Sparse LBL, also known as range-aiding, is a hybrid positioning technique that leverages the combination of minimal acoustic ranges from seabed transponders with an inertial navigation system (INS), such as SPRINT or SPRINT-Nav.

This technique was developed to combine the positional integrity of LBL with the efficiency of requiring far fewer transponders. Fewer transponders are required because you are no longer calculating position with ranges, you are constraining INS drift.

This makes it suitable for a wide array of subsea operations, particularly those with high demands for speed and accuracy in deep water.

If you want to know the basic principles of LBL, read them here.

Primary applications and project types of Sparse LBL or range-aiding

Range-aiding is primarily used in the energy/oil and gas sector but also has applications in ocean science and defence.

Specific high-precision tasks include:

Subsea structure installation and construction – Sparse LBL is used for subsea structure installation where an ROV can be rigidly docked on to a structure. This approach is used in major offshore field development operations.

• Metrology – It is applied for high-precision measurement tasks, such as metrology (precise measurement of distances between structures). A comparison against conventional metrology showed that using Sparse LBL with SLAM resulted in differences as low as 8 mm in horizontal distance and 16 mm in depth difference.
• Pipeline work – The technique is crucial for operations involving pipelines, including pipeline positioning and monitoring. Sparse LBL has been successfully used to reduce the number of Compatts required for pipeline installation arrays by 50–66%. It can support survey-grade tasks such as determining pipeline out of straightness.
• Survey and IMR – It is adopted in inspection, maintenance and repair (IMR) operations and light construction. It is also used for environmental surveys, site survey and characterisation.

Vehicle navigation and hybrid functions

Sparse LBL significantly enhances the performance and efficiency of vehicle navigation by reducing the dependency on dense acoustic arrays:

• ROV and AUV navigation – It is used for ROV navigation and AUV (autonomous underwater vehicle) navigation. For AUVs, Sparse LBL is particularly adapted because it enables them to cover large areas with a minimum of deployed acoustic hardware. The robust positioning and high update rates provided by the integrated INS/acoustic solution are typically what an ROV pilot requires.
• Dynamic positioning (DP) reference – Sparse arrays can provide stable, depth-independent position data to the vessel’s DP system. This serves as a valuable alternative or back-up to GNSS, especially when operating near infrastructure that might compromise satellite signals.
• Overcoming line-of-sight dependency – The inertial aiding allows the ROV to maintain precise navigation even when acoustic ranges are momentarily lost or interrupted (acoustic dropouts), which happens frequently when operating near subsea structures

What are the benefits of using Sparse LBL for underwater positioning and navigation?

The use of Sparse LBL is driven by efficiency and cost reduction:

• Reduced costs: It reduces the number of transponders needed (by 30% or even 50-66% for certain pipeline arrays). This, in turn, reduces hardware requirements and costs, and streamlines logistics.
• Vessel time savings: It leads to significant vessel time savings. Since fewer transponders are required, the time spent deploying them and performing calibration routines is greatly diminished.
• Real-time calibration (SLAM): Sparse LBL operations can be calibrated using simultaneous localisation and mapping (SLAM), especially with Fusion 2 software. Real-time SLAM calibration can often be performed concurrently with other ROV survey operations (such as pre-lay surveys), effectively removing the time-consuming traditional baseline calibration workflow from the project schedule.
• Remote operations support: Using Fusion 2 and our Remote Operations Access Module (ROAM), Sparse LBL operations, including real-time SLAM calibration, can be supported or operated remotely by onshore surveyors, mitigating logistical risks and travel restrictions.

LBL (Long BaseLine) underwater positioning is an acoustic positioning technique that provides high precision underwater positioning and navigation.

It involves the use of acoustic transponders placed on the seabed, typically hundreds or thousands of meters apart (hence “Long Baseline”).

Core principles of LBL

Ranging and positioning: LBL systems determine the relative position of a target, such as a remotely operated vehicle (ROV), by sending and receiving acoustic signals to transponders with known positions (relative to each other) on the seabed. The system calculates the distance to each transponder by measuring how long the signals take to reach them. Using these distances, the system computes the target’s location, much like a GPS system.

