DRDC Atlantic Research Articles

Efficient Identification of Naval High-Speed Craft Shock Mitigation Seat Modal Parameters From Drop-Test Data

Naval high-speed craft (HSC) operating in moderate to high seas experiences high-g and repeated shock loading at the seat–deck interface. These conditions are known to pose a serious potential for injury to the occupants. While various shock-mitigating seats are commercially available; their designs are in many cases quite different, and quantifying their shock attenuation characteristics can be challenging. The need for a standard test platform and experimental analysis methodology to investigate HSC seat effectiveness is a major objective of research being conducted by Carleton University's Applied Dynamics Laboratory (ADL) in partnership with Defence Research and Development Canada-Atlantic (DRDC Atlantic). A drop tower was designed and manufactured for testing HSC seats in order to characterize their shock-mitigating effectiveness by simulating the severe conditions of a slam impact at sea. Further, in order to identify seat dynamic parameters from drop-test data, the eigensystem realization algorithm (ERA), a modal-analysis-based system identification method, was applied to efficiently extract the modal parameters. The technique was shown to successfully extract the damping ratio as well as the damped and undamped natural frequencies of the seats from impact test data. The evaluated dynamic properties of the seats can subsequently inform decisions related to the design and/or procurement of commercially available seats.

Improved Active Sonar Performance Using Costas Waveforms

Costas waveforms are a class of waveforms having the form of frequency-hopped pulse trains. When used as a transmit waveform in an active sonar system they may provide superior performance to conventional waveforms such as continuous-wave (CW) and frequency-modulated (FM) pulses, as each Costas waveform provides both range and speed information for a target echo. Matched-filtering identifies individual Costas waveforms from among a set of Costas waveforms that are received. This characteristic allows for more frequent ensonification of a water mass for targets at moderate ranges, giving a higher rate of return echoes for improved target tracking. Alternatively, it offers the potential for overlapping bandwidths and waveform types to be used in a multisonar environment. In this work, the Costas wideband ambiguity, cross- and autocorrelation functions are derived. The range and speed resolutions of Costas waveforms are compared to CW and FM pulse resolutions both via a broadband pulse propagation model and using data obtained during the Defence Research And Development Canada-Atlantic (DRDC Atlantic) Towed Integrated Active-Passive Sonar (TIAPS) deep sea trials in September 2003. Costas waveforms are shown to provide simultaneous range and speed resolution in deep- and shallow-water environments, and Costas waveforms are shown to be individually identifiable at the receiver. The performance of Costas waveforms in reverberation is also briefly examined, and found to be similar to that of FM pulses.

Extracting bottom information from towed-array reverberation data: Part I: Measurement methodology

Long-range reverberation data were gathered on towed arrays during the NATO Rapid Response exercises (1996–1998) and the Boundary Characterization experiments (2000–2004). The primary tools were various SACLANTCEN/NURC arrays towed by the NATO Research Vessel Alliance. Most of the sites were in the Mediterranean, in both deep and shallow water, but there were sites on the east and west Atlantic continental shelves. This paper describes the experimental measurements, data processing, and quick-look analyses that were used in our REA (rapid environmental assessment) approach. Polar plots of the beam time series, overlaid on the bathymetry of the region, were extremely useful in providing an immediate analysis of the area. The reverberation highlights are usually correlated with bottom bathymetry and known scattering features. However, it was found that the polar plots often showed scattering features not on the charts, but which could often be identified by detailed follow-on bottom surveys. Thus the polar-plot technique is a valuable survey tool. Examples from a number of the sites are illustrated. The reverberation beam time series data, from areas with few scattering features, were used with model predictions to extract estimates of bottom loss and backscattering strength. An example is given here, but details of the modelling procedure and parameter extraction are left to a companion paper. [Work supported in part by the US Office of Naval Research, DRDC Atlantic and NATO SACLANTCEN/NURC].

