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Beam Performance Test 7.6

By Harold Orlinsky, General Manager

If you’ve ever been asked – how good is your multibeam system, don’t fall into the trap of just looking at the system specification for the sonar. It’s not the answer you need. You need to consider everything that goes into the system to come up with an answer.

One method is the Beam Performance Test, a new module in SonarWiz 7.6. The program provides the user with information about each beam and measures differences along the swath, providing a snapshot of how the overall system is expected to perform during your survey operations.

A sonar that is promoted at a 150-degree system – 7.4x water depth, will provide you a depth measurement across the seabed. Your job is to determine how good all that data is. How far off nadir will the depth be accurate enough to use? Can you make improvements to the system to increase the usable swath? 

Using the performance test is straightforward. We create a reference surface with the system and use an independent check line to compare each beam to a known depth. The output is viewed in a graphical display, to provide you the expected error across the sonar swath:

One critical part of the process is to create a proper reference surface. Find a flat area (slope less than 3%) and collect sonar data in the along track and across track direction, oversampling the bottom. In processing reduce the swath to 45 degrees (the inner section of the swath is more accurate). The area doesn’t need to be large, but needs to ensure the reference line falls inside the area.

Survey lines for reference area, line spacing 20m
Check line run orthogonal to the survey area, ensuring all biases are removed.

The output of the Beam Performance Test in SonarWiz provides the user with useful information for the system, including IHO pass/fail criteria, data histograms, reject ratio and information for each beam (the latter would be useful if a single beam has a failure).

Taken together, this information should be used for planning your next survey. If there is an IHO Special Order requirement, and the system fails after 52 degrees (104 degree swath), line planning should use that criteria. The nice thing about the Beam Performance Test is that the results are good for a while – and only need to be redone when a sensor is moved or changed out.

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