L.Ginzkey, C. Kubaczyk, J. Ehrlich...DSP is switched on and evaluates the data in the memory. If predetermined levels and signal lengths were found the data is pushed into a permanent

Bundeswehr Technical Centre for Ships and Naval Weapons, Naval Technology and ResearchResearch Department for Underwater Acoustic and Marine Geophysics

Wehrtechnische Dienststelle für Schiffe und Marinewaffen, Maritime Technologie und Forschung

Forschungsbereich für Wasserschall und Geophysik · Klausdorfer Weg 2-24 · D-24148 Kiel

Tel.: +49 431 6 07-0 · Fax: +49 431 6 07-41 50 · E-Mail: [email protected] · Internet: http://www.FWG-Kiel.de

Hydroacoustic monitoring for CTBTO with a mobile buoy system Past experiences and new concept

L.Ginzkey, C. Kubaczyk, J. Ehrlich

BackgroundThe hydroacoustic network of CTBTO has an inherent possible vulnerability against station breakdown. A mobile buoy system as a temporary replacement for a hydroacoustic sensor can be a valuable asset for CTBTO. In 2000 FWG drafted a concept for such buoy and built a prototype buoy which was successfully tested in 2002. Although the buoy was never in operation for CTBTO, the concept is still interesting, as the recent breakdown of several hydroacoustic sensors and stations show. We present a concept for an updated version of the buoy that benefits from the technological improvements made since 2002 and overcomes the shortfalls of the old buoy system.

Updated conceptThe new design is based on the old, proven system but it has several improvements due to technological advances made in the last few years.

Sensor Unit?Data acquisition and preparation with field programmable analog arrays (FPAA)?Signal based amplification and intelligent filtering?16 bit A/D conversion with adaptive adjust

ment of dynamic range?Network based transmission of digital data

allows application-oriented interconnection of different sensors

Sensor Unit Network?Different sensor units may be combined within a network?Analog data are locally prepared and converted?Both acoustic and non-acoustic data (temperature, salinity,

static pressure, etc.) can be sampled?Synchronization through trigger line or NTP server

Communication Unit?Data transmission over Iridium satellite network?Position is determined by GPS?GPS based time synchronization?Reliable connection to subsurface units with fiber optic network ?Data rate up to 2400 bps with standard Iridium service?Integration of Iridium OpenPort high speed connection possible (max. 128 kbps)

Signal processing and storage• Micro-Controller (RT-Kernel) for communication with sensor

and communication unit?Power management?Digital signal processing?Ring buffer with 32 MB flash memory (24 h of data)?Solid state disk for data storage (8 GB for data from 6 months)

ConclusionsA mobile buoy system as a temporary replacement for a CTBTO hydroacoustic sensor or station is proposed. The experimental buoy system of FWG from 2002 has proven itself as a capable and reliable system for this task, although it was never in operational duty for CTBTO due to shortcomings of the signal processing and transmission capabilities at the time. A new concept for a buoy system is presented that is based on the proven design but uses the advances made in signal processing, computing power and satellite communications since 2002. This allows the storage and transmission of large amounts of original time series data. The intelligent data management ensures that all possible noticeable nuclear events are detected and resolved properly. Additionally, arbitrary time series data can be retrieved via radio control. Continuous operation and data storage for more than 6 months is possible, depending on disc space and battery capacity. A buoy system according to this design could be a very versatile and useful system for CTBTO at acceptable costs for periods of reduced fixed stations in operation.

The spar buoy with the communication unit is basically the same except for the satellite transceiver that is replaced by a IRIDIUM modem which supports a higher data rate and thus allows the transmission of time series signals. The signal processing unit consists of new hardware, a solid state disc for stor-age of large amounts of original data and a ring buffer storage for the most recent data.

The sensor unit with the hydrophone now also holds the A/D converter and the transmission of the signals to the processing unit is done digitally with fiber optic cables. The communication and data transport across the different units of the system is completely based on ethernet protocol.

Buoy prototypeA prototype buoy was built by FWG and tested in several occasions, the longest of which was a period of 4 months in the Skagerak in 2002. The buoy was anchored at a depth of approximately 500 m and drifted within a radius of 200 m.

Concept of buoy from 2002The buoy consisted of three main parts. A spar buoy that is floating on the surface holds the communication equipment with a GPS receiver and the ORBCOMM satellite transceiver and antenna. The spar buoy design was chosen because it decouples the acoustic units from the sea surface motion. The satellite connection via the ORBCOMM service allowed bidirectional email-based communication with low data rate. That implied that no complete time series could be transmitted. The data had to be analyzed and preprocessed within the buoy.

This was done in a subsurface buoy that floats at around 100 m depth and holds the A/D converter, a microcontroller, a DSP, RAM memory and a power supply. The receiving unit floats in the SOFAR channel at around 1000 m depth and holds the hydrophone, amplifiers and filters. The buoy is anchored at the appropriate depth with an anchor stone and can be released by an acoustic releaser. The power supplies allowed a continuous operation time of at least one year.The main drawback of this system was that the satellite connection forbid the transmission of complete time series and therefore pre-processed results had to be transmitted.