Extracting bottom information from towed-array reverberation data. Part II: Extraction procedure and modelling methodology

Reverberation data were gathered on the NATO Research Vessel Alliance, using the SACLANTCEN/NURC towed arrays, during the Rapid Response Exercises (1996–1998) and the Boundary Characterization experiments (2000–2004). Most of the sites were in the Mediterranean, but there were shallow-water sites on the east and west Atlantic continental shelves. A companion paper discusses the data gathering, polar-plot displays, and at-sea rapid environmental assessment of the reverberation and scattering features. This paper describes our efforts to use reverberation data from beam time series without prominent scattering features to extract estimates of bottom loss and scattering strength. The emphasis of this paper is on the procedure for extraction of the geoacoustic properties of the seafloor and on the modelling methodology used for the model-data comparisons. Towed-array beam patterns are included in the modelling, so polar plots of model-data differences can be used to provide a crude scattering map of the area. A manual inversion procedure was used at sea during the experiments for extracting bottom properties; the results compared reasonably well with post-trial measurements and estimates using an automated version of the extraction procedure. [Work supported in part by the US Office of Naval Research, DRDC Atlantic, and NATO SACLANTCEN/NURC].

Tools and techniques to measure environmental parameters that improve mid-frequency sonar performance predictions

The performance of mid-frequency active sonar systems in littoral waters depends upon several environmental factors and their variability in space and time. These factors include seabed scattering strength, bottom loss, and the water column sound speed profiles. DRDC Atlantic has developed experimental apparatus and techniques to measure these environmental parameters in order to enhance the accuracy of sonar performance estimates in the mid-frequency band, 1 to 10 kHz. Experiments have been conducted during sea-trials on the Scotian Shelf and Strataform area on the New Jersey Shelf. In some instances, these experiments have been in conjunction with international research partners to provide an inter-comparison between experimental techniques and/or a comparison of rapid environmental assessment techniques with more traditional, and time-consuming, methods. In this talk, we will demonstrate that predictions of sonar system performance improve when the environmental parameters obtained with these techniques are used, rather than parameters from archival sources.

Control Task Analysis for Applied Settings

As part of ongoing research at DRDC Atlantic, Cognitive Work Analysis (CWA) is being used to design enhanced capabilities for naval command and control systems. Typically, the first two steps (work domain analysis (WDA) and control task analysis (CTA)) are conducted, followed by the identification of design seeds for the development of new system concepts. To date, there has been little guidance in the literature regarding how to perform both WDA and CTA, although many practitioners have performed WDA. Fewer have performed CTA, so the interested analyst has few examples to draw from, in particular for studies in industry. The work discussed in this paper involved the use of CTA to analyse scenario-based data in order to characterize expert performance in control tasks. By understanding expert performance according to a decision ladder (DL) model, it is possible to accurately support the required level of performance in complex, dynamic, open domains. This paper presents an overview of the work and includes some examples of the analyses.

Acoustic backscatter measurements from littoral seabeds at shallow grazing angles at 4 and 8 kHz

Direct measurement of acoustic scattering from the seabed at shallow grazing angles and low kilohertz frequencies presents a considerable challenge in littoral waters. Specifically, returns from the air-water interface typically contaminate the signals of interest. To address this issue, DRDC Atlantic has developed a sea-going research system for measuring acoustic scatter from the seabed in shallow-water environs. The system, known as the wideband sonar (WBS), consists of a parametric array transmitter and a superdirective receiver. In this paper, backscatter measurements obtained with the WBS at two sandy, shallow-water sites off North America's Atlantic coast are presented. Data were collected at 4 and 8 kHz at grazing angles from 3 degrees-15 degrees. The backscattering strength is similar at both sites and, below about 8 degrees, it appears to be independent of frequency within the statistical accuracy of the data. The measurements show reasonable agreement with model estimates of backscatter from sandy sediments. A small data set was collected at one of the sites to examine the feasibility of using the WBS to measure the azimuthal variability of acoustic scatter. The data set--although limited--indicates that the parametric array's narrow beamwidth makes the system well-suited to this task.

Twenty years of barrel-stave flextensional transducer technology in Canada

The barrel-stave flextensional transducer, a compact underwater sound source, was conceived at DRDC Atlantic in 1986 [G. W. McMahon and D. F. Jones, U.S. Patent No. 4,922,470 (1 May 1990); Canadian Patent No. 1,285,646 (2 July 1991)]. Over the years, five barrel-stave designs belonging to three flextensional classes were built and tested at DRDC Atlantic. Three Class I transducers with operating frequencies ranging from 800 to 1600 Hz were integrated into submarine communications buoys, low frequency active horizontal projector arrays, and a broadband sonar towbody. A high-power Class II and broadband (1–7 kHz) Class III transducer were deployed under the ice in the Lincoln Sea for research related to rapidly deployable surveillance systems. These barrel-stave flextensional transducers have also supported a variety of marine mammal studies including vocal mimicry in long-finned pilot whales, coda dialects in sperm whales, and the R&D of acoustic detection and tracking systems for endangered northern right whales. In August 2004 a barrel-stave transducer was used to lure a trapped juvenile humpback whale to the sluice gates of a tidal generating station on the Annapolis River in Nova Scotia by transmitting humpback whale calls underwater. The acoustic performance parameters for all 5 transducers will be presented.

