open water results

A brief window of opportunity presented itself for open watertesting prior to the Antarctic sea trials. Simple ranging tests wereconducted in Holyrood harbor, Newfoundland and Labrador,Canada with the base system as described in Section 21. The testconfiguration consisted of suspending the acoustic beacon from a

stic receiving systems operating in parallel.

Table 1Horizontal ranges for open water acoustic detection trials.

Test Latitude Longitude Range(m) Electronicdetection

Audibledetection

1 47123034.700N 53107057.700W 508 Yes Yes

2 47123035.000N 53107058.000W 515 Yes Yes

3 47123048.100N 53107056.100W 917 Yes Yes

4 47123048.500N 53107056.600W 927 Yes Yes

5 47123048.800N 53107057.000W 935 Yes Yes

6 47124005.400N 53108003.000W 1433 Yes Yes

7 47124005.600N 53108003.300W 1439 Yes Yes

8 47124006.000N 53108003.700W 1451 Yes Yes

9 47124028.700N 53108005.000W 2152 Yes Yes

10 47124029.000N 53108009.300W 2163 Yes Yes

11 47124029.400N 53108005.600W 2174 Yes Yes

12 47125009.700N 53107049.900W 3435 No Yes

13 47125042.100N 53107027.600W 4492 No Yes

Fig. 4. Parameters for acoustic beacon trials.

Table 2Parameter values for the acoustic ranging under

sea ice tests.

Parameter Unit Values

a km 1–8 incl.

b m 1,10,20,50,100

c m 1,5,25,50,80

R.S. Lewis et al. / Ocean Engineering 43 (2012) 56–63 59

wharf and deploying the receiver from a small craft. The condi-tions were fair and winds light. The small craft sailed outwardfrom the wharf into deeper water (50–60 m). The water depth atthe beacon location was approximately 5 m and it was suspendedat 2 m depth at a distance of 2 m from the wharf. The hydrophonewas suspended 5 m into the water. A Matlab script was written toautomatically detect the acoustic signal and time stamp itsarrival. When the range limit was reached for automated electro-nic detection, the hydrophone was suspended an additional 10 minto the water. The signal was still audibly detectible at this point;however, the Matlab script did not automatically detect it. Thescript was basic in functionality and based detection on thresholdand frequency. No sophisticated filtering algorithms wereemployed to improve these early results.

The test results are given in the following table. The acousticbeacon was located and fixed at 47123019.1000N, 531805.3000W forthe entire testing sequence. The data in Table 1 are point sampleswhere an audible signal (a ping) was either interpreted electro-nically or not. The data collection approach utilized for the sea icetrials differed slightly and is described in that section.

4. Sea ice tests

4.1. General description

The motivation was to determine the maximum range that alow frequency acoustic beacon could be tracked from the RV

Aurora Australis in sea ice and to investigate the Aurora Australis’suitability as a support platform for sea ice operations with alarge AUV. The Aurora Australis is approximately 3900 t and is92 m in length. The Australian Antarctic Division chartered thevessel for resupply of the Australian Stations in Antarctica and forAntarctic marine science. The Aurora Australis was being consid-ered for deployment of the Memorial Explorer AUV to achievecertain research goals of the Antarctic Climate & EcosystemsCo-operative Research Centre with respect to sea ice mass volumecharacterization and habitat assessment (King et al., 2010).

The testing involved placing an acoustic noise source, thebeacon, through sea ice at some range to the Aurora Australis asdepicted in Fig. 4. The two hydrophones were deployed from theaft trawl deck beneath the keel of the Aurora. The three para-meters of interest to the measurements were the distancebetween the hydrophone and beacon; the depth of the beaconbeneath the sea ice; and the distance of the hydrophones beneaththe keel of the ship.

One day was originally scheduled for the acoustic measure-ments. There were several hours allocated in the morning to

collect noise data and ensure optimal functionality of the receiv-ing components of the trials and develop the sound velocityprofile. Then in the afternoon the acoustic beacon would be flownout onto ice flows via helicopter, deployed though ice holes atsubsequent locations and the ranging component of the exercisewould be undertaken. The test values for the parameters are givenin Table 2.

4.2. Test procedure

The proposed data collection procedure was to deploy theacoustic beacon at some range (a) to the Aurora Australis.Measurements were to be taken at each value of (b). It wasplanned that variation in (c) would actually be minimal becausethe Aurora Australis would be declutching its main propeller forthe tests.

In advance of the test it was made clear that there may not bean opportunity to test incrementally at the different values of(a) in order to establish a range limit due to limited access onhelicopter resources resulting from poor visibility and weatherconditions in the ice pack. The decision tree in Fig. 5 was devisedto resolve a limit of up to 8 km in four values of (a). The goal wasto establish an upper bound on the horizontal distance of the LFacoustic signal propagation.