Note: The final positioning accuracy is dependent on several factors that begin with the GPS signals received at the vessel’s antennae. Antennae must be located so that there are minimal obstructions from vessel masts and superstructure, which will provide a clear line of sight to the GNSS satellites. Accurate offsets from the antennae to USBL transceiver and on the ROV are then required so that the best possible “real world” coordinates of the ROV’s common reference point (CRP) can be determined.

Is LBL depth independent?

A key benefit of LBL is that its precision (how consistently the system can reproduce the same position measurement) is independent of water depth.

This is because the LBL array is in a fixed position at the seabed. Distance measurements relative to these fixed points are unaffected by water depth variations.

It is used when the highest level of subsea positioning accuracy (how close the measured position is to the targets true position) is required.

How many ranges are required for LBL positioning?

Theoretically, LBL requires a minimum of three ranges plus knowledge of depth to calculate a unique position. However, in real-world applications, at least four, and ideally five or more ranges, are typically necessary to ensure redundancy, which helps to detect and reject erroneous readings (outliers) and compute a high integrity position.

• Acoustic operation: The system interrogates the transponders acoustically, they respond, and the time of travel is measured. This is then used to calculate the distance and then relative position.
• Speed of sound (SV): The most critical part of this calculation is the speed of sound, or sound velocity (SV). Unlike radio signals, where the speed of light is constant, the speed of sound in water is variable.

Sound speed in water varies primarily due to changes in temperature, salinity and pressure (depth). Warmer temperature and higher salinity increase sound speed, while increasing pressure with depth also raises the speed. These factors vary spatially and with depth, causing sound speed to be variable in water.

If the SV used in the calculation is incorrect, all measurements will be wrong. Sound velocity sensors (like those found in Compatt transponders) are used to measure the SV directly from the environment.

What equipment is involved in an LBL operation?

The hardware:

LBL systems use robust acoustic hardware like our Compatt 6+ transponders, which serve as a seabed reference (once calibrated as an array), and a ROVNav 6+ acoustic transceiver, which is typically installed on the ROV.

While classic (full) LBL operations typically require a dense seabed array, often consisting of six or more transponders, Sparse LBL, often referred to as range-aiding, is a hybrid approach developed to achieve high integrity positioning using a minimal number of transponders.

This reduction in transponders is primarily enabled by inertial aiding, which involves combining acoustic ranging data with an inertial navigation system (INS), such as our SPRINT or SPRINT-Nav. Sparse LBL relies heavily on the INS to provide precision positioning while using minimal acoustic range data.

The software:

Sonardyne’s LBL and Sparse LBL operations are managed using Fusion 2, which provides a single software suite to control LBL, Sparse LBL and SPRINT INS projects, streamlining workflows and maximizing efficiency. Fusion 2 also unlocks the potential of Wideband 3 signal technology, which is optimized for 6G+ instruments.

LBL and its variations are used for numerous applications in the energy, ocean science, and defence sectors:

• Subsea structure installation
• Metrology (highly precise measurement of distances between structures)
• Pipeline positioning and monitoring
• ROV, AUV and towfish navigation
• Deepwater nodal positioning

If you want to know more about Sparse LBL, click here.

iWand is a rugged handheld acoustic testing and configuration unit, which you can use to do all of your Fetch pre-deployment tests. Make sure it’s charged up and then you’re ready to go.

Once your Fetch instrument and iWand are powered up, you first want to identify the Fetch and inspect its acoustic parameters.

Gently push the end of the ‘antenna’ onto the Fetch transducer and then select “Ping Check” from the iWand menu. You’ll see that the display has turned round so it’s not upside down.

Press ENT a couple of times to answer the questions, then iWand will try all the available communications bands until it gets a response. When you hear a chirping noise, that’s the Fetch answering a request from the iWand.

At this stage the iWand has auto-discovered the Fetch and it indicates the Fetch acoustic address on its display. A tip about the acoustic address – new instruments have their acoustic address printed on the label, but it can easily be changed by the user so it’s always best to check.

Take a note of the address as it is the key to talking to the instrument. Other basic information collected from a Fetch as part of a “Ping Check” are UID and battery capacity remaining. UID is the Unique Identifier that is different for every single Sonardyne instrument and cannot be changed. This is required for some operations (such as releasing from the seabed) to ensure that the operation can only affect the correct instrument.

