Sérgio Cunha

An Autonomous Boat Based Synthetic Aperture Sonar

This paper describes a Synthetic Aperture Sonar (SAS) system being developed at the University of Porto to be used in a small autonomous boat for the survey of shallow water environments, such as rivers, deltas, estuaries and dams. Its purpose is to obtain high resolution echo reflectivity maps through synthetic aperture techniques, taking advantage of the high precision navigation system of the boat. In the future the production of bottom tomography maps is also considered through the use of interferometric imaging techniques.

Shallow water height mapping with interferometric synthetic aperture sonar

Height mapping of shallow water areas is an important task for many commercial and scientific applications like river navigability, infrastructure maintenance or natural resource monitoring. The use of an autonomous boat presents several advantages that ease the use of synthetic aperture images to create three-dimensional topographic maps through interferometric techniques. Sample data obtained during test trials illustrate how synthetic aperture can be used to generate imagery and bathymetry data.

Synthetic Aperture Techniques for Sonar Systems

about this segment is still very far away from that of the land segment. Synthetic aperture is a technique that enables high resolution through the coherent processing of consecutive displaced echo data. Instead of using one static large array of transducers, it uses the along-track displacement of the sensors to synthesize a large virtual array. The resolution thus obtained is in the order of the transducer size and, most importantly, independent of the range between sensor and target. While a modern high frequency real-aperture sonar system can have a beam width below 1o, this translates into a resolution of half a meter at a range of just 25m. A synthetic aperture system using the same transducer can obtain a resolution of abou t 5cm across the whole range. Moreover the transducers used for synthetic aperture can be much simpler and so a good deal less expensive. Because there is no need to have a small real aperture, the frequency employed can be considerably lower, which enables longer reach due to the better propagation of lower frequencies in water. This potential resolution increase comes at th e cost of algorithm complexity in the image formation. The sonar must also describe a move ment with tight tolerances with respect to deviations from known velocity and path. Also, the platform maximum velocity is a function of the pulse repetition rate and sensor size. This limit relates to the resolution of the image that if not respected will lead to aliasing. The most used platform for synthetic aperture sonar is the tow-fish. Good designs enable smooth motion, but the inability to use satellite navigation technology leads to expensive solutions that integrate high grade inertial navigation units and data extracted from the sonar array itself. This only works for arrays with a high count of elements that operate at the nominal or above the nominal pulse repetition frequency. The sonar can also be mounted on the hull of a ship, providing access to high precision GPS navigation that can be integrated with data from moderate cost inertial systems to further refine the navigation solution. Nevertheless a ship is seldom easy to manoeuvre and presents considerable operation and maintenance costs of the sonar system itself. An autonomous boat arises as an interesting solution for these problems. It can be used as standalone or with the support of a ship. It enables the use of GPS and inertial navigation units efficiently. Moreover, its path

Sub-band processing of synthetic aperture sonar data

High frequency synthetic aperture sonar systems require demanding tolerances in motion errors and medium phase stability. This article proposes a new method that mitigates the problems associated with small wavelength related errors. By dividing the received signal bandwidths in to several smaller ones and conjugate complex multiplying them, a new signal is obtained with longer effective wavelength, thus reducing the impact of motion errors and medium phase fluctuations.

An algebraic approach to synthetic aperture sonar image reconstruction

A new approach for synthetic aperture image formation is presented in this paper. With the presented method image formation is regarded as a signal arrangement that can be described by a matrix. This method integrates the sonar platform motion in the image formation process but more importantly it acknowledges the non ideal data gathering process and implements means to mitigate these shortcomings. This method is illustrated with real data obtained in test mission in the Douro River, Portugal by a synthetic aperture sonar developed at the University of Porto.

Shallow Water Surveying Using Experimental Interferometric Synthetic Aperture Sonar

An interferometric synthetic aperture sonar system that uses an unmanned surface vehicle customized for shallow water environments as a basis is described in this paper. High resolution imagery that is independent of range through sonar platform displacement use for large virtual array creation is enabled through synthetic aperture sonar. Three-dimensional topographic map construction is possible if two geometrically displaced images are obtained through interferometric technique use. Several advantages are presented through unmanned surface vehicle use through which some problems related to synthetic aperture image formations are alleviated, mainly related to the fact that for boat control and sonar imagery motion compensation, a precise navigation can be used. In terms of cost of operation, maintenance, and ease of deployment, a small unmanned surface vehicle is also advantageous. For many commercial and scientific applications like natural resource monitoring, infrastructure maintenance, and river navigability an important task is shallow water area mapping. This system allows these tasks to be done efficiently. How synthetic aperture can be used to generate bathymetric data and underwater imagery is illustrated through sample data obtained during test trials.

A global motion control strategy using dynamic programming

A computational method to obtain autonomous marine vehicle guidance strategies is proposed. It was developed so that good performances are achieved for cheap computational effort. Strategies are obtained by solving a control optimization problem. The performance index under optimization includes energy consumption, risk of collision and travel time. The main contribution of the article is the stochastic formulation of this problem, which enables the modelling of collision risk in a natural way. By taking into account the knowledge of a given path between the start and finish points, it is possible to establish a precedence relationship between any two points in the vehicle circulation domain. This relationship enables the efficient solution of the problem by using dynamic programming.<<ETX>>