Each node is a possible value for (a) for successive testlocations Test 1–Test 4. The ranges are resolved into integer

R.S. Lewis et al. / Ocean Engineering 43 (2012) 56–6360

kilometer ranges. The shaded area captures node ranges where achange in bearing relative to the hydrophone is to occur. Thischange of bearing is based on the assumption that there is aphysical obstruction affecting signal transfer such as an ice keel.

Fig. 5. Decision tree for horizontal range parameter based on a pass (P) or fail

(F) for signal detection. Nodes indicate at which range test is to be performed and

the shaded area constitutes a test location with a changed bearing.

Fig. 6. Temperature profile generated using CTD and XBT data (left) and salinity profile

salinity in parts per million.

5. Sound velocity and simulation prediction for sea ice trials

CTD casts were made at the start and end of the sea-ice trial.The first casts were made at 11:00 [þ7:00 GMT] in the vicinity of64110030.700S, 83102008.500E and the last casts were made at 15:30[þ7:00 GMT] at the conclusion of the acoustic data collection.The XBT cast followed immediately after the second set of CTDcasts. Cast results are displayed in Fig. 6.

The sound velocity profile was compiled and generated usingmultiple data sources for the entire depth of the ocean where thesea-ice acoustic trials occurred. It was assumed that for depthbeyond approximately 350 m the salinity was constant at34.5 ppt and that for depth beyond 750 m the temperature wasconstant at 0.995 1C. The following sound velocity was generatedbased on the CTD and XBT data collected and the aforementionedassumptions using Clay and Medwin (1977):

CðT ,S,zÞ ¼ 1449:2þ4:6T�0:055T2þ0:00029T3

þð1:34�0:010TÞðS�35Þþ0:016z

where c¼speed (m/s), T¼temperature (1C), S¼salinity (ppt), andz¼depth (m).

The generated sound velocity profiles are shown in Fig. 7. Themelting water from the sea ice results in cold surface water andthe noticeable minimum velocity there. The relatively constanttemperature and salinity below the surface produce a generallylinear sound velocity profile of an upwardly refracting soundpressure wave environment as expected (Lurton, 2002). Simula-tion predicted a sound channel close to the surface, likely arisingfrom the temperature minimum in that area. However, theroughness of this boundary resulted in complex reflections andnoise that caused channel degradation and increased transmis-sion losses when compared to the predicted propagation rangeswell beneath the surface where losses would mostly be attributedto spreading and attenuation. There were irregular acousticshadow pockets or holes predicted in the surficial sound channel.It is not known if such holes arose from the boundary assump-tions employed (high sea state, high winds) in lieu of an actual seaice boundary layer, or if they are an artifact of the simulation andnot an actual observable environmental phenomenon.

6. Noise measurements

The ambient environmental noise levels and self-noise of theAurora Australis were unknown prior to the sea trials. It was

from CTD (right) with depth is in meters, temperature in degrees Centigrade and

Fig. 7. Generated sound velocity profile for full ocean depth at test location (left) and generated sound velocity profile for first 1000 m (right).

Table 3Noise levels for increasing hydrophone vertical distances from ship’s hull.

Hydrophonedepth (m)

Measured noise(dBV/OHz)

Noise level(dB re 1 lPa/OHz)

Contourcolor

10 �82 77 light blue

15 �82 77 blue

35 �88 71 pink

60 �93 66 red

80 �95 64 yellow

R.S. Lewis et al. / Ocean Engineering 43 (2012) 56–63 61

necessary to measure the noise levels to improve understandingof the acoustic beacon signal transmission performance, todetermine the optimal hydrophone immersion depth (c) forreliable acoustic ranging to the beacon and gain an understandingof this acoustic environment for the future AUV deployment withrespect to communication, navigation and localization.

For these tests, the Aurora Australis’ main propeller wasdeclutched and the 12 kHz depth sounder was shutdown for themeasurements. The ship was made as acoustically quiet aspossible, but it was not possible to turn off all the onboardsystems. The two generators were isolated and it was determinedthat their noise contributions were similar. One generator had toremain on at all times to maintain power for the ship’s coresystems. The generator resulted in some noise in the frequencyband of interest for the acoustic beacon data collection. Measure-ments were made using the acoustic support hardware describedin Section 2.3 and were processed with Spectra Lab. The resultsare captured in Table 3. The conversion from dBV/OHz to dBmPa/OHz was achieved by adding 159 dB Fig. 8.

After a general reduction in the noise level was achieved acrossthe frequency spectrum a periodic noise was detected in thefrequency band of interest. The noise reduction was achieved viaan iterative process of engaging the vessel’s crew to identify andisolate noise sources. The periodic noise was determined to beexpansion and contraction of steam pipes. Once this noise wasisolated it was identified and eliminated for the remainder of thetrial. The time series for the steam pipes is shown in Fig. 9.

7. Ranging test and signal detection

The sea ice acoustic ranging tests were conducted on Novem-ber 12, 2009 north of Australia’s Davis Station, Antarctica in thevicinity of 64110017.0400S, 82159029.1600E.

Poor flight conditions and heavy ice conditions delayed thestart of the acoustic experiment and nearly canceled the work.A small suitable operational window enabled some data collec-tion. The ice conditions were approximately 80% coverage with acombination of multiyear floes and first year floes. Fig. 10 showsthe ice conditions for the test area. Flight operational criteriadictated that helicopter landings could only occur on ice thickerthan 60 cm and flow size larger than 100 m by 100 m. These twofactors resulted in three stations achieved for range resolution.The measurement of additional parameters ‘b’ and ‘c’ was waivedin order to maximize time to collect data to support quantifyingof ‘a’.

The results of the data collection are summarized in the tablebelow. It should be noted that there is a difference to the datapresented below and that data collected for the preliminary openwater trials of the acoustic beacon system. The samples in thetable below constitute recordings at each station as opposed topoint samplings. If for a 5 min recording period consisting ofeither 7 or 8 pings a predetermined number (majority) of pingswere audible and electronically detected in this 5 min recordingspan, then the result was deemed to be positive.

After successful signal reception at station one and station twoit was determined that only one additional measurement wouldbe possible due to deteriorating flying weather conditions. Anactive decision was made to make measurement at an approx-imate 4 km range; however, there were no suitable floes forlanding and the next closest floes of suitable size were 5 km fromthe Aurora Australis. The decision was based on simulation and adesire to establish the bound on horizontal signal propagationrange, that is, there would be less benefits for a positive result forupper bound estimation. Simulation based on the sound profileand established noise of the Aurora resulted in confidence that3 km range would be successful, hence, the decision to establish astation at 4 km from the vessel. The losses at this distancecoupled with the noise of the Aurora Australis resulted in anegative result for station 3, the beacon was not detected audiblyor electronically at this range. The ranging data and results arecontained in Table 4.

Post experiment simulation using CHRISAR Software Technol-ogies: ACSRAY was utilized to assess acoustic performance. Thegenerated results with this parameter configuration were com-parable with the data collection results once the contributions ofthe self-noise of the Aurora Australis were factored into thesimulations. The simulation results indicate the maximum rangefor signal reception to be approximately 3 km.

Fig. 9. Juxtaposed plots of the acoustic noise resulting from steam pipe expansion and contraction over time (left). The noise is absent once it was identified during noise

measurements and the system was shutdown (right).

Fig. 8. Sound level (dBV/OHz) as a function of frequency (Hz) for different hydrophone depths.

R.S. Lewis et al. / Ocean Engineering 43 (2012) 56–6362

8. Conclusions

A low frequency (10 kHz) acoustic source was used for soundpropagation experiments in a sea ice environment. The primarypurpose of these experiments was to determine the maximumhorizontal range for reliable acoustic signal detection. The resultsof this data would then be used to determine acceptable missionconfiguration parameters to execute a sea ice AUV deployment.

The range would impact the proposed mission tracks, the max-imal safe working distance for the AUV from the ship and searchresolution in case of vehicle loss.

A secondary motivation for the trials was to assess the RV

Aurora Australis suitability as a deployment platform for an AUVsea ice mission. The prescribed tests included measuring noiselevels of the Aurora in the sea ice environment in conditions thatmirrored an actual AUV deployment and conducting a series of

Table 4Results for sea ice acoustic ranging tests.

Stn Ship Latitude Ship Longitude Station Latitude Station Longitude Range (m) Elect. detect Aud. detect

1 64110030.700S 83102008.500E 64110014.600S 83100048.500E 1190 Yes Yes

2 64110035.700S 83102011.400E 64110002.400S 82159035.200E 2350 Yes Yes

3 64110041.700S 83102014.900E 64109036.700S 82156026.600E 5119 No No

Fig. 10. Aerial view of sea ice environment for the acoustic trials.

R.S. Lewis et al. / Ocean Engineering 43 (2012) 56–63 63

helicopter supported tests deploying the low frequency beacon atincrementally larger ranges from the Aurora Australis until it wasno longer possible to receive the acoustic signals.

Acoustic signals were successfully received at a range ofapproximately 2300 m when the hydrophone was suspended80 m beneath the keel of the Aurora and the pinger wassubmersed to a depth of 100 m. The authors were able todetermine simulation parameters that suggest a 3 km signalreception range is possible. It is expected that noise reductionin the range of the receiver or a power increase to the source willresult in an increased range.

Successful follow up testing was conducted in late 2010 onV1-2010. The depth capability of the hydrophone was increasedto reduce Aurora Australis noise in an attempt to improve on themaximum receivable range for the same 10 kHz source.

Acknowledgments

The authors would like to thank the Australian Antarctic Division;the P&O Maritime Crew of the RV Aurora Australis; and the pilots andengineers of Helicopter Resources Ltd. for the fine logistical and moralsupport during Voyage 1-2009. Financial support for equipment,travel expenses and shipping was provided from the AustralianMaritime College and the University of Tasmania through a SpecialProjects Fund—SPF: AUV-Under Ice Development. Ron Lewis issupported by a Canada NSERC PGS-D Scholarship.

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