Fletcher A. Blackmon

Buoyant device for bi-directional acousto-optic signal transfer across the air-water interface

A buoy system bi-directionally communicates in-air and underwater. A buoy having a shell to float on water has an upper portion in air and a lower portion in water. An array of acoustic transducers which is disposed in the lower portion receives acoustic signals and transmits acoustic signals. A dome-shaped retro-reflective coating on the upper portion is vibrated for retro-reflecting impinging laser illumination as data signals in air. An array of photo-detectors on the upper portion are responsive to impinging laser control signals and/or signals which may be transmitted as acoustic signals in water.

Iterative equalization and decoding techniques for shallow water acoustic channels

We investigate the application of iterative equalization and decoding techniques to the shallow water acoustic channel. The first receiver is a joint decision feedback equalizer (DFE) and turbo decoder. The second receiver is a turbo equalizer, which jointly estimates the channel, performs MAP equalization, and decodes received symbols. Although the MAP equalizer is optimum for a known channel, channel estimation errors degrade the performance of the turbo equalizer. We compared the performance of both receivers under real channel conditions.

Performance comparison of RAKE and hypothesis feedback direct sequence spread spectrum techniques for underwater communication applications

Direct sequence spread spectrum (DSSS) techniques for low probability of intercept (LPI) and multi-user communications applications for underwater acoustic communications are investigated. Two promising receivers, a RAKE based receiver and a hypothesis-feedback equalization based architecture are presented. The performance of the proposed receiver structures are compared based on simulations and also actual field test data.

Experimental demonstration of remote, passive acousto-optic sensing

Passively detecting underwater sound from the air can allow aircraft and surface vessels to monitor the underwater acoustic environment. Experimental research into an optical hydrophone is being conducted for remote, aerial detection of underwater sound. A laser beam is directed onto the water surface to measure the velocity of the vibrations occurring as the underwater acoustic signal reaches the water surface. The acoustically generated surface vibrations modulate the phase of the laser beam. Sound detection occurs when the laser is reflected back towards the sensor. Therefore, laser alignment on the specularly reflecting water surface is critical. As the water surface moves, the laser beam is reflected away from the photodetector and no signal is obtained. One option to mitigate this problem is to continually steer the laser onto a spot on the water surface that provides a direct back-reflection. Results are presented from a laboratory test that investigates the feasibility of the acousto-optic sensor detection on hydrostatic and hydrodynamic surfaces using a laser Doppler vibrometer in combination with a laser-based, surface normal glint tracker for remotely detecting underwater sound. This paper outlines the acousto-optic sensor and tracker concepts and presents experimental results comparing sensor operation under various sea surface conditions.

Iterative equalization, decoding, and soft diversity combining for underwater acoustic channels

The performance of iterative equalization, decoding, and soft diversity combining in underwater acoustic channels is studied. A decision feedback equalizer (DFE) is employed instead of a MAP equalizer to reduce the complexity of the iterative algorithm. The performance of a soft diversity combining scheme, where multiple DFEs are employed for spatial diversity or time diversity as well as a DFE with multiple feedforward filters is presented. The proposed receiver structures are tested and compared using experimental data.

Performance comparison of iterative/integral equalizer/decoder structures for underwater acoustic channels

The use of an adaptive decision feedback equalizer (DFE) in the demodulation of high speed data transmitted through underwater acoustic channels has been well established. In many channels, however, the performance obtained with the conventional DFE and decoder is not adequate for particular applications. This paper considers four different iterative equalization/decoder techniques for improving the performance of the receiver. One technique uses the hard decisions from the decoder output to feed back to the DFE for making additional passes through the data. The second technique uses the soft outputs from the decoder output to feed back to the DFE. The third technique, termed an integral iterative equalization scheme, is designed to mitigate and correct the errors being fed back to the DFE in a block fashion within the data packet. Finally, the fourth technique, called a turbo equalizer, is an iterative scheme which employs a MAP equalizer and a MAP decoder. These iterative/integral equalization/decoding techniques are applied to convolutionally encoded BPSK and QPSK data received during several field tests. The performance of the iterative equalizer/decoder algorithms is compared on the basis of bit error rate and packet statistics.