The next thing to test are the sensors. Select “Quick Check” from the menu and the iWand will interrogate Fetch for its full status. On completion, select “View Sensors” to see the results.

If the readings can be seen in green, this means that the measurement is valid and less than 10 seconds old; if the readings are in orange, the measurement is more than 10 seconds old; and if the readings are in red, the measurement has been reported in error. You can hit refresh if the readings are not green the first time.

If the Fetch is fitted with a release you can also test that with the iWand by selecting “Test SCR Release”.

Basic testing of your Fetch using iWand is now complete, and you can use the iWand 6G Configurator app to generate a test report.

Note that AZA testing via the iWand can only be done using SAM software and PIES testing can only be done in water.

You find out how to perform basic testing for Fetch using an iWand by watching the video below.

Also, check out our other Fetch FAQs and videos for tips on testing and preparing Fetch for deployment.

When working with a Sonardyne instrument there must be a way to identify it. This is where the acoustic address and UID come in.

Every Sonardyne Wideband instrument has a configurable acoustic address. This address is a 4-digit identifier for communication with that specific device.

Meanwhile, an instrument’s Unit ID or UID is a unique, non-configurable, permanently assigned 6-digit number.

The acoustic address and the UID are necessary for tasks such as programming, diagnostics and communication. They can be checked using NFC or the iWand.

Note that new instruments have their acoustic address printed on their label, but since this can easily be changed, it’s always best to check!

Watch the video below for more information

There are a number of options for connecting to a Fetch instrument for testing and configuration purposes before deployment, this article discusses how to connect using the Bluetooth in Fetch.

To enable connection to a Fetch with Bluetooth first make sure the Fetch is powered on. This can be done by simply pulling the battery disconnect fob out of the holder (taking care not to allow it to hit the glass sphere) and storing it somewhere safely. A red indicator light will appear for 30 seconds to signal the instrument is powered up.

Fetch’s Bluetooth module is powered and ready for pairing when the red indicator light is on, but since this is only for 30 seconds after powering up a Fetch, you may need to command Bluetooth to turn on via 6G Terminal lite.

To do this with the 6G Terminal, use acoustic command OPT:AAAA;W1,Uhhhhhh,EXTP_REG or the direct serial command OPT:Uhhhhhh,EXTP_REG where AAAA is the acoustic address and Uhhhhhh is the Unique ID.

Bluetooth must be disabled before deployment to prevent unnecessary battery drain and for this use acoustic command OPT:AAAA;W1,Uhhhhhh,EXTP_OFF or the direct serial command OPT:Uhhhhhh,EXTP_OFF.

To pair to Bluetooth, make sure Bluetooth is enabled on your PC and the red indicator light is showing on your Fetch. From your PC Bluetooth settings, pair the Fetch unit with the PC. Note that the name of the Fetch will be Fetch_XXXXXX where XXXXXX is the unit ID.

Bluetooth creates 2 new COM Ports, one for CPU and one for DAS, and once a COM port is in use, the blue LED light will come on.

Run 6G Terminal on your PC, click connect and select the applicable Bluetooth COM port. You will see the blue LED light, so now 6G Terminal Lite can communicate directly with the Fetch instrument.

In the 6G Terminal Lite software, navigate to the 6G Setup tab where you can start your checks and generate a test report.

Note: you cannot test the acoustic communications with Bluetooth. 

You can find out more about connecting to your Fetch by reading the knowledge base articles or watching the video below.

There are a number of options for connecting to a Fetch instrument for testing and configuration purposes before deployment, this article discusses how to connect using an RS232 test cable.

Connecting to a Fetch with an RS232 cable is only an option when there is an optional bulkhead 8-way female connector fitted on the lower sphere, normally used for release or external sensors.

Simply connect the CPU connector of the serial cable to a PC COM port, run 6G Terminal Lite on your PC, click connect, select the applicable RS232 COM port and the connection is complete. 6G Terminal Lite can now communicate directly with the Fetch instrument. To run your checks and generate a test report, navigate to the 6G Setup tab in the software.

Note: you cannot test the acoustic communications or acoustic release with an RS232 cable. To do this we recommend using an iWand.

