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How do I calibrate my sparse LBL array ?

21 June 2021


What are the steps for a sparse LBL Calibration in Fusion 2

Option 1 2D

Step-by-step guide – How-to

Establish USBL drop coordinates for the array in planning and confirm when deployed (assign semi major / semi minor errors to the drop coordinates the default is 10m but this can be changed depending on the spread observed when doing the box-ins).

Compatts should be placed so that a decent angle of cut can be achieved to the location that you want positioning in 60° being the minimum.

Box in the first and last Compatt if working along a route.

Currently the system can handle three Compatts in the SLAM algorithm at one time so plan accordingly. Starting with a box-in Compatt moving past that and along three uncalibrated Compatts and then finishing that SLAM before starting a new SLAM. Then moving three Compatts along and then finishing by moving past a boxed-in Compatt.

For best results start in an LBL array or on a known location. When I say on a known location, I don’t mean close to I mean cm from or perfectly on any error in the initial start point can translate into future error in Compatt position, depending on the scale of the error.

2D array calibrations do not need a DQ loops but they need a definition of depth from the ROV on deployment this can be achieved by placing the bumper bar up against the Transducer on deployment and then reducing this for tide once all Compatts have been deployed.

High accuracy measurements are not needed as all 2D array Compatts should be a minimum of 150m away from the route that is to be surveyed. This range minimises the effects of any depth error that might be present.

Option 2 3D

Step-by-step guide – How-to

Establish USBL drop coordinates for the array in planning and confirm when deployed (assign semi major / semi minor errors to the drop coordinates the default is 10m but this can be changed depending on the spread observed when doing the box-ins).

Compatts should be placed so that a decent angle of cut can be achieved to the location that you want positioning in 60° being the minimum.
Box-in your reference Compatts or at least establish a Compatt on a known location.

Make sure that you have decent geometry from your planned circular trajectory to your boxed-in Compatt / reference Compatt.

On this note it is important to stress that if your reference Compatt is on an operation structure you may have to contend with background noise and (if the Compatt in question is low down) Line of sight issues. So be aware.

When performing your circular SLAM / trajectory with good geometry to the reference Compatt make sure that the ROV is aware of the dangers of catching the Compatt in the tether and accidentally moving it.

Please see the you tube link below for a better understanding: https://www.youtube.com/watch?v=kc99iiuk0io

For more information there is a guidance note that can be downloaded link to SLAM Guidance note

Points to remember 2D

  • 10m semi major /semi minor is a guide not a standard, redefine this depending on box-in spread. Depth definition from ROV on deployment reduced by tidal curve later, no DQ survey needed.
  • Start with a good definition of ROV location where possible.
  • 150m from the route to be surveyed.
  • Can be performed during other ops such as pre lay etc.
  • Can be post processed in Janus to improve things.

Points to remember 3D

Depth definition from ROV on deployment reduced by tidal curve later, no DQ survey needed as 3D SLAM also SLAM calibrated the depth as well as the X and Y relative to its position.
Good geometry needed during calibration.


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