Chris Capus

Short-time fractional Fourier methods for the time-frequency representation of chirp signals

The fractional Fourier transform (FrFT) provides a valuable tool for the analysis of linear chirp signals. This paper develops two short-time FrFT variants which are suited to the analysis of multicomponent and nonlinear chirp signals. Outputs have similar properties to the short-time Fourier transform (STFT) but show improved time-frequency resolution. The FrFT is a parameterized transform with parameter, a, related to chirp rate. The two short-time implementations differ in how the value of a is chosen. In the first, a global optimization procedure selects one value of a with reference to the entire signal. In the second, a values are selected independently for each windowed section. Comparative variance measures based on the Gaussian function are given and are shown to be consistent with the uncertainty principle in fractional domains. For appropriately chosen FrFT orders, the derived fractional domain uncertainty relationship is minimized for Gaussian windowed linear chirp signals. The two short-time FrFT algorithms have complementary strengths demonstrated by time-frequency representations for a multicomponent bat chirp, a highly nonlinear quadratic chirp, and an output pulse from a finite-difference sonar model with dispersive change. These representations illustrate the improvements obtained in using FrFT based algorithms compared to the STFT.

The fusion of large scale classified side-scan sonar image mosaics

This paper presents a unified framework for the creation of classified maps of the seafloor from sonar imagery. Significant challenges in photometric correction, classification, navigation and registration, and image fusion are addressed. The techniques described are directly applicable to a range of remote sensing problems. Recent advances in side-scan data correction are incorporated to compensate for the sonar beam pattern and motion of the acquisition platform. The corrected images are segmented using pixel-based textural features and standard classifiers. In parallel, the navigation of the sonar device is processed using Kalman filtering techniques. A simultaneous localization and mapping framework is adopted to improve the navigation accuracy and produce georeferenced mosaics of the segmented side-scan data. These are fused within a Markovian framework and two fusion models are presented. The first uses a voting scheme regularized by an isotropic Markov random field and is applicable when the reliability of each information source is unknown. The Markov model is also used to inpaint regions where no final classification decision can be reached using pixel level fusion. The second model formally introduces the reliability of each information source into a probabilistic model. Evaluation of the two models using both synthetic images and real data from a large scale survey shows significant quantitative and qualitative improvement using the fusion approach.

Bio-inspired wideband sonar signals based on observations of the bottlenose dolphin (Tursiops truncatus)

This paper uses advanced time-frequency signal analysis techniques to generate new models for bio-inspired sonar signals. The inspiration comes from the analysis of bottlenose dolphin clicks. These pulses are very short duration, between 50 and 80 micros, but for certain examples we can delineate a double down-chirp structure using fractional Fourier methods. The majority of clicks have energy distributed between two main frequency bands with the higher frequencies delayed in time by 5-20 micros. Signal syntheses using a multiple chirp model based on these observations are able to reproduce much of the spectral variation seen in earlier studies on natural dolphin echolocation pulses. Six synthetic signals are generated and used to drive the dolphin based sonar (DBS) developed through the Biosonar Program office at the SPAWAR Systems Center, San Diego, CA. Analyses of the detailed echo structure for these pulses ensonifying two solid copper spherical targets indicate differences in discriminatory potential between the signals. It is suggested that target discrimination could be improved through the transmission of a signal packet in which the chirp structure is varied between pulses. Evidence that dolphins may use such a strategy themselves comes from observations of variations in the transmissions of dolphins carrying out target detection and identification tasks.

Analysis and classification of broadband echoes using bio-inspired dolphin pulses

To date most sonars use narrow band pulses and often only the echo envelope is used for object detection and classification. This paper considers the advantages afforded by bio-inspired sonar for object identification and classification through the analysis and the understanding of the broadband echo structure. Using the biomimetic dolphin based sonar system in conjunction with bio-inspired pulses developed from observations of bottlenose dolphins performing object identification tasks, results are presented from experiments carried out in a wave tank and harbor. In these experiments responses of various targets to two different bio-inspired signals are measured and analyzed. The differences in response demonstrate the strong dependency between signal design and echo interpretation. In the simulations and empirical data, the resonance phenomena of these targets cause strong notches and peaks in the echo spectra. With precision in the localization of these peaks and dips of around 1 kHz, the locations are very stable for broadside insonification of the targets and they can be used as features for classification. This leads to the proposal of a broadband classifier which operates by extracting the notch positions in the target echo spectra.

High-resolution sonars: what resolution do we need for target recognition?

