Sonar Concepts Research Articles

A sonar experiment to study sound propagation through flames

Disorientation is a major cause of firefighter death and injury. When a firefighter is trapped in a structure, there is a small window of time for rescue teams to locate a downed firefighter. We propose an acoustic navigation system based on sonar concepts to augment existing techniques (e.g. thermal imaging cameras). As a first step toward this system, a better understanding of acoustic wave propagation through the fire environment is needed. Therefore, a sonar experiment was developed to measure the distance through a flame. Since information in the literature suggests that transmission loss through fire increases with frequency, a parametric array was used to maintain narrow source directivity while remaining in the low frequency/low attenuation regime. Results of the sonar experiment will be presented for various source functions and flame conditions.

My personal perspective of sonar research at the NATO Undersea Research Centre

As the first German Director of the NATO Undersea Research Centre I was contemporary to the fall of the Berlin Wall in November 1989 and the unification of Germany in October 1990, the breathtaking termination of the Cold War without bloodshed. NATO's Undersea Research Centre, thirty years of age when the Berlin Wall fell, has been a key player and focal point of a most important and demanding field of defense research. It has been exemplary in combining customer oriented applied research on sonar concepts up to system demonstrators with basic research to understand, to model and to predict the variability of the ocean environment, the most difficult challenge of successful surveillance and reconnaissance. Some unique capabilities enable the Centre to serve the nations and the NATO commands: Above all, the ability to attract first class scientists to work together at the Centre for a few years and then return to their home countries, forming an ever growing international network of intense scientific cooperation. The advanced sea trial related technology and data treatment realized by a capable and flexible technical staff is no less decisive. The many high-ranking visitors, scientific, military and political underline the international standing of the Centre.

Teaching sonar methods and technologies using a practical, real-life research environment

The results and current developments of an educational outreach website and CD-ROM which allows students and instructors to follow the day-by-day progress of an ocean research expedition aboard the R/V Thomas G. Thompson are presented. Sonar concepts, techniques, and technologies are explained through daily updates, science objectives, data results, and lesson outlines. As the greatest source of data collected on the expedition, sonar technologies play a large role in the outreach effort. Students are exposed to hullmounted bathymetry data collection instrumentation that includes the EM300 and Hydrosweep, multibeam sonar systems. The concept of deep towed, side scan-sonar is introduced through the DSL-120 system (WHOI). Students are also introduced to sonar technologies incorporated into remotely operated vehicles like Jason II/ Madea. These include the Imagenex 855 scanning sonar, the RDI bottom-tracking Doppler velocity logger, as well as navigational transponders. Each instrument and educational concept is detailed using video, Quick Time Virtual Reality, html text, and/or images. Using a Marisat satellite link, students are able to ask the researchers questions about their work and findings. [Work supported by a supplement to NSF/OCE-0002584 project: Collaborative Research: Studies of Deep-sourced Mud Volcanism in the Mariana Forearc: A DSL120, Jason ROV, and Coring Program.]

New virtual sonar and wireless sensor system concepts

Recently, exciting new sensor array concepts have been proposed which, if realized, could revolutionize how we approach surface mounted acoustic sensor systems for underwater vehicles. Two such schemes are so-called ‘‘virtual sonar’’ which is formulated around Helmholtz integral processing and ‘‘wireless’’ systems which transfer sensor information through radiated RF signals. The ‘‘virtual sonar’’ concept provides an interesting framework through which to combat the dilatory effects of the structure on surface mounted sensor systems including structure-borne vibration and variations in structure-backing impedance. The ‘‘wireless’’ concept would eliminate the necessity of a complex wiring or fiber-optic external network while minimizing vehicle penetrations. Such systems, however, would require a number of advances in sensor and RF waveguide technologies. In this presentation, we will discuss those sensor and sensor-related developments which are desired or required in order to make practical such new sensor system concepts, and we will present several underwater applications from the perspective of exploiting these new sonar concepts. [Work supported by ONR.]

