Doppler Velocity Log – Operating Concept

The DVL estimates velocity relative to the sea bottom by sending acoustic waves from the four angled transducers and then measure the frequency shift (doppler effect) from the received echo. By combining the measurements of all four transducers and the time between each acoustic pulse, it is possible to very accurately estimate the speed and direction of movement.

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2
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4
1

Sound wave sent

Acoustic narrow beam sound wave is sent from each of the four transducers.

2

Sound wave echo

The sound wave will bounce off the bottom and the transducers will pick up the echo

3

Measurements performed

The DVL computer measures the echo and the AHRS/IMU

4

Velocity calculated

Estimated distance travelled, time of flight and AHRS headings are all fed into our Kalman filter which in turn will output the velocity of the DVL.

1. Sound wave sent

Acoustic narrow beam wave is sent from each of the four transducers.

2. Sound wave echo

The sound wave will bounce off the bottom and the transducers will receive the echo.

3. Measurements performed

The DVL computer measures the received echo and the IMU:

  • Doppler measurement. The difference in frequency between transmitted and received signal is the “Doppler effect” which relates a change in frequency to a change in velocity.
  • Time of flight. This gives a measurement of the distance between the transducer and the bottom (altitude).
  • AHRS/IMU. The onboard Attitude and Heading Reference System (AHRS) / Inertial Measurement Unit (IMU) reads its triaxial gyroscope, accelerometer, and compass sensors to determine orientation.

4. Velocity calculated

Estimated distance travelled, time of flight and AHRS headings are all fed into our Kalman filter which in turn will output the velocity of the DVL.

Use Cases

Being fully self contained without the need for topside communication or any external computers, the Water Linked DVL-A50 is a very useful tool for several areas of underwater navigation.

ROV

Utilizing a Water Linked DVL, the ROV operator will feel a whole new level of stability and control of the ROV. Professional assignments can be carried out with full confidence. The quality of the video will increase dramatically as the ROV will be significantly more stable.

Examples:
  • Maintaining stability while operating ROV tools or performing detail inspections
  • Station keeping in challenging situations like ocean currents or tether pull
  • Terrain following
  • General velocity feedback for vehicle control

AUV

Autonomous Underwater Vehicles are usually operating without tethers or direct input from the surface. Long range assignments creates challenges for acoustic positioning systems. Therefore the most used navigation sensor for an AUV is a DVL in combination with an inertial sensor (IMU/INS).

Examples
  • Long range assignments
  • Bathymetry surveys
  • Military subsea patroling

Diving

Military, Police enforcement and commercial divers are examples of divers that often relies on very accurate dive paths to follow. This can easily be achieved with acoustic positioning like Underwater GPS, but when very long range is needed, you have to look for an integrated solution without topside support. This is where the DVL comes in.

Examples
  • Low/no visibility diving
  • Long range dives
  • Diver waypoint navigation

Why size matters

A lot of effort was put into making sure the DVL-A50 would be a truly disruptive product.

We used modern computing power, designed our own transducers and removed every millimeter of space not absolutely needed. The result is a DVL which totally places the competition in the shadow!

Size Comparisons – Side By Side

“Teledyne” and “Nortek” are registered trademarks and the property of their respective owners. All company, product and service names used in this article are for identification purposes only. Use of these names,trademarks and brands does not imply endorsement.

Mini ROV Mounting Ability

Trying to use one of the legacy DVLs on a modern mini ROV like the BlueROV2 clearly shows why the size of the Water Linked DVL is so important.

In stark contrast to the Teledyne and Nortek DVLs, the Water Linked DVL A50 can be mounted on a BlueROV2 or any other mini ROV/AUV without making significant impact to the behavior of the vehicle!

Competitor Comparisons

As the table below shows, the Nortek is 19 times larger and the Teledyne is 91 times larger than the A50!

“Teledyne” and “Nortek” are registered trademarks and the property of their respective owners. All company, product and service names used in this article are for identification purposes only. Use of these names,trademarks and brands does not imply endorsement.

Verifying Performance

A Water Linked Doppler Velocity Log is a highly accurate instrument. To document this, we have performed validation tests.

Worlds first DVL with Artificial Intelligence?

Classic DVLs calculate the velocity by looking at the measured frequency shift (aka the Doppler effect) between the transmitted and received sound wave. This technique is well documented and works well. But modern technologies enables us to take it several steps further!

No frequency shift calculations

The Water Linked DVL algorithms actually does NOT calculate the frequency shift at all. Instead we incorporate a wide range of techniques normally not used in DVLs. We do this because we wanted to bring to the market a DVL with very robust performance while keeping the size/price small.

Machine Learning / Artificial Intelligence

A countless number of test results has been collected and fed into our own machine learning AI algorithms where the DVL as a result has “learned” how different seabeds and other reflective areas “sounds like”.

Multiple side-by-side comparisons between classic algorithms and our AI engine has proven that AI is significantly better at coming to the correct conclusion.

Having AI in the core also enables us to keep feeding samples into the machine learning modules making the DVL become even better to estimate the correct velocities, bottom lock etc. The new performance levels will arrive to the customers directly through software upgrades.

Navigation with DVL only?

A stand-alone DVL provides the ability for the underwater vehicle to do Dead Reckoning Navigation. This is the process of calculating the vehicles position by applying speed, time and directon of travel compared to the last known position.

The challenge by doing dead reckoning is that small velocity errors are integrated up and will over time result in a less accurate position estimate. To mitigate this, the DVL may be combined with other sensors which reduce this effect. An example of such a sensor is an acoustic positioning systems like Underwater GPS.

Underwater GPS integration

To remove the issue of long term errors (position drift), it will be possible to integrate the DVL with Underwater GPS. This will be offered as a software upgrade and will feature auto-discovery between the two systems. By integrating the two systems, their negatives will more or less be nullified by each other and you will be left with an amazing navigation solution.

DVL vs Underwater GPS

These two systems are both used for underwater navigation. But they have different strengths and weaknesses. However, if you combine them on one vehicle, you can more or less eliminate several of the weaknesses of the separate systems.

DVLUnderwater GPS
Position will drift over timeYESNO (Locator A1/D1)
Extremely stable position (millimeter variations)YESNO
Requires topside communicationNOYES
Requires initial setupNOYES
Limited range NOYES
Limited altitudeYESNO

Available DVL Products

DVL A50

1 MHz, 4-beam convex Janus array

0.05 – 50m altitude

Worlds – by far – smallest DVL!