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Oceanic Engineering, IEEE Journal of

Issue 1 • Date Jan. 1997

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Displaying Results 1 - 22 of 22
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  • X-band low-grazing-angle ocean backscatter obtained during LOGAN 1993

    Page(s): 18 - 26
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    This paper reviews X-band ocean microwave backscatter data from the LOGAN (LOw Grazing ANgle) experiment conducted on the Chesapeake Light Tower by the Naval Air Warfare Center. The data were collected under varied wind, sea, and swell conditions that provide some new insights into low-grazing-angle backscatter phenomena. Transient backscatter peaks called “sea spikes” have long been associated with deep-water breaking waves; however, they have yet to be fully reconciled with backscatter and hydrodynamic theories. New analysis techniques have been applied to the LOGAN data that take advantage of the unique characteristics of sea spikes and their dynamics. High-resolution Doppler spectra are organized relative to the space-time centroids of the sea-spike clusters and conditionally averaged by RCS strength. The mean Doppler variation of the strongest sea spikes then map the breaking-wave structure just as Doppler histories measured at moderate grazing angles map the dynamics of the dominant linear surface-wave components. While breaking waves are manifest to some degree in backscatter data at all grazing angles, a non-Bragg-scatter mechanism accentuates the crest scattering at low grazing angles. The phenomena potentially can be exploited for remote ocean sensing and imaging View full abstract»

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  • A voting-based approach for fast object recognition in underwater acoustic images

    Page(s): 57 - 65
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (196 KB)  

    This paper describes a voting-based approach for the fast automatic recognition of man-made objects and related attitude estimation in underwater acoustic images generated by forward-looking sonars or acoustic cameras. In general, the continuous analysis of sequences of images is a very heavy task for human operators and this is due to the poor quality of acoustic images. Hence, algorithms able to recognize an object on the basis of a priori knowledge of the model and to estimate its attitude with reference to a global coordinate system are very useful to facilitate underwater operations like object manipulation or vehicle navigation. The proposed method is capable of recognizing objects and estimating their two-dimensional attitude by using information coming from boundary segments and their angular relations. It is based on a simple voting approach directly applied to the edge discontinuities of underwater acoustic images, whose quality is usually affected by some undesired effects such as object blurring, speckle noise, and geometrical distortions degrading the edge detection. The voting approach is robust, with respect to these effects, so that good results are obtained even with images of very poor quality. The sequences of simulated and real acoustic images are presented in order to test the validity of the proposed method in terms of average estimation error and computational load View full abstract»

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  • Active impulsive echo discrimination in shallow water by mapping target physics-derived features to classifiers

    Page(s): 66 - 80
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (344 KB)  

    One of the most difficult challenges in shallow-water active sonar processing is false-alarm rate reduction via active classification. In impulsive-echo-range processing, an additional challenge is dealing with stochastic impulsive source variability. The goal of active classification is to remove as much clutter as possible while maintaining an acceptable detection performance. Clutter in this context refers to any non-target, threshold-crossing cluster event. In this paper, we present a clutter-reduction algorithm using an integrated pattern-recognition paradigm that spans a wide spectrum of signal and image processing-target physics, exploration of projection spaces, feature optimization, and mapping the decision architecture to the underlying good-feature distribution. This approach is analogous to a classify-before-detect strategy that utilizes multiple informations to arrive at the detection decision. After a thorough algorithm evaluation with real active sonar data, we achieved over an order of magnitude performance improvement in clutter reduction with our methodology over that of the baseline processing View full abstract»

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  • Imaging algorithms for a strip-map synthetic aperture sonar: minimizing the effects of aperture errors and aperture undersampling