Discrimination of amplitude-modulated synthetic echo trains by an echolocating bottlenose dolphin.

Bottlenose dolphins (Tursiops truncatus) have an acute ability to use target echoes to judge attributes such as size, shape, and material composition. Most target recognition studies have focused on features associated with individual echoes as opposed to information conveyed across echo sequences (feature envelope of the multi-echo train). One feature of aspect-dependent targets is an amplitude modulation (AM) across the return echoes in the echo train created by relative movement of the target and dolphin. The current study examined whether dolphins could discriminate targets with different AM envelopes. "Electronic echoes" triggered by a dolphin's outgoing echolocation clicks were manipulated to create sinusoidal envelopes with varying AM rate and depth. Echo trains were equated for energy, requiring the dolphin to extract and retain information from multiple echoes in order to detect and report the presence of AM. The dolphin discriminated amplitude-modulated echo trains from those that were not modulated. AM depth thresholds were approximately 0.8 dB, similar to other published amplitude limens. Decreasing the rate of modulation from approximately 16 to 2 cycles per second did not affect the dolphin's AM depth sensitivity. The results support multiple-echo processing in bottlenose dolphin echolocation. This capability provides additional theoretical justification for exploring synthetic aperture sonar concepts in models of animal echolocation that potentially support theories postulating formation of images as an ultimate means for target identification.

Bistatic synthetic aperture imaging of proud and buried targets from an AUV

The use of autonomous underwater vehicles (AUVs) for the detection of buried mines is an area of current interest to the Mine CounterMeasures (MCM) community. AUVs offer the advantages of lower cost, stealth, reduced operator risk, and potentially improved coverage rates over more traditional mine hunters. However, AUVs also come with their own set of difficulties, including significant error in navigation and low communication rates with the mother platform and each other. In the case of bistatic detection scenarios, AUVs will therefore have difficulty knowing where exactly in space they are and the trigger time of sources on other platforms, be they ships or other AUVs. However, the potential improvement in detection and coverage rates offered by bistatic sonar concepts makes resolution of these issues a high priority. In this paper, the problems of inaccurate navigation and source timing information are addressed for the Generic Oceanographic Array Technology data set. In this experiment, conducted off Marciana Marina during June 1998, a MIT AUV with a SACLANTCEN acoustic array and acquisition system was used together with a TOPAS parametric sonar to explore issues of buried target detection using AUVs. In this paper, solutions to the navigation and timing problems are proposed which enable the effective use of bistatic synthetic aperture sonar (SAS) concepts for the detection of buried objects in the mid-frequency regime of 2-20 kHz.

Multi-platform sonar concepts for buried target detection and classification

Buried target classification is of paramount importance in mine countermeasures (MCM) applications. The Generic Oceanographic Array Technology Sonar (GOATS) project approaches this fundamental problem by exploiting the assets of autonomous underwater vehicles (AUVs). Recent developments in unmanned vehicle technology have opened the door for new sonar design concepts that may be applied to create alternative target detection and classification methods in complex environments. Adaptive vehicle behaviors provide the potential for the sonar system to adjust its geometry for optimal performance. The vehicles can also access very shallow water environments, which allows for more complete multi-aspect views of targets of interest than are possible with remote towed array systems. In this paper, it is demonstrated that the multi-platform system can provide improved detection and classification capability, while at the same time reducing the computational requirements. Data from prior GOATS experiments (1998 and 2000) are used to illustrate the effectiveness of the multi-platform sonar concepts, and real-time simulations are used to show the planned implementation of these techniques on-board the AUVs for GOATS’02.

New sonar concepts and the sensor-related developments they require

Recently, exciting new approaches to the underwater vehicle acoustic sensing problem have been proposed which, if realized, could revolutionize how we approach underwater sonar. Two such schemes are the so-called ‘‘virtual sonar’’ which is formulated around Helmholtz integral processing and ‘‘wireless’’ systems which transfer sensor information through radiated rf signals. The full implementation of such new conceptual approaches, while presenting exciting possibilities, will nevertheless require a number of sensor-related technology advances. In this presentation, we will discuss those sensors and sensor-related developments which are desired or required in order to make practical such new sensor system concepts, and we will indicate the status of such developments. Finally, we will present several underwater applications from the perspective of exploiting these new sonar concepts. [Work supported by ONR.]

