This article aims to help planners avoid the standard pitfalls of planning a Sparse LBL array.

Planning an array ahead of time

It is best practice to plan a transponder array deployment before it is placed on the seabed. This can be for a number of reasons and can highlight potential issues before the job has started:

• Avoid areas such as overage curves anchor scour locations areas deployment corridors etc
• An array plan will include an SV analysis that will define the expected range of your Compatt transponders at different depths. This will avoid  a situation where a Compatt that you need, maybe  is out of range
• The seabed is not always flat, strategically placing Compatts at the top of mounds for example, can lead to less Compatts being needed so less vessel time is needed and money is saved.
• Do you need specialist software to do an array plan, arguably yes. Though Sonardyne does offer an inhouse array planning service and will soon offer a cloud-based planning portal for all those who want to create their own arrays.

In order to perform an array calibration, you will need:

• A Sound velocity profile (SVP) taken at the location in question (preferably at the same time of the year) and covering all depths at that location (SV smoothing / QC may be necessary depending on the quality of your SVP)
• A .xyz Digital Terrain Model (DTM) of the area in question preferably under a Gb in size .
• A .dxf drawing that covers all relevant subsea infrastructure. Preferably one that does not have every nut and bolt included as the smaller the drawing the more efficiently the system will run.
• A list of x y coordinates for the Compatt along with transducer heights (usually depending if you are using stands / floats or a combination of) (Z is usually defined by the system as it plots the x y and adds the transducer heights).

Once you have everything it is worth working out the distance that you want to cover and defining the type of array that you are going to be using:

• Standard LBL,
• This will require two box-ins or two known coordinates, at least one SV Compatt and a standard ROV setup
  • No INS needed.
• 2D SLAM calibrated LBL,
• This will require two box-ins or two known coordinates at the beginning and end of a predefined route and no more than 6 Compatts between box-ins
• Or if working in an array you could conceivably calibrate the array relative to one Compatt/boxin
  • INS needed.
• 3D SLAM calibrated LBL
• This will at least one box-in or a known coordinate.
  • INS needed.

Contact [email protected] for more information.

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.

Contact [email protected] for more information.

What is SLAM Calibration in Fusion 2?

SLAM stands for Simultaneous Localisation And Mapping and is a process of positioning yourself in what is initially, an unknown environment, or in our case positioning a Compatt transponder in an unknown environment.

SLAM in Fusion 2, uses a SPRINT-Nav to record the ROV position changes, and acoustic ranges from a ROVNav 6+ transceiver are used to locate the Compatt as the ROV moves around the environment.

Clear and concise diagnostic information displayed during the data collection, along with real-time processing, show you when you have enough confidence in your position to meet specification.

The SPRINT-Nav can be aided using a variety of sources, depending on the task at hand, these include LBL & USBL.

For a run through of the Fusion 2 SLAM setup and collection process, please watch this video.

Contact [email protected] for more information.

We listened to your feedback, and we’ve made big changes to Micro-Ranger 2. It’s now more flexible and easier to use than ever. Find out more in this video.

CASIUS result can indicate more about the system than just the transceiver misalignment values. Quick checks can reveal information on Ranger 2 USBL or Marksman LUSBL system behavior.

CASIUS (Calibration of Attitude Sensors in USBL Systems) primarily computes the misalignment between the transceiver and the ships motion reference units. It also provides an indication of system performance, errors in dimension control, deployment pole integrity, GNSS quality, MRU errors and noise interference sectors. From these result corrections can be made to improve the systems performance an accuracy.

The scatter distribution is summarised as a statistic compared to water depth so 100m depths should result in a scatter of approximately 10cm 1DRMS. If the calibration lines exceed ½ the water depth horizontal offset, refraction errors can bias the data.

Calibrating in shallow water <50m tends not to produce consistent results because the GNSS and MRU error is invariably larger than the acoustic jitter and misalignments are lost in the noise.

• Check the computed offsets don’t differ significantly from measured values.
• Check computed sound speed agrees with expected. This is often scaled in error to attempt to resolve data outside ½ waterdepth.
• Check processing results agree with the tidal model result confirming reliable GNSS height.
• Check the error result is within expected error budget.
• Check all clusters are similar levels of noise as some observations, using astern propulsion, can suffer aeration.
• Check which data collection points have outliers and does this correlate to certain vessel headings?
• Is the data more spread in a certain axis or geometry as this indicates whether Pitch and Roll out perform Heading error.
• Poor repeatability can indicate pole movement.

Larger spread on lines exceeding ½ water depth.

In this video we run through how to release a Sonardyne RT 6 transponder using Ranger 2 USBL software. No additional software or kit is needed if you already have a Micro, Mini or Standard Ranger 2 USBL topside.

If you need more assistance see the other YouTube videos in this series or contact Customer Support Team.

The default acoustic update rate in Ranger 2 is two seconds. This suits most operations you’re planning and you can leave it untouched.

However, for certain operations, you’ll find you need to change it. Maybe you’re tracking a fast-moving target and need a location update every second. Or perhaps your task is to monitor the position of a target over an extended period, and you’re keen to conserve the battery in your transponder. Changing the update rate in Ranger 2 couldn’t be easier. You can even select different update rates for each beacon in your job.

Watch this short video on YouTube to find out how to set your acoustic update rate.