    Page(s): 27 - 39
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    Imaging the sea floor using high-precision synthetic aperture sonar (SAS) techniques is at the stage where the efficiency and the robustness of the various imaging algorithms are of concern. There have been several block processing algorithms developed for relatively narrow-band-, narrow swath-, and narrow beamwidth synthetic aperture systems mainly for use by the synthetic aperture radar (SAR) community. These algorithms are summarized and their relevance to the slower speed of propagation, broad-band, broad swath-, and broad beam-width sonar equivalents are noted. Additional algorithms intended to ameliorate distortions injected by motion errors of the tow fish and medium turbulence are also assessed. One of the significant differences between the sonar and radar systems is that most synthetic aperture sonars travel faster than that required to meet the spatial sampling criterion and so the aperture is under- or insufficiently sampled. The digital spotlighting approach can be shown to reduce the grating-lobe images generated by this undersampling to a significant degree. The operational effectiveness of these various algorithms are shown on real data as collected by an ocean-going, boat-towed, rather than a rail or otherwise guided, sonar. What is important is that these algorithms in various combinations can ultimately produce near diffraction-limited imaging on real data. Typical results are shown when using the Kiwi-SAS to image point retro-reflectors (either as isolated targets or deployed in pairs) on a sea floor of bland silt. To date, no unclassified SAR or SAS uses the range or along-track spatial bandwidths employed by the Kiwi-SAS. The final SAS image resolution of 16 cm×5 cm is a considerably finer resolution than achieved by any SAR of equivalent carrier wavelength. The fine resolution is due to the correspondingly high spatial bandwidths covered by the system; that of range due to the chirp bandwidth coupled with the slow speed of sound in water and that of along-track due to the small real apertures employed. Access to this wide spatial bandwidth makes the applicability of normal SAR algorithms uncertain and we explore some of the trade-offs View full abstract»

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  • Practical use of video imagery in nearshore oceanographic field studies

    Page(s): 81 - 92
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    An approach was developed for using video imagery to quantify, in terms of both spatial and temporal dimensions, a number of naturally occurring (nearshore) physical processes. The complete method is presented, including the derivation of the geometrical relationships relating image and ground coordinates, principles to be considered when working with video imagery and the two-step strategy for calibration of the camera model. The techniques are founded on the principles of photogrammetry, account for difficulties inherent in the use of video signals, and have been adapted to allow for flexibility of use in field studies. Examples from field experiments indicate that this approach is both accurate and applicable under the conditions typically experienced when sampling in coastal regions. Several applications of the camera model are discussed, including the measurement of nearshore fluid processes, sand bar length scales, foreshore topography, and drifter motions. Although we have applied this method to the measurement of nearshore processes and morphologic features, these same techniques are transferable to studies in other geophysical settings View full abstract»

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  • DIET AWSUM: a fluctuation-based deconvolution technique for enhancing signal-to-noise ratio and resolution

    Page(s): 119 - 127
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (148 KB)  

    A technique for simultaneously improving resolution and signal-to-noise ratio gain, the directivity improved estimation technique (DIET) combined with the advanced WISPR summation (AWSUM), or DIET AWSUM, is introduced. The DIET AWSUM method combines a straightforward deconvolution technique with the highly fluctuation-sensitive AWSUM filter. DIET AWSUM results generated for ocean acoustic data are compared with estimates produced using the maximum entropy method (MEM), a well-tested standard. The DIET AWSUM method achieved superior results in both increased resolution and improved gain for stable signal identification chiefly because of its greater ability to discriminate between stable signals and fluctuating signals and noise View full abstract»

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  • Acoustic tomography at basin scales and clock errors

    Page(s): 143 - 150
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    A basin-scale acoustic tomography simulation is carried out for the northeast Pacific Ocean to determine the accuracy with which time must be kept at the sources when clocks at the receivers are accurate. A sequential Kalman filter is used to estimate sound-speed fluctuations and clock errors. Sound-speed fluctuations in the simulated ocean are estimated from an eddy-resolving hydrodynamic model of the Pacific forced by realistic wind fields at daily resolution from 1981-1993. The model output resembles features associated with El Nino and the Southern Oscillation, as well as many other features of the ocean's circulation. Using a Rossby-wave resolving acoustic array of four fixed sources and twenty drifting receivers, the authors find that the percentage of the modeled ocean's sound-speed variance accounted for with tomography is 92% at 400-km resolution, regardless of the accuracy of the clocks. Clocks which drift up to hundreds of seconds of error or more for a year do not degrade tomographic images of the model ocean. Tomographic reconstructions of the sound-speed field are insensitive to clock error primarily because of the wide variety of distances between the receivers from each source. Every receiver “sees” the same clock error from each source, regardless of section length, but the sound-speed fluctuations in the modeled ocean cannot yield travel times which lead to systematic changes in travel time that are independent of section length. The Kalman filter is thus able to map the sound-speed field accurately in the presence of large errors at the source's clocks View full abstract»