Bistatic synthetic aperture target detection and imaging with an AUV

The acoustic detection and classification of completely and partially buried objects in the multipath environment of the coastal ocean presents a major challenge to the mine countermeasures (MCM) community. However, the rapidly emerging autonomous underwater vehicle (AUV) technology provides the opportunity of exploring entirely new sonar concepts based on mono-, bi- or multi-static configurations. For example, the medium frequency regime (1-10 kHz) with its bottom penetration advantage may be explored using large synthetic apertures, where acoustic information is accumulated over a series of sonar pings. The performance of such approaches is highly dependent on accurate platform navigation and timing, which poses a significant challenge to AUV developers, particularly because the navigation procedures are themselves dependent on the complicated multipath acoustic environment. Data from the GOATS'98 experiment have been analyzed to investigate the feasibility of combining seabed scattering data from consecutive pings of a fixed parametric source to form a bistatic synthetic aperture for target localization and imaging with an AUV based receiving platform. The paper describes different levels of bistatic processing including both incoherent and coherent beamforming and very large aperture interferometric approaches, and the associated performance tradeoffs are discussed.

Bistatic target detection, interferometry, and imaging with an autonomous underwater vehicle platform

The acoustic detection and classification of completely and partially buried objects in the multipath environment of the coastal ocean presents a major challenge to the underwater acoustics community. However, the rapidly emerging autonomous underwater vehicle (AUV) technology provides the opportunity of exploring entirely new sonar concepts based on mono-, bi-, or multistatic configurations. For example, the medium frequency regime (1–10 kHz) with its bottom penetration advantage may be explored using large synthetic apertures, where acoustic information is accumulated over a series of sonar pings. The performance of such approaches is highly dependent on accurate platform navigation and timing, which poses a significant challenge to AUV developers, particularly because the navigation procedures are themselves dependent on the complicated multipath acoustic environment. Using experimental data from the GOATS’98 SACLANTCEN/MIT experiment, this paper describes an investigation into the feasibility of combining seabed scattering data from consecutive pings of a fixed parametric source to form a bistatic synthetic aperture for target localization and imaging with an AUV-based receiving platform. The paper describes different levels of bistatic processing including both incoherent and coherent beamforming and very large aperture interferometric approaches, and the associated performance trade-offs are discussed. [Work supported by ONR and SACLANT.]

Physics of 3-D scattering from rippled seabeds and buried targets in shallow water

A new wave theory model (OASES-3D) providing consistent modeling of seabed insonification, three-dimensional target scattering, and rough seabed reverberation has been used to investigate the spatial and temporal characteristics of the multistatic scattering and reverberation from rippled, shallow-water seabeds with buried targets. It is shown that the highly polarized spectral characteristics of ripple fields is associated with a reverberation environment which is highly sensitive to both the frequency and insonification aspect relative to the ripples. The theoretical study has suggested that significant gains in subcritical detection performance for buried objects can be achieved by band-limiting the processing to frequencies below typically 3–5 kHz. The results of 3-D target scattering and reverberation measurements performed by SACLANTCEN in shallow-water environments with rippled, sandy bottoms have confirmed this hypothesis. However, the experiments have consistently shown higher low-frequency reverberation than predicted by the model. Here, OASES-3D has been used to demonstrate that a very small amount of low-wave-number roughness components superimposed on the sand ripples can account for the observed reverberation. This in turn suggests that the use of new low-frequency sonar concepts for buried target detection and classification must be accompanied by adequate environmental assessment incorporating the long-wavelength (λ>25 cm) seabed roughness components. [Work supported in part by SACLANTCEN and ONR, Ocean Acoustics.]