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Can I perform a USBL verification in shallow water without an ROV to deploy or recover the beacon?

24 June 2021

Yes you can.
You can deploy the beacon to the seabed using an arrangement similar to that below.
USBL Verification
The beacon, for example a WSM 6+, must be held vertical by a small float about 10 m above its transducer to prevent acoustic masking, with a (approximately) 1 m strop length from the beacon to a 20 kg clump weight (the strop length is important to reduce the possibility of the beacon hitting the weight and sustaining damage when it is lowered but not too long that the beacon ‘waves’ above it).
A second 20 kg clump weight (approximately twice the water depth away) acts as the tether point for a larger location buoy which will be at the surface well clear of the vessel’s turning circle. The surface float’s strop should be long enough to compensate for tidal variations to prevent it from being pulled below the surface.

Spin Test (DP Vessels):
Track the transponder on the seabed with the vessel directly over the target. Set a way point for the transponder position and set the measure tool from the way point to the transponder or increase the trail history, ensure GNSS is being fed to the system.

Slowly rotate the vessel over the top of the transponder, using the USBL pole as the centre of rotation whilst observing the snail trail on the Chart display for the mobile target.

If the transponder trail history draws a doughnut shape greater than expected from the 1DRMS value, an error may be present in the pitch and roll installation corrections.

Heading test:
Track the transponder on the seabed with the vessel directly over the target and set a way point as the transponder position. Set the measure tool to measure from the way-point to the transponders reported position and ensure GNSS is being fed to the system. Sail away from the transponder in any direction at a constant speed.

Watch the distance of the transponder from the way-point.

If the mobile target moves perpendicular to the vessel heading there is a possible heading alignment error between the USBL transceiver and the gyro.


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