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  • Comments on “A low sidelobe technique for the direct measurement of scattering functions”

    Page(s): 189 - 190
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    The author comments on the work of Ricker & Gustafson (ibid., vol. 21, pp. 14-23, 1996) and raises the issue of signal design for UPF (uncertainty product scattering) applications and scattering function estimation in general. Ricker & Gustafson reply to the comments View full abstract»

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  • Detection of objects on the sea bottom using backscattering characteristics dependent on the observation point

    Page(s): 40 - 46
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    Sector-scanning sonar systems image the sea bottom to detect objects that can be distinguished from the background structure of the sea bottom. In current systems, images are displayed and discarded as new image data become available, In this paper, a method for improving sonar detection by utilizing all images in a sequence is investigated. The proposed method requires that sonar data are acquired with a sector-scanning sonar in a side-looking configuration. It is demonstrated that these data can be used to detect observation-point-dependent changes in sea-bottom backscattering characteristics. These changes provide additional cues for discrimination that can improve the detection of objects on the sea bottom. Results of applying the method to experimental data are presented View full abstract»

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  • Lossless compression of hydroacoustic image data

    Page(s): 93 - 101
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    Despite rapid progress in improving mass-storage density and digital communication system performance, compression of hydroacoustic image data is still significant in many engineering and research areas since it can overcome data storage and transmission bandwidth limitations. In this paper, we present a novel and effective approach for lossless compression of hydroacoustic image data which consists of two stages. The first stage reduces the information redundancy. We propose several new techniques to remove redundancy between data samples, data blocks, and data frames. The second stage uses a newly developed cascade coding scheme. This simple scheme can achieve an efficiency of 97%. A decomposition algorithm is presented for finding the optimal cascade coding parameters. The algorithm decomposes a multivariable optimization problem into a series of one-variable optimizations. Our two-stage algorithm offers a compression ratio of 2-3 and provides an exact recovery of the original data. Because of its simplicity, the algorithm can be incorporated into a variety of echo sounder systems. The compression algorithms can also be implemented using low-level assembly language to meet the requirements of real-time applications View full abstract»

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  • Wavelet analysis of satellite images for coastal watch

    Page(s): 9 - 17
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    The two-dimensional wavelet transform is a very efficient bandpass filter, which can be used to separate various scales of processes and show their relative phase/location. In this paper, algorithms and techniques for automated detection and tracking of mesoscale features from satellite imagery employing wavelet analysis are developed. The wavelet transform has been applied to satellite images, such as those from synthetic aperture radar (SAR), advanced very-high-resolution radiometer (AVHRR), and coastal zone color scanner (CZCS) for feature extraction. The evolution of mesoscale features such as oil slicks, fronts, eddies, and ship wakes can be tracked by the wavelet analysis using satellite data from repeating paths. Several examples of the wavelet analysis applied to various satellite images demonstrate the feasibility of this technique for coastal monitoring View full abstract»

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  • Model-based enhancement of internal wave images

    Page(s): 1 - 8
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    The measurement of internal-wave signatures using synthetic aperture radar (SAR) or real aperture radar (RAR) techniques is an emerging technology that offers a viable means of locating and tracking surface ship wakes by their unique signatures. Under the assumption that the image measured by the radar is dominated by the underlying dynamics of the internal wave, we develop model-based techniques for enhancement based on a recently developed generic dispersive-wave processor. Using images synthesized by a sophisticated propagation model, it is shown that the processor not only offers a unique approach compared to the more traditional image-processing techniques that do not incorporate the propagation model, but is also capable of providing reasonable enhancement of the noisy measurements View full abstract»

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  • The AWSUM filter: a 20-dB gain fluctuation-based processor

    Page(s): 110 - 118
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    The advanced WISPR summation (AWSUM) filter, a natural extension of the WISPR filter for higher filter order numbers, is presented and its performance is compared to the performance of the WISPR filter and the conventional summation processor. It is shown that the AWSUM filter achieves substantial gains in various measures of processor performance above those of the other two processors in spatial and spectral resolution, minimum detectable level (MDL), clutter reduction, and signal-to-noise ratio (SNR) gain. The important processing parameters are shown to be the percentage of overlap of the voltage time series and the number of FFT's averaged. SNR gains in excess of 20 dB were shown to be achievable for low-fluctuation amplitude tonals using measured data View full abstract»