Spar BuoySpar Buoy

Cable, 200 mCable, 200 mKevlar, low twistKevlar, low twist

HydrophoneHydrophone

Buoyancy ElementsBuoyancy Elements

Synthetic CableSynthetic CableLength depends on DepthLength depends on Depth

Acoustic ReleaserAcoustic Releaser

Anchor Stone 1 tAnchor Stone 1 t

Subsurface Buoy, 0.5 tSubsurface Buoy, 0.5 t

Cable, 1000 mCable, 1000 mwith steel, low twistwith steel, low twist


Satellite AntennaSatellite AntennaData, PositionData, Position

Swivel Swivel contactlesscontactless data transmissiondata transmission


Buoy ParameterBuoy ParameterBuoy Parameter

Power Supply 1Power Supply 1Power Supply 1

FLASH-RAMFLASHFLASH--RAMRAM

Ring RAMRing RAMRing RAM

ADC 24 bitADC 24 bitADC 24 bit


Micro-ControllerMicroMicro--ControllerController

Control Unit/Processor„Seakat“

Control Unit/ProcessorControl Unit/Processor„„SeakatSeakat““

GPS - ReceiverGPS GPS -- ReceiverReceiver

ORBCOMMSatellite-Modem

ORBCOMMORBCOMMSatelliteSatellite--ModemModem

Digital Signal Processor (DSP)Digital Signal Processor (DSP)Digital Signal Processor (DSP)

HydrophoneAmplifier, Filter

HydrophoneHydrophoneAmplifier, FilterAmplifier, Filter

Spar BuoySpar Buoy











































-400

-300

-200

-100

0

100

200

300

400

-400 -300 -200 -100 0 100 200 300 400

Movement of the buoy27.Feb.- 25.Juni 2002

Distance in m

Dis

tan

ce

inm

Center position: 10° 7,79 E58° 25,27 N

PresentationFrequency splitting Downsampling 1:3 Data reduction: 11 points

Spar BuoySpar Buoy
















Solid State DriveSolid State DriveSolid State Drive


Ring BufferRing BufferRing Buffer

CommunicationUnit

CommunicationCommunicationUnitUnit

IRIDIUMSatellite-Transceiver

IRIDIUMIRIDIUMSatelliteSatellite--TransceiverTransceiver


Sensor UnitSensor UnitSensor Unit



(Power Supply)(Power Supply)(Power Supply)

Spar BuoySpar Buoy



















CommunicationUnit















CommunicationUnit









Recording and signal processingThe acoustic front end consisted of one hydrophone, preamplifier and an A/D converter. It feeds the received data into a ring memory storage that could keep data from approx. 64 minutes. Every hour the DSP is switched on and evaluates the data in the memory. If predetermined levels and signal lengths were found the data is pushed into a permanent memory for further analysis.

Due to the limited transmission capacity a preprocessing of the received signals had to be done before transmission. The signal processing consisted of 4 steps. In the detection step a decision is made based on long time/short time averages and signal duration. In the next step the possible signals that were detected are divided into 6 frequency bands (1-2 Hz / 3-5 Hz /6-10 Hz / 11-20 Hz / 21-40 Hz / 41-100 Hz). Afterwards a downsampling to 80 Hz is performed by averaging 3 time bins into one point. Then a data reduction of the peak to 11 points is performed. These data and the corresponding peak and mean energy levels are transmitted via the ORBCOM satellite link.

Communication Unit

GPS

µCOE

IridumModem

Signal Processing and StoragingSignal Processing and Storaging

µC

DCPOE

Solid State Disk (8GB)

Flash (32MB)

Sensor UnitSensor Unit

µC

Controller

FPAA

OAED

Sensor Unit #1

Sensor Unit #2

Sensor Unit #N

ET

HE

RN

ET

Sensor Unit #1

Sensor Unit #2

Sensor Unit #N

ET

HE

RN

ET

µC

Controller

FPAA

OAED

µC

Controller

FPAA

OAED

µC

Controller

FPAA

OAED

Sig

na

l/dB

Sig

na

l/dB

Sig

na

l/dB

Sig

na

l/dB

Sig

na

l/dB

Sig

na

En

erg

iel/d

B

Sig

na

En

erg

iel (

-20

dB

)/d

BSig

na

l/dB

Sig

na

l/dB

Sig

na

l/dB

Sig

na

l/dB

Sig

na

l/dB

Sig

na

l/dB

Timebins Timebins TimebinsTimebins Timebins Timebins

Frequency/Hz

Frequency/Hz

Time/s

Frequency/Hz

Documents

L.Ginzkey, C. Kubaczyk, J. Ehrlich...DSP is switched on and evaluates the data in the memory. If predetermined levels and signal lengths were found the data is pushed into a permanent