Multibeam Sonar Theory of Operation

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Multibeam Sonar Theory of Operation Detection Processing and Range Calculations Figure Chapter 4 - -11: Parabola Fitting for High-Resolution Angle Estimation A time slice with no strong echoes might produce no angle estimates, if all measured amplitudes are below the detection threshold. In these cases, however, the strongest amplitude value and its associated beam angle are saved for use in the sidescan data processing. Each angle/amplitude pair that emerges from the angle estimation process is called a hit. A hit is composed of three pieces of data: its angle, its amplitude, and the time it occurred (equal to the time associated with the time slice). Motion Compensation The Angle Estimation process produces a list of hits including their amplitudes and angles for each time slice. However, the angles have been measured with respect to the hydrophone array, and the hydrophone array is mounted on a moving platform, a vessel at sea. Before the reported echoes can be translated into measurements of the sea floor, and before they can be compared between time slices (the hydrophone array will be in different positions and at different angles in different time slices), the motions of the hydrophone array must be measured and taken into account. This process is called motion compensation, or beam stabilization. The motions of a ship at sea and the hydrophone mounted to it are highly complex and somewhat unpredictable. Some of these motions are deliberate, such as the ship’s forward progress, or turns. However, most are due to the effects of waves. All motion of a ship can be described by using a point called the center of mass. The center of mass of a ship is exactly that— it is an imaginary point in space that represents the center of the weight distribution of the ship. It is also the point about which a ship will rotate if any external force is applied to it. Around the center of mass a three dimensional coordinate system is constructed (see Figure Chapter 4 - -12). The three coordinates are: • The x, or acrosstrack direction – This is the direction perpendicular to the ships track and parallel to the surface of the sea. • The y, or alongtrack direction – This is the direction parallel to the ships direction of travel, or track. • The z, or vertical direction – This is the direction perpendicular to the surface of the sea. Copyright © 2000 L-3 Communications SeaBeam Instruments Page 4-13 No portion of this document may be reproduced without the expressed written permission of L-3 Communications SeaBeam Instruments
Detection Processing and Range Calculations Multibeam Sonar Theory of Operation Figure Chapter 4 - -12: Ship-Motion Coordinate System A ship at sea experiences translational motion in each of these three directions. Fore and aft motion in the alongtrack direction is called surge. Port and starboard motion in the acrosstrack direction is called sway. Up and down motion in the vertical direction is called heave. Page 4-14 Copyright © 2000 L-3 Communications SeaBeam Instruments No portion of this document may be reproduced without the expressed written permission of L-3 Communications SeaBeam Instruments
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Multibeam Sonar Theory of Operation

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