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  • Design of an integrated shallow water wave experiment

    Page(s): 184 - 188
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    The experimental design and instrumentation for an integrated shallow-water surface gravity wave experiment is discussed. The experiment required the measurement of the water surface elevation, meteorological parameters, and directional spectra at a number of locations on a shallow lake. In addition, to acquire data under a wide range of conditions, an experimental period of three years was required. A system of telephone and radio modem links were installed to enable real-time monitoring of instrument performance at eight separate measurement locations on the lake. This system also enabled logging sessions to be optimized to ensure the maximum possible data return from this extended experiment View full abstract»

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  • Remote sea current sensing using HF radar: an autoregressive approach

    Page(s): 151 - 155
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    We present and demonstrate a new method for accurately determining the radial component of sea current, using information derived from HF radar returns. The method uses autoregressive modeling and constrains the estimated frequency separation of the first-order Bragg lines to be its required theoretical value of 2√(gf/πc), where f is the radar frequency View full abstract»

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  • The structure of ocean-surface currents measured by Doppler radar

    Page(s): 156 - 167
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    The spatial structure of surface currents collected using a Doppler Radar system off the Florida Keys has been investigated. Surface current vectors were acquired within a rectilinear grid containing 700 nodes, where each node was spaced 1 km apart. Principal component analyses indicate that at least 63% of the variance of the surface currents at the 700 nodal locations can be accounted for by only three principal components. The principal modes of circulation during two separate experiments were extracted and found to be basically the same, although the first two modes were reversed between the two experiments. Coherence spectra were determined for nodal spacings between 1.76 km and 10.48 km, and the results of these analyses indicate that over most of the experimental area, approximately 60% of the variance is coherent over separations of 10.48 km. Application of a synoptic time-series model indicates that accurate prediction of the mean currents over each of the 65-h subrecord lengths was the dominant factor in controlling model performance, and that on the order of 10% error could be expected in using the time-series model to predict the low-frequency fluctuations. This indicates that time-series modeling of surface currents may be feasible and useful in estimating the long-term mixing characteristics of contaminants transported in the surface layer View full abstract»

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  • Application of an improved self-starter to geoacoustic inversion

    Page(s): 102 - 109
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    The self-starter is improved using the operator of the split-step Pade solution. In addition to providing greater stability and being applicable closer to the source, the improved self-starter is an efficient forward model for geoacoustic inversion. It is necessary to solve only O(10) tridiagonal systems of equations to obtain the acoustic field on a vertical array located O(10) wavelengths from a source. This experimental configuration is effective for geoacoustic inverse problems involving unknown parameters deep in the ocean bottom. For problems involving depth-dependent acoustic parameters, the improved self-starter can be used to solve nonlinear inverse problems involving O(10) unknown sediment parameters in less than a minute on the current generation of workstations View full abstract»

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  • Mapping climatic temperature changes in the ocean with acoustic tomography: navigational requirements

    Page(s): 128 - 142
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    In eddy-resolving hydrodynamic models, first-mode baroclinic Rossby waves linked to El Nino/Southern Oscillation are the dominant features which change basin-wide temperatures below the seasonal thermocline in the northeast Pacific at periods less than a decade. Simulations are carried out in which Rossby waves are mapped using acoustic tomography. Based on the model which propagated these waves, a Kalman filter is used to map temperature signals for a year. The modeled data are taken from a dense network of acoustic tomography sections. At 300-m depth, where the temperature perturbations associated with Rossby waves are about ±1°C, 80% to 90% of the model variance is accounted for with tomographic estimates. The corresponding standard deviations of the estimates are less than 0.1°C at 400-km resolution. About 80% of the model variance is accounted for with tomography when the navigational errors of the sources and receivers are as poor as one kilometer. Consequently, it may be unnecessary to accurately navigate actual tomographic instruments to map climate change. Modeling results are insensitive to: 1) a reduction in data due to a significant number of instruments which fail; 2) whether the instruments are mobile or fixed; 3) the detailed trajectories of mobile receivers; 4) the shape of the a priori spectrum of ocean fluctuations; 5) the corrections to the acoustic travel-time biases; and 6) the errors in the sound-speed algorithm. In basin-scale arrays, the modeled variance of acoustic travel time depends on the horizontal wavenumber of temperature as k-5.5. Because sound has little sensitivity to small wavelengths, modeled Rossby waves can be mapped in a day from a few sources and of order ten receivers. The results only depend on the model having large scales in space and time View full abstract»