Laser-based acousto-optic uplink communications technique

An apparatus for enabling acousto-optic communication comprising an in-water platform comprising means for emitting an acoustic signal to an acousto-optic interaction zone, an in-air platform comprising the ability for transmitting a first optical interrogation beam, the ability for receiving a portion of the first interrogation beam and a second laser beam formed from the reflection of the first interrogation beam off of the acousto-optic interaction zone, the ability for measuring and outputting a plurality of optical interferences between the portion of the first interrogation beam and the second reflected beam, and a signal converter receiving as input the plurality of optical interferences and outputting an electrical signal representing the received acoustic telemetry signal at the interrogation point at the air-water interface.

Experimental detection and reception performance for uplink underwater acoustic communication using a remote, in-air, acousto-optic sensor

Covert communications between underwater and aerial platforms would increase the flexibility of surface and air vehicles engaged in undersea warfare by providing a new netcentric warfare communications capability and could have a variety of commercial and oceanographic applications. Research into an acousto-optic sensor shows promise as a means for detecting acoustic data projected toward the water surface from a submerged platform. The laser-based sensor probes the water surface to detect perturbations caused by an impinging acoustic pressure field. Experimental studies were conducted to demonstrate acousto-optic sensor feasibility for obtaining accurate phase preserved recordings of communication signals across the air-water interface. The recorded surface velocity signals were transferred to an acoustic communications receiver that used conventional acoustic telemetry algorithms such as adaptive equalization to decode the signal. The detected, equalized, and decoded bit error rate performance is presented for hydrostatic and more realistic, hydrodynamic water surface conditions

Linear optoacoustic underwater communication.

The linear mechanism for optical-to-acoustic energy conversion is explored for optoacoustic communication from an in-air platform or surface vessel to a submerged vessel such as a submarine or unmanned undersea vehicle. The communication range that can be achieved is addressed. A number of conventional signals used in underwater acoustic telemetry applications are shown to be capable of being generated experimentally through the linear optoacoustic regime conversion process. These results are in agreement with simulation based on current theoretical models. A number of practical issues concerning linear optoacoustic communication are addressed that lead to a formulation of a linear-regime optoacoustic communication scheme. The use of oblique laser beam incidence at the air-water interface to obtain considerable in-air range from the laser source to the in-water receiver is addressed. Also, the effect of oblique incidence on in-water range is examined. Next, the optimum and suboptimum linear optoacoustic sound-generation techniques for selecting the optical wavelength and signaling frequency for optimizing in-water range are addressed and discussed. Optoacoustic communication techniques employing M-ary frequency shift keying and multifrequency shift keying are then compared with regard to communication parameters such as bandwidth, data rate, range coverage, and number of lasers employed.

Experimental investigation of optical, remote, aerial sonar

Detecting underwater objects such as debris fields, submarines or mines for littoral area clearance, while in the air, would increase the autonomy and flexibility of subsurface, surface and air vehicles engaged in undersea warfare and could have a variety of commercial and oceanographic applications. Experimental research into a laser-based active sonar concept is being conducted for remote, aerial detection of submerged objects and for sonar mapping of an undersea area. An airborne high-energy, pulsed laser is used to remotely generate underwater acoustic energy. The acoustic wave propagate through the water and are reflected from underwater objects. The acoustic signals are then detected as they reach the water surface using a low-powered laser interferometer device. An array of surface detections of the acoustic field reflected from the underwater objects will then be analyzed using traditional time-delay beamforming techniques to locate underwater objects. The combination of these optical technologies provides a means for stealthy, remote, active as well as passive sonar that does not currently exist. Results are presented from a controlled laboratory test using commercial laser devices, which demonstrate the feasibility of the sonar concept for remotely searching for underwater objects. This paper outlines the aerial sonar concept and provides the initial experimental results of the underwater object illumination, detection and localization.