Target recognition in sonar imagery has long been an active research area in the maritime domain, especially in the mine-counter measure context. Recently it has received even more attention as new sensors with increased resolution have been developed; new threats to critical maritime assets and a new paradigm for target recognition based on autonomous platforms have emerged. With the recent introduction of Synthetic Aperture Sonar systems and high-frequency sonars, sonar resolution has dramatically increased and noise levels decreased. Sonar images are distance images but at high resolution they tend to appear visually as optical images. Traditionally algorithms have been developed specifically for imaging sonars because of their limited resolution and high noise levels. With high-resolution sonars, algorithms developed in the image processing field for natural images become applicable. However, the lack of large datasets has hampered the development of such algorithms. Here we present a fast and realistic sonar simulator enabling development and evaluation of such algorithms.We develop a classifier and then analyse its performances using our simulated synthetic sonar images. Finally, we discuss sensor resolution requirements to achieve effective classification of various targets and demonstrate that with high resolution sonars target highlight analysis is the key for target recognition.

Real-time sidescan simulator and applications

New generations of sonars appeared in the last decade. The major interest in SAS systems and high frequency sonars is in the improvement of the sonar resolution and the reduction of noise level. Sonar images are distance-images but at high resolution they tends to appear visually as optical images. Usually the algorithms developed for sidescan were specific for sonar images due to the poor resolution essentially. With high resolution sonars, algorithms developed in the image processing field for natural images became applicable. In this paper we present a real-time and realistic sidescan simulator, and test image-based classification algorithms (such as PCA and eigenface algorithms) with synthetic images in order to characterize the precision necessary for these image-based algorithm to work.

The application of bioinspired sonar to cable tracking on the seafloor

Marine mammals have developed highly effective sonar systems for detecting, identifying, and following underwater objects. In this paper we demonstrate that bio-inspired wideband sonar offers great capability for tracking cables on the seafloor. The analysis of biological signals, including dolphin clicks, suggests two approaches. The first is to use a wideband signal, integrating the response of an object over many frequencies. For simple forms, this is known to give access to shape and material information. The second idea is to use intelligent signals designed to elicit information from specific target types. In this paper results are presented from sets of experiments using bio-inspired wideband sonar. The aim of these experiments is to determine the feasibility of tracking small diameter marine communications cables using the wideband responses. Echoes from four different cable types are analysed using a variety of signals. Experiments using bio-inspired pulses illustrate the benefits of using this type of wideband signal for detection and recognition. A strong correspondence between theoretical and experimental echoes is shown.

Detection of buried and partially buried objects using a bio-inspired wideband sonar

The Ocean Systems Laboratory is developing bio-inspired wideband acoustic sensing methods for underwater target detection and tracking. The wideband sensors themselves are based on bottlenose dolphin sonar, covering a frequency band from around 30kHz to 150kHz and having a frequency dependent beamwidth considerably larger than conventional imaging sonars. The entire system is relatively compact and is suitable for mounting on a variety of platforms including small scale autonomous underwater vehicles (AUVs). In this paper we overview recent efforts applying the sonar to the detection and tracking of various underwater cables, and to the detection and classification of these cables in shallow burial, based on their midwater responses.

Target detection using statistical MIMO

MIMO systems have raised a lot of interests in the recent years especially in the radar community. For MIMO systems with widely spaced antennas for example it has been shown that channel matrices are decorrelated from one another. The view diversity of an illuminated target is then increased. And as a consequence the detection probability of statistical MIMO systems increases thanks to this gain diversity. In this paper we present the finite scattering point model introduced by Haimovich and demonstrate the equivalence between the MIMO scattering problem using the finite scattering point model and the random walk problem. Studying the convergence of the central limit theorem applied to this problem, we demonstrate that it is possible to estimate the scattering points density for a low number of scatterers in the resolution cell. Finally we show that statistical MIMO system suppresses the interferences between scatterers and maximises the target response.

Wideband sonar system for autonomous surveys using REMUS

Many new roles are being proposed for Autonomous Underwater Vehicles (AUVs) to carry out hazardous tasks in harsh or remote locations and to free up valuable resources required for manned missions. These roles include marine environmental survey, target detection and classification and tracking of underwater pipes and cables. Suitable sensing and processing packages must be provided and building on recent wideband sonar research, the Ocean Systems Laboratory (OSL) is putting together a wideband system for deployment on board a REMUS AUV. The prototype sensor package comprises paired projectors and receivers mounted in a side‐looking arrangement to provide complementary information to the standard REMUS sidescan modules. The sensor bandwidths cover a range from 30‐130kHz similar to those used by the bottlenose dolphin. Data is gathered autonomously with a dedicated AMD Geode based PC104+ PC controlling a 4‐channel 800kHz simultaneous sampling data acquisition module. Acquisition is triggered to coincide wi...