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  • Automatic interpretation of sonar image sequences using temporal feature measures

    Page(s): 47 - 56
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    This paper reports the development of a system for the automated interpretation of sector scan sonar data. It proposes the use of a new combination of feature measures derived from sequences of sonar scans to characterize the behaviour of targets' returns over time. Previous research used grey-scale and shape descriptors derived from single sonar scans. However, problems were experienced with targets whose return varied significantly over time (such as divers, UUV's, and ships' wakes). Hence a new set of temporal feature measures has been developed by combining existing one-dimensional temporal measures and two-dimensional object descriptors. These new features provide a quantitative description of the behaviour of a target's two-dimensional returns over a sequence of sonar scans. Experiments with a limited but real data set have shown that classification accuracy can be significantly improved by the use of these new features. The use of “static” feature measures (derived from a single scan) was observed to give classification errors of between 7% and 10% when they were applied to the data set. In contrast, the use of temporal measures reduced this error rate to 1% or 2% and in some cases reduced it to zero View full abstract»

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  • Perceptually optimum gray-scale transformation of multibin time history data

    Page(s): 179 - 183
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    Gray-scale displays of a time history of data in multiple channels are encountered in various applications. The signal tracks in a noisy display often have to be detected by a human operator. To exploit the pattern-recognition capability of the human “receiver”, the perceptual factors influencing visual discrimination needs to be incorporated into the gray-scale transformation scheme of the data. To this end, an objective function, viz. the total perceived entropy, is proposed in this paper for optimum gray-scale quantizer design. Two types of quantizers, one designed with unconstrained maximization of the objective function and another which maximizes the objective function with the constraint enforced by Weber's law, are proposed. It is demonstrated, using simulated spectrogram data, that these quantizers provide better signal track discrimination than Lloyd-Max quantization View full abstract»

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  • A closed-form solution to bearings-only target motion analysis

    Page(s): 168 - 178
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    Bearings-only target motion analysis is a nonlinear state estimation problem in which the noise corrupted angle of arrival measurements of an emitted signal are used to obtain estimates of the source's range, bearing, course, and speed. The estimation process is complicated by unusual observability properties that render the quality of the estimate highly dependent on both the measurement noise levels and the source-observer geometry. Solutions that use recursive Kalman filtering approach or batch-style algorithms have been reported. The nonlinear batch style estimators for this process require iterative solution methods and under certain scenarios can be sensitive to initial conditions. Pseudolinear solutions that alleviate some of the difficulties with the iterative batch algorithms have been proposed. Although early versions of the pseudolinear filter suffered from biased estimates, subsequent improvements appear to have reduced the bias problem. This paper discusses a new pseudolinear solution based on the observable parameters from individual data segments defined by periods of constant observer velocity (termed “legs”). This solution is a true closed-form solution to the bearings-only target motion analysis problem. Although theoretically interesting, the technique does suffer under conditions of poor observability. A practical pseudolinear estimate, that does not suffer from the same observability problems, is developed and related to the first solution. Algorithm performance results, obtained from computer simulation, are presented. For the scenarios examined, the technique provides good state estimates under conditions of high observability. As observability conditions deteriorate, the solution does develop biases. However, it may still be useful for initializing an iterative nonlinear batch-style estimation algorithm View full abstract»

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Aims & Scope

The IEEE Journal of Oceanic Engineering (ISSN 0364-9059) is published quarterly by the IEEE Oceanic Engineering Society (IEEE OES). The scope of the Journal is the field of interest of the IEEE OES, which encompasses all aspects of science, engineering, and technology that address research, development, and operations pertaining to all bodies of water. This includes the creation of new capabilities and technologies from concept design through prototypes, testing, and operational systems to sense, explore, understand, develop, use, and responsibly manage natural resources.

Full Aims & Scope

Meet Our Editors

Editor-in-Chief
N. Ross Chapman
School of Earth & Ocean Sciences
University of Victoria
3800 Finnerty Road
Victoria, BC V8P 5C2 Canada
chapman@uvic.ca