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The Imagenex 881A Imaging Rotary Sonar devices area multi-frequency digital scanning sonars.
http://www.imagenex.com/Products/881A/881A_Imaging/881a_imaging.html
http://www.imagenex.com/Products/881A/881A_Profiling/881a_profiling.html
The data products for Imaginex Imagenex 881A sonars are described here. Currently, there are two types of Imagenex Rotary 881A Sonar scans supported: Sector (also known as Imaging) and Profile. Sector scans represent a horizontal rotation of the sonar, like a radar, and profile describe a vertical rotation, like a radar turned on it's edge or a freewheel. The data products for both are the same, except that imaging plots have a direction arrow to indicate true north (see the device's metadata for the heading), while the profiling plots will contain a mention that a tilt angle offset was applied to orient them vertically (see the device details page (example) -> additional attributes -> sonarProfileTiltAngle). In either case, the plots will also note if there's no heading or tilt angle available (for the profiling plots without the tilt angle offset, the seabed echo may appear upside-down reversed and/or off-horizontal - this will be improved as the tilt angles are updated). Also note that the tilt angle offset is applied for all Imagenex Rotary Sonar data products for profiling sonars, when it is defined. In general, these data products contain the same information that is available through the manufacturer's software; primarily backscatter amplitude, range, head angle and time, plus various metadata.
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