You can find out more about connecting to and running tests on your Fetch by reading the knowledge base articles or watching the video below.

There are a number of options for connecting to a Fetch instrument for testing and configuration purposes before deployment, this article discusses how to connect using an iWand.

iWand has a small colour display screen and five simple navigation buttons as well as an internal rechargeable battery that you charge from a USB port and will provide many hours of operation. Most importantly, iWand has an ‘antenna’ with a small acoustic transducer built in that actually does the communicating. iWand can quickly establish acoustic communication with a Fetch instrument and identify the acoustic address of it with just a few button presses. It will then allow you to inspect the operating parameters.

The first thing to do to enable connection to a Fetch with an iWand is to make sure the Fetch is powered on. This can be done by simply pulling the battery disconnect fob out of the holder (taking care not to allow it to hit the glass sphere) and storing it somewhere safely. A red indicator light will appear for approximately 30 seconds to signal the instrument is powered up. This light will go out, but don’t worry about this as the Fetch will remain powered up until the magnet is returned to its holder.

Next, turn on your iWand by pressing the central ENTER button for a second or so. As soon as the iWand has completed its start-up sequence, a menu will show, and then you’ll be good to start the sequence for basic testing of Fetch with an iWand.

You can find out more about how to run tests on your Fetch with an iWand by reading more knowledge base articles or watching the video below.

There are a number of options for connecting to a Fetch instrument for testing and configuration purposes before deployment, this article discusses how to connect using an acoustic transceiver.

Connecting to a Fetch to run some basic tests before deployment can be done with any 6G transceiver, for example a Dunker 6 or one of Sonardyne’s USBL High Performance Transceivers (HPTs).

First, connect the transceiver to a power source and PC COM port. Make the connection between the transceiver and computer via the RS232 link from a PC running the 6G Terminal Lite software.

With this software running on your PC, click connect and select the applicable COM port. Hold your transceiver close (within 10 cm) to the Fetch transducer and you can start communicating directly with the Fetch instrument.

When testing Fetch with a transceiver, perhaps the easiest way to go is to use manual commands via 6G Terminal Lite. Open the manual commands tab and first set a low power level and low linear gain in your transceiver for in-air testing by applying the “CS:LG#,TPL#” command where # represents the numbers you select for power and gain (example: CS:LG6,TPL170).

To then test your Fetch with the transceiver, input manual commands into the input field such as those detailed in the table below.

Where aaaa = acoustic address; W1 = wake-up signal 1; and hhhhhh = UID

Alternatively, testing Fetch with an acoustic transceiver can be done by navigating to the 6G Setup tab in the 6G Terminal Lite software as described in the ‘Functional testing’ section in your Fetch manual.

You can find out more about connecting to and running tests on your Fetch by reading other knowledge base articles or watching the video below.

Before deploying a Fetch instrument for a long-term measurement campaign you want to confirm it is working nicely and to do this, there are some basic tests you can conduct. These include checking acoustic communications, battery status, that the sensors are working and for a Fetch with an acoustic release and/or AZA mechanism, checking those too. Fetch instruments aren’t fitted with display screens and they generally don’t have connectors to allow a computer to be plugged in, so how can you connect to a Fetch to run some basic testing?

When a Fetch is out of the water, there are a number of options for connecting to it:-

1. Using an iWand
2. Using Bluetooth
3. Using an RS232 test cable (if connector is fitted to Fetch, e.g. release option)
4. Using an acoustic transceiver.

We recommend using the first option since an iWand is suitable for all elements of testing and it is convenient. The table below summarises which elements of testing can be conducting when connecting to Fetch in different ways.

You can find out more about how to connect to Fetch with an iWand, Bluetooth, RS232 cable or acoustic transceiver by reading more Fetch knowledge base articles or watching the video below.

When you’re ready to start data harvesting from your Fetch instrument it is good practice to make a note of the real time from your watch or a clock, but what’s the benefit of this?

By recording the real time from your watch/a clock when you start data harvesting, you can compare it with the reported time given in Subsea Array Manager (SAM), noting the time given in SAM is in UTC/GMT.

This way, if a time error occurs, you can use these time figures to work out what has happened and adjust your harvested data accordingly.

You can find out more detail on how to harvest data from Fetch instruments by watching the video below.