This Appendix was reproduced from the Echoview® help file with permission from Echoview Software Pty Ltd, Australia.
Transducer geometry refers to the configurable location and orientation of transducers. This Appendix covers:
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Overview of transducer geometry
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About transducer location
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About transducer orientation
Each transducer may be located in space and oriented as desired. Illustrated in Figure 1 is a schematic display of relative platform system reference point, a GPS antenna and a transducer.
Transducers are always associated with a platform (Figure 1). The system reference point of the platform is at (0, 0, 0) by definition and defines the position of the platform in the real world (that is, the platform is considered to be, in the real world, wherever it’s reference point is).
Please note that the positive Z direction is downwards and the X-Y plane is horizontal (considered to be on a rigid platform that does not pitch and roll).
For many applications, such as a typical ship based echo-integration survey with multiple downward looking transducers, the only aspect of transducer geometry required is the definition of transducer depth (draft, shown in Figure 1), if desired. Other applications, such as multiple frequency target strength techniques, surveys with non-vertical transducers, and applications that require the position of samples to be precisely located in the world.
Locations are all defined relative to a system reference point (Figure 1). The coordinate system utilises three axes (X, Y and Z) and their orientation depends upon whether the platform is fixed or mobile.
The platform system reference point (0, 0, 0) is in a platform space where:
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The X axis is defined to run south-north (positive northwards, negative southwards)
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The Y axis is defined to run west-east (positive eastwards, negative westwards)
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The Z axis is defined to run vertically (positive downwards, negative upward)
The geographic location for the platform system reference point is to a specified latitude, longitude and altitude. Positive altitude is upwards.
The platform system reference point (0, 0, 0) is in a platform space where:
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The X axis is defined to run alongship (positive towards the bow, negative towards the stern)
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The Y axis is defined to run athwartship (positive towards starboard, negative towards port)
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The Z axis is considered to run vertically (positive downwards, negative upwards)
The geographic location for the platform system reference point (X = 0, Y = 0) is the latitude and longitude of the GPS antenna as measured by a Global Positioning System (GPS) device. The location of the GPS antenna relative to the system reference point is specified in X, Y coordinates in meters. The altitude of the system reference point (Z = 0) may be specified. Positive altitude is upwards.
To determine the geographic location of a sample point or a single target in geographic coordinates it is also necessary to define the orientation of the transducer as detailed in the following section.
Transducers are not only located, but also oriented - that is, they point somewhere. Like location, orientation requires three parameters to be specified, in this case angles rather than coordinates.
The X-Y-Z axes as defined above are taken as a reference for orientation. In summary:
For fixed platforms:
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For mobile platforms:
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Two angles are used to define the direction in which the acoustic axis is pointing (either elevation and azimuth angles or alongship and athwartship angles). A third angle called the rotation defines the direction of the minor axis of the transducer relative to a vertical plane passing through the beam axis. The rotation of the transducer can only be determined after the definition of the beam direction.
The angles are defined as follows:
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Elevation is the angle between the beam axis and the positive Z axis.
Valid range is 0° to 180°.
0° |
defines a vertically downward pointing beam |
90° |
a horizontal beam |
180° |
a vertically upward pointing beam |
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Azimuth is the angle between the beam axis and the positive X axis (measured clockwise when viewed in the positive Z direction).
Valid range is 0° to 360° .
0° |
defines a northward (or forward) pointing beam |
90° |
eastward (or starboard) pointing |
180° |
southward (or aft) pointing |
270° |
westward (or port) pointing |
If the elevation is 0° or 180° then Azimuth is equivalent to a rotation.
The angles are defined as follows:
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Alongship is the angle between the beam axis and the Y-Z plane.
Valid range is -180° to 180° .
0° |
defines a downward pointing beam in the Y-Z plane |
-90° |
a horizontal aft pointing beam |
90° |
a horizontal forward pointing beam |
-180° |
an upward pointing beam in the Y-Z plane |
180° |
an upward pointing beam in the Y-Z plane |
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Athwartship is the angle between the beam axis and the X-Z plane.
Valid range is -180° to 180° .
0° |
defines a downward pointing beam in the X-Z plane |
-90° |
a horizontal port pointing beam |
90° |
degrees a horizontal starboard pointing beam |
-180° |
an upward pointing beam in the X-Z plane |
180° |
an upward pointing beam in the X-Z plane |
Note: Not all combinations of Alongship and Athwartship angle are valid. If one angle defines a downward pointing beam (-90° to 90°) and the other an upward pointing beam (-180° to -90° or 90° to 180°) they cannot be describing the same direction!
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Rotation is the angle between the positive minor-axis of the transducer and the vertical plane running through the beam axis (measured in the clockwise direction as seen from the transducer).
Valid range is 0° to 360°.
0° |
an upward pointing positive minor-axis |
180° |
a downward pointing positive minor-axis |
To determine the three coordinates defining the beam orientation, do the following:
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Determine the pointing direction of the beam axis
Use your choice of either elevation-azimuth angles or alongship-athwartship angles.
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Determine the rotation angle of the transducer
Remember that the zero reference for the rotation angle is the vertical plane running through the beam axis and therefore that the rotation coordinate can only be meaningfully determined after you have defined the orientation of the beam axis.
Examples:
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A transducer beam pointing to starboard at an angle of 45 degrees with the positive minor axis of the transducer pointing forward is defined by either:
elevation = 45° , azimuth = 90° , rotation = 270°
-OR-
alongship = 0° , athwartship = 45° , rotation = 270° -
A transducer beam pointing to port at an angle of 45 degrees with the positive minor axis of the transducer pointing forward is defined by either:
elevation = 45° , azimuth = 270° , rotation = 90°
-OR-
alongship = 0° , athwartship = -45° , rotation = 90°
Notes:
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You may define the pointing direction of the transducer with whichever pair of angles is most convenient for your application but the rotation angle will be the same, whichever pair of angles you choose to define the pointing direction.
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For a transducer with an elevation of 0° (that is, vertically downward pointing), the azimuth angle is logically equivalent to the transducer rotation.