Design of a Low-cost Acoustic Modem for Moored

Oceanographic Applications

Bridget Benson, Grace Chang, Derek Manov, Brian Graham, Ryan Kastner

University of California Santa BarbaraWUWNet PresentationSeptember 25th 2006

Outline

• Motivation

• Requirements

• Design

• Experimental Results

• Meeting the Requirements for the SB CHARM

• Future Work

Motivation

• To understand the ocean we must be able to observe large regions for long periods of time

• Such data could help predict earthquakes, tsunamis, or spread of a toxic algal bloom

• Platforms to measure large sets of data for long periods have been and are being established

Cabled Observatories

NEPTUNELEO

Martha’s Vineyard Coastal Observatory

Moorings

MBARI Ocean Observing System (MOOS)

• EOM cable in this mooring system parted in a storm on December 15, 2002.

Mooring Modem Requirements•Sensor to Modem Interfacing•Packaging•Power•Low Cost

•Data Transmission Range

•Data Transmission Rate

•Oceanic Motion Resistance

•Acoustic Interference

Mooring Modem Design

Transmitter:

Receiver:

Mooring Modem Specifications

• Power Supply: 12 V• Sampling Frequency:

8 kHz• Data Rate: 80 bps• Carrier Frequency: 50

kHz• Frequency Separation:

320 Hz• Transducer Beam

Width: 20o

Experimental Set-Up

Experimental Results

Distance (ft)

Direction Bit Errors

Data Errors

BER DER

4 Vertical 2 1 0.002 0.033

9 Vertical 10 10 0.010 0.333

10 Horizontal 0 0 0.000 0.000

17 Horizontal 4 4 0.004 0.133

19 20 Degrees

10 4 0.010 0.133

25 Horizontal 17 6 0.018 0.200

Experimental Results

Spreading Loss

0

100

200

300

400

500

600

700

800

900

1000

0 5 10 15 20Distance (ft)

Rec

eive

Vo

ltag

e (m

Vp

p)

Transmit 1.35Vpp

Transmit 10Vpp

Ideal 1/x^2 for 1.35Vpp

Ideal 1/x^2 for 10Vpp

SB CHARM

Meeting the Requirements for the SB CHARM

Requirement SB CHARM Mooring Modem

Sensor-to-Modem Interfacing

RS-232 Data Protocol X

Packaging Water proof and biofoul resistant

X

Power Mooring batteries must last 6 months

X

Low-Cost Minimum 9 modems 9 x $100.00 = $900.00

Data Transmission Range

6 meters (~20 ft) 25 ft

Data Transmission Rate

200 bps 80 bps

Oceanic Motion Resistance

Speeds of 1 m/s contribute to frequency deviation of 1 – 40 Hz

Bandpass Filters designed to withstand 40Hz deviation

Acoustic Interference ADCP operates at 300 kHz

Fmark=1040 Hz Fspace=1360 Hz

Fc = 50 kHz

Future Work• Determine actual power consumption and battery

life of mooring modems• Reduce size of the Mooring Modem design and

waterproof the electronics• Implement error correcting codes to reduce the

BER• Determine how to network the Mooring Modems• Interface the Mooring Modem with oceanographic

sensors using RS-232 protocol• Test the Mooring Modem on the SB CHARM at

depths 0, 4, 10, and 22m

Acknowledgements

• W.M Keck Foundation

• Hua Lee and Ronald Iltis