109
antenna to receive further information. Although some experimental progress in
underwater radio transmission in the 1–5 MHz band has been reported (Lucas and
Yip 2007), communication via electromagnetic means between submerged Tx/Rx
antenna pairs to date remains commercially unavailable. Acoustic links constitute a
viable (albeit slow) alternative to radio communications which, for reasons discussed above, are unviable in the subsurface ocean environment. The ocean is however an inherently noisy environment with acoustic signals ranging from 10 to
100 kHz and intensities of up to 100 dB from sources as diverse as earthquakes,
explosions waves, wind, rain, shipping, ocean exploration, and the marine biota
(Marsh 1969). Coastal underwater environments are particularly noisy due to the
profusion of marine life and intense human activity. The challenges of acoustic
communications in the noisy marine environment have been to some point overcome using digital wide band/spread spectrum communication schemes implementing multiple frequency shift keyed modulation or phase shift keyed modulation,
data packaging, and multipath correction to achieve error-free data transmission. A
recent breakthrough (Shi et al. 2017) adds an additional modulation dimension by
impressing orbital angular momentum to the acoustic beam thus generating helical
acoustic vortex beams.
Instruments for underwater data communication incorporating such features are
known as acoustic modems. Bottom mounted acoustic wave and current meters and
deep ocean tsunami monitors both communicate with surface buoys making use of
acoustic modems as do deep-water moorings incorporating multiple instruments.
Commercially available acoustic modem links, widely used in the offshore industry,
operate at frequencies between about 10 and 50 kHz achieving ranges beyond
4000 m Sendra et al. (2016).
Sea bottom emplacements and ODAS buoys systems require periodic retrieval
for instrument and equipment maintenance. Shallow water emplacements can be
accessed by divers or buoy tender vessels but alternate means are necessary at
greater depths. The acoustic release is an electronic instrument designed to allow
recovery of buoys or sea bottom emplacements moored at depths beyond the reach
of divers and buoy tenders. Their remote operation involves a complex mix of disposable ballast, and integrated buoyancy besides the acoustic release. Plastic
encased spherical glass floats ensure the required buoyancy upon anchor release.
The acoustic release acts as a structural component, essentially a shackle, but incorporates an acoustic transducer and circuitry tuned to a specific acoustic command
sequence that prompts the opening of the mechanical component. In practice, the
recovery vessel lowers a hydrophone into the water to deliver a coded signal unique
to the instrument being addressed. To minimize malfunction contingencies, highvalue packages may be secured by paired acoustic releases in a configuration such
that release by either of the pair achieves release of the buoyed instrument package
leaving behind only the disposable ballast. In general, shallow water instruments
operate in the range of 20–50 kHz. Deeper deployments require lower frequencies
to achieve better propagation in the seawater medium.
5.4 Satellite-Aided and Autonomous Underwater Navigation for Ocean Observing
antenna to receive further information. Although some experimental progress in
underwater radio transmission in the 1–5 MHz band has been reported (Lucas and
Yip 2007), communication via electromagnetic means between submerged Tx/Rx
antenna pairs to date remains commercially unavailable. Acoustic links constitute a
viable (albeit slow) alternative to radio communications which, for reasons discussed above, are unviable in the subsurface ocean environment. The ocean is however an inherently noisy environment with acoustic signals ranging from 10 to
100 kHz and intensities of up to 100 dB from sources as diverse as earthquakes,
explosions waves, wind, rain, shipping, ocean exploration, and the marine biota
(Marsh 1969). Coastal underwater environments are particularly noisy due to the
profusion of marine life and intense human activity. The challenges of acoustic
communications in the noisy marine environment have been to some point overcome using digital wide band/spread spectrum communication schemes implementing multiple frequency shift keyed modulation or phase shift keyed modulation,
data packaging, and multipath correction to achieve error-free data transmission. A
recent breakthrough (Shi et al. 2017) adds an additional modulation dimension by
impressing orbital angular momentum to the acoustic beam thus generating helical
acoustic vortex beams.
Instruments for underwater data communication incorporating such features are
known as acoustic modems. Bottom mounted acoustic wave and current meters and
deep ocean tsunami monitors both communicate with surface buoys making use of
acoustic modems as do deep-water moorings incorporating multiple instruments.
Commercially available acoustic modem links, widely used in the offshore industry,
operate at frequencies between about 10 and 50 kHz achieving ranges beyond
4000 m Sendra et al. (2016).
Sea bottom emplacements and ODAS buoys systems require periodic retrieval
for instrument and equipment maintenance. Shallow water emplacements can be
accessed by divers or buoy tender vessels but alternate means are necessary at
greater depths. The acoustic release is an electronic instrument designed to allow
recovery of buoys or sea bottom emplacements moored at depths beyond the reach
of divers and buoy tenders. Their remote operation involves a complex mix of disposable ballast, and integrated buoyancy besides the acoustic release. Plastic
encased spherical glass floats ensure the required buoyancy upon anchor release.
The acoustic release acts as a structural component, essentially a shackle, but incorporates an acoustic transducer and circuitry tuned to a specific acoustic command
sequence that prompts the opening of the mechanical component. In practice, the
recovery vessel lowers a hydrophone into the water to deliver a coded signal unique
to the instrument being addressed. To minimize malfunction contingencies, highvalue packages may be secured by paired acoustic releases in a configuration such
that release by either of the pair achieves release of the buoyed instrument package
leaving behind only the disposable ballast. In general, shallow water instruments
operate in the range of 20–50 kHz. Deeper deployments require lower frequencies
to achieve better propagation in the seawater medium.
5.4 Satellite-Aided and Autonomous Underwater Navigation for Ocean Observing