Doppler Velocity Log (DVL)

Designed and developed to make high specification DVL performance available at an affordable price, while ensuring it’s simple for anyone to use. Enabled by proprietary transducer design, developing advanced algorithms and removing every millimetre of space that’s not absolutely needed.

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The smallest DVL in the world

From 0.105kg and 25mm x 66mm.

World's shortest blanking distance

Operate virtually at the seabed, as low as at 5cm altitude.

Unbeatable price/ performance

From less than US$ 8,000.

How it works:

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 measures 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.

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 is used to calculate the velocity.
- Time of flight. This gives a measurement of the distance between the transducer and the seabed (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.

DVL in use:

Use cases of Water Linked DVLs

An onboard computer removes the need for external computers or topside communication, enabling the Water Linked DVL to be used as a navigation sensor on a range of underwater platforms.

ROV
By utilizing a Water Linked DVL, the ROV operator will experience a whole new level of stability and control when operating the ROV. Thanks to the increased stability, the quality of the video will improve dramatically as the ROV pilot carries out his works with increased confidence.

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 (AUV) are usually operating without tethers or with any direct input from the surface. While undertaking long range assignments the AUV is typically also unable to relay on an acoustic positioning system for navigation information.

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 patrolling.

Diving
Military, police and commercial divers are examples of divers that often have to follow a very accurate dive path.

This can be achieved with reliable acoustic positioning such as our Underwater GPS, however when very long range is needed, you have to look for a solution that does not require topside support. This is where the DVL comes in.

Examples:
- Low visibility diving.
- Long range dives.
- Diver waypoint navigation.

Why size matters

A lot of effort was put into making sure the Water Linked DVL would be truly unique and provide benefits never seen before in a DVL.

We have used modern computing power, designed our own transducers and removed every millimetre of space that was not absolutely needed. The result has been a DVL design that has disrupted the market by using the latest technology.

Competitor comparisons

Image: Water Linked DVL size has set a new standard for high performance DVLs.

Manufacturer	Water Linked		Teledyne		Nortek
Model	A50	A125	Wayfinder	Pathfinder	DVL1000 G3	DVL500	Nucleus 1000
Depth rating (meters)	600	4000	300	500	300	300	300
Minimum altitude (meters)	0.05	0.05	0.5	0.15	0.1	0.1	0.1
Maximum altitude (meters)	50	125	60	89	75	200	50
Diameter (mm)	66	125	115	229x102	114	186	90
Height (mm)	25	30	70	71	158	203	42
Weight in air (kg)	0.17	0.98	0.85	1.9	1.7	3.5	0.54
Weight in water (kg)	0.1	0.5	0.51	?	0.3	0.5	0.3

“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 qualify the DVL we carried out a range of tests to validate the DVL performance.

Test A: How stable is the ROV when operating in a strong current

Benchmark: What level of control is there when performing a task in a strong current

Setup: The DVL was mounted to a ROV which was submerged in a water tank. The water tank dimensions were 210x110cm. A camera was mounted 2 meter above the ROV, filming and documenting any movements of the ROV. Another camera was mounted at the bottom of the water tank. A Blue Robotics T-200 thruster was mounted on the side of the tank pointing directly towards the ROV. The thruster was used to create currents in the water.

Performing the test: The ROV was set in “station keep” mode. Altitude was ~5 cm. The thruster creating current was set to about 40% power.

Results: The ROV was easy to control while approaching the target to be picked up. The critical moment when the gripper connected to the target was performed in a controlled manner.

Test B: Station keeping