Martin Siderius

The Kauai Experiment

The Kauai Experiment was conducted from June 24 to July 9, 2003 to provide a comprehensive study of acoustic propagation in the 8–50 kHz band for diverse applications. Particular sub‐projects were incorporated in the overall experiment 1) to study the basic propagation physics of forward‐scattered high‐frequency (HF) signals including time/angle variability, 2) to relate environmental conditions to underwater acoustic modem performance including a variety of modulation schemes such as MFSK, DSSS, QAM, passive‐phase conjugation, 3) to demonstrate HF acoustic tomography using Pacific Missile Range Facility assets and show the value of assimilating tomographic data in an ocean circulation model, and 4) to examine the possibility of improving multibeam accuracy using tomographic data. To achieve these goals, extensive environmental and acoustic measurements were made yielding over 2 terabytes of data showing both the short scale (seconds) and long scale (diurnal) variations. Interestingly, the area turned out...

High‐Frequency Geoacoustic Inversion of Ambient Noise Data Using Short Arrays

Ocean ambient noise is generated in many ways such as from winds, rain and shipping. A technique has recently been developed (Harrison and Simons, J. Acoust. Soc. Am, Vol. 112 no. 4, 2002) that uses the vertical directionality of ambient noise to determine seabed properties. It was shown that taking a ratio of upward looking beams to downward produces an estimate of the reflection loss. This technique was applied to data in the 200–1500 Hz band using a 16‐m vertical array. Extending this to higher frequencies allows the array length to be substantially shortened and greatly reduces interference from shipping. If array lengths can be reduced to about 1 m then it may be possible to hull‐mount or tow such an array from a surface ship or submerged vehicle (e.g. an autonomous underwater vehicle). Although this seems attractive the noise is primarily generated by wind which in turn causes a rough sea‐surface and bubbles and these factors combined with increased volume attenuation may degrade this type of reflec...

Combination of Acoustics with High Resolution Oceanography

The variability of underwater sound observed on a moving platform in littoral waters is a combination of the effects of temporal and spatial environmental variations, which are intermingled and obscured by the change of the platform position. Sound transmission experiments over a fixed range are an approach to separate different sources of environmental impact on acoustic propagation. Even in a fixed geometry time variability experiment, the most demanding observations are for the spatial distributions of controlling parameters. On a short time scale, the variation of spatial distributions is responsible for acoustic variability, while mean, range averaged conditions may stay unchanged. The instrumentation for monitoring the ocean environment during the ASCOT 01 acoustic trial included moored instruments for the measurement of temperature and current profiles, tidal and surface waves. By means of wave dispersion relations, moored records of ocean variability may be transformed into guessed spatial fine structure. But critical results such as correct slope statistics of sound channel boundaries and iso-velocity surfaces cannot be guaranteed. A 40-sensor CTD chain was continuously towed parallel to the acoustic range. The measured 2-dimensional sound velocity field has 2 m vertical and 4 m horizontal resolution. A primitive model, which propagates rays through measured high resolution sound velocity fields is able to explain observed multipath arrival structures on a vertical hydrophone array.

Impact of Thermocline Variability on Underwater Acoustic Communications: Results from KauaiEx

In July 2003, the KauaiEx, high‐frequency acoustic experiments were conducted off the coast of Kauai, Hawaii. Both acoustic communications signals and probe signals (to measure the channel impulse response) were transmitted in the 8–50 kHz band. These signals were transmitted over several days from fixed and moving platforms and were received at multiple ranges and depths using vertical arrays and single hydrophones. Extensive environmental measurements were made simultaneous to the acoustic transmissions (e.g. measurements of the water column temperature structure, wind speed and surface wave heights). The experimental site has a relatively reflective seabed made up of sand that was combined with highly variable oceanographic conditions which led to communications performance closely tied to source/receiver geometry. In this paper, the correlation between environmental factors and communications performance will be discussed. The focus is on communications signals in the 8–13 and 14–19 kHz frequency band...

Broadband Acoustic Signal Variability in Two “Typical“ Shallow-Water Regions

Successful sonar performance predictions in shallow-water regions are strongly dependent on accurate environmental information used as input to numerical acoustic prediction tools. The sea-surface and water-column properties vary with time and this time variability of the ocean introduces fluctuations in received acoustic signals. The lack of knowledge of the environmental changes results in uncertainty in predictions of the acoustic propagation. SACLANTCEN has recently conducted two experiments to quantify the impact of the time-varying ocean on broadband acoustic propagation in “typical” shallow-water regions. Extensive oceanographic data were collected during the acoustic transmissions. Broadband acoustic signals were transmitted every minute over a fixed propagation path up to 18 h. The signals were received on a vertical array at fixed ranges of 1 to 10 km from a moored source. The variability of the oceanographic and acoustic data is presented for the two experimental areas. Numerical modelling of the sound propagation using the measured environmental data is shown and compared to the acoustic data. The possibility of predicting the received signals with an extensive knowledge of the underwater environment is discussed.

Results from the Elba HF‐2003 experiment

In October of 2003, a high‐frequency propagation and acoustic communications experiment was conducted off the Italian island of Elba. The experiment followed closely a previous experiment off Kauai (Hawaii Islands). In particular, a 5 km propagation path along the 100‐m isobath was selected. Relative to the Kauai Experiment, the Elba test was significant both in terms of what was similar and what was different. The experiment geometry was identical and a similar mixed layer structure was expected. However, since NURC has worked extensively in this area in past tests we were able to confidently select two sites, one with a very soft bottom and one with a very hard bottom. The comparison between measurements at the two sites in Elba and in Kauai is very illuminating in terms of the propagation conditions and the performance of the acoustic communications scheme. A final significant change was the inclusion of multiple input/multiple output (i.e. using source/receive arrays) communications schemes. We summar...

Source Localization in A Highly Variable Shallow Water Environment: Results from Ascot-01

Variability in the ocean environment can have a big impact on acoustic propagation. Acoustic receptions often contain multipath contributions with fluctuations that vary significantly from the direct path to the higher order multipath. Matched-field methods take advantage of the multipath to extract information about the source location and seabed properties. Matched-field processing is generally successful in environments that are not highly range dependent and do not vary significantly in time. However, in some cases the environmental conditions are too extreme for good propagation predictions and matched field results suffer. The ASCOT-01 acoustic experiments were conducted in June 2001 specifically to explore the limits of matched field methods in highly variable environments. Measurements were made over several days between a sound source and a moored vertical line array of receivers. Results characterizing the difficulties of matched field processing at this site will be presented. Source localization results using standard estimators will be compared with those using new alternatives intended to be robust against harsh environmental conditions.

High-Frequency Propagation for Acoustic Communications

In recent years there has been great progress in developing undersea wireless networks. The physical layer of these networks is generally an acoustic link operating in the 10–50 kHz band. Interestingly, our understanding of the acoustic propagation has not kept up with the elegant signaling schemes used to transmit the information. For instance, one common system uses adaptive equalizers to not only recombine the ocean multipath but to track the ocean dynamics. The tracking occurs on a millisecond time scale. Predicting system performance then requires an understanding of the transmission loss, the noise background, and the dynamics of the multipath. Here we use the term ‘transmission loss’ loosely, glossing over subtleties about the coherent and the incoherent field, which also play an important role in the system performance. This talk will summarize the issues and our knowledge about them, drawing upon results from an extensive set of sea tests conducted under the SignalEx program.