Broadband source localization using matched correlation processing

For many years, model-based signal processing algorithms using Matched Field Processing (MFP) techniques have been analyzed with the goal of improving the capability of passive sonar systems for localizing quiet underwater sources. Recently, researchers at DRDC Atlantic have been investigating Matched Correlation Processing (MCP) as a faster alternative to MFP. In this method, the cross-correlations for a source as measured with a pair of hydrophones in a horizontal array are matched with those generated with a correlation model for many candidate ranges and depths along a candidate bearing. These matches are carried out with a number of hydrophone pairs to form many ambiguity surfaces. The maximum on the average of these surfaces is assumed to yield the best estimate of the source position. By carrying out this procedure over a number of candidate bearings, a full 3-D search for the source location is achieved. Since 2002, a number of localization trials have been carried out east of Nova Scotia, Canada. During those trials, an array was deployed on the sea floor and used to collect acoustic signals from various broadband sources. In this paper, we describe the broadband MCP localization technique and show some localization results from those trials.

Azimuthal variability of acoustic backscatter from 3 to 8 kHz on the New Jersey continental shelf

The acoustic backscattering strength of the seabed has been demonstrated to be one of the key inputs required in sonar performance prediction (SPP) models. Extending range independent SPP models to full 3-dimensional—or even the simpler N×2-dimensional—models requires measurements of the azimuthal variability of the backscattering strength. DRDC Atlantic’s Wide Band Sonar (WBS) system, which consists of a parametric transmitter and a superdirective receiver, is ideally suited to make these measurements. In the spring of 2003 the system was used to measure the azimuthal variability of acoustic backscatter at a site referred to as the New Jersey Strataform. Data were collected at 3, 5, and 8 kHz at grazing angles spanning 10–40. Interpretation of the data set is enhanced with swath bathymetry measurements made during the experiment as well as 3-D seismic profiles made by other researchers [C. S. Duncan, J. A. Goff, J. A. Austin, Jr., and C. S. Fulthorpe, Marine Geology 170, 395–421 (2000)]. In this paper the experimental geometry is described and the backscatter measurements are presented.

Measurements of acoustic backscatter at shallow grazing angles at low kHz frequencies

The acoustic backscattering strength of the seabed has been demonstrated to be one of the key inputs required in sonar performance prediction models. Yet, direct measurement of acoustic scattering from the seabed at shallow grazing angles presents a considerable challenge in littoral waters. The DRDC Atlantic (formerly DREA) Wide Band Sonar system, which consists of a parametric transmitter and a superdirective receiver, is ideally suited to make this measurement. The system was used to measure backscatter as a function of grazing angle and azimuth on the Scotian Shelf off the coast of Nova Scotia and on the Strataform site off the coast of New Jersey. Interpretation of the data set is enhanced with swath bathymetry measurements made at one of the experimental sites. In this paper the experimental geometry is described and the backscatter measurements are presented and discussed in light of the swath bathymetry results.

Geoacoustic characterization of seabed scattering experiment locations

Measurements of acoustic forward and backscattering have been made by DRDC Atlantic during collaborative sea-trials on the Scotian Shelf off the coast of Nova Scotia and on the Strataform site off the coast of New Jersey. In this paper, the geoacoustic properties and roughness parameters that are necessary to interpret and model the scattering measurements are presented. They have been determined using complementary in situ and acoustic techniques. The in situ measurements have been made using grab samples and a free fall cone penetrometer that has been fitted with a resistivity module. The probe provides two independent means of calculating the undrained shear strength, an empirical sediment classification and sediment bulk density. The acoustic measurements include inversions for geoacoustic parameters using the WARBLE [Holland and Osler, J. Acoust. Soc. Am. (2000)] and normal incidence sediment classification [Hines and Heald, Proc. Inst. Acoust. (2001)] techniques. At the scattering experimental locations, these measurements have been combined with surveys using commercial equipment: sidescan sonar, multibeam bathymetry, and subbottom profilers to characterize the seabed.