An Overview And Lessons Learned From The Five Octave Research Array (FORA) With Some

Perspectives For Future Towed Array

John R. Prestona, Kyle M. Beckerb, Paul T. Shultzc, Jeffrey R. McIlvainc, George M. Ruhlmannd

aARL, Penn State University, P.O. Box 30, MS 6110D, State College, PA 16804bNURC, Viale San Bartolomeo 400, 19126 La Spezia, Italy

cL-3 Chesapeake Sciences Corp., 1127B Benfield Blvd., Millersville MD 21108dWaveTech Engineering, LLC., 3200 Dunbratton Court, Waldorf, MD 20601

Purpose/Background

• Built to support the ONR Ocean Acoustics Program’s 6.1 experimental research efforts.

• MACE program at ONR was also a sponsor/user of this system.

• After its shakedown cruise in 2002, FORA has completed ten very successful sea trials as of Sept. 2010.

Tow CableDeck Cable

TATS

Fore VIM TripletModule

Fore LF/MF MF/HF Aft LF/MF Aft VIM

25.0m 88m

277 m (triplet 89 mm dia, linear 77 mm. dia.)

33.5m 45.0m25 m 30.0m

ULF

30.5m

FORA System Module Layout*

950 m long, 27.4 mm dia.

Array & WinchWt: 22,000 lbs.

Tow CableDeck Cable

Towed Array Test Set (TATS)

Controller

Forw. VIM TripletModule

Forw. LF/MF MF/HF Aft LF/MF Aft VIM

Drogue305 m long,11 mm dia.

88m33.5m 45.0m30.0m

(Aft) ULF

30.5m

*not the same as acoustic apertures

30 m long, 15 mm dia

Nas Nodes

FORA with new level wind

FORA Acoustic AperturesAcoustic

Apertures

No. of phones Design freq (Hz)/spacing (m)

ULF 64 250 / 3.0

LF 64 500 / 1.5

MF 64 1000 / 0.75

HF 64 2000 / 0.375

Triplet section 78*3 3750 / 0.2

FORA SAMPLING CONFIGURATIONS(Total channels =396)

Config.

number

Fwd Triplet

Aft Triplet All Linear

Apertures

1 25 kHz 12.5 kHz 6.25 kHz

2 12.5 kHz 12.5 kHz 6.25 kHz

3 16.0 kHz 8.0 kHz 8.0 kHz

4 8.0 kHz 8.0 kHz 8.0 kHz

5 2.0 kHz 2.0 kHz 2.0 kHz

FORA on R/V KNORR

Deployment

FORA Overboard Sheave

FORA Capabilities• Large frequency coverage 50 - 3750 Hz with beamformer• Very high dynamic range 24 bit A/D, 19 bits actual• 4 NAS suites - each with heading, pitch, roll, depth and

temperature• Removable ULF section (88 m)• Can also operate as short/long vertical array suspended

from own ship (drifting or moored only)• Five sampling configurations from 2 kHz - 25 kHz

• Highest frequency aperture is a triplet array for left-right ambiguity resolution

• Desensitized phone to capture direct arrival waveforms

Limitations/Considerations• Drogue was designed for 4-5 kt tows but the split drogue

mod. allows tows up to 10 kts.• Maximum depth at 3 kts is ~350 m (cable limit).• Array uses ATM/Sonet fiber optic data link so input data

rate fixed at 155 mb/s but recording is done separately for triplet or linear sections.

• The FORA acquisition system can record all 4 linear sections or the full triplet section without problems.

• Acquisition system is based on archiving to SATA disks from Linux workstations.

Newer Sea Trial Summaries• NPAL//SPICE/LOAPEX 04 - long range propagation and azimuthal

scattering studies: FORA operated for the entire experiment without any days lost but ~1s one-way timing errors were later discovered.

• OREX 05 - scattering and inversion studies: FORA weather + equipment problems caused 1-2 days of data loss (for first time). New FORA limit is SS4 for operations.

• Gulf of Maine 06 - remote sensing of fish schools: FORA operates without equipment failures for entire trial.

• Clutter07/BASE07 FORA operates without major problems again (all desired data collected successfully).

• During the course of FORA’s ten sea trials, it was requested to collect data for a total of 138 days and missed only 3.5 days due to FORA equipment problems (including the winch).

Summary of FORA Sea Trials

Sample of Reverberation from the FORA LF aperture on the New Jersey Shelf

Sample reverberation from the FORA cardioid aperture on the New Jersey Shelf

Triplet BF Near Mud Volcanoes

FORA Acquisition Software Problem History/Fixes

• The original FORA acquisition software was adapted from other active systems and worked well in active experiments.

• In 2006-2007, post analysis of the NPAL04 experiment showed one-way timing errors of >=0.5 s with no way of correcting them.

• In 2008-2009 several improvements reduced one-way error to ~8ms.

• In 2010 the acquisition system was completely rewritten. In the course of improving the FORA acquisition system for the LRAC10 experiment, GPS time tags have been added for each data block, and all data dropouts are logged with location and duration.

• The new acquisition software worked flawlessly during the remainder of the LRAC10 experiment recently completed (no dropouts). Now one-way timing errors are down to approximately 1 ms.

Acknowledgements• ONR Ocean Acoustics Program

• ONR MACE Program

• Chesapeake Sciences Corp. & Teledyne Inst.

• Digital Systems Research (DSR) Inc.

• Lillimar (Lillie) Rulhmann, Nick Makris (MIT)

• R. Hollett (SACLANTCEN), Eddie Scheer (WHOI)

• A. C. Hill (Hill Consulting)

• WaveTech Engineering, LLC (George Ruhlmann)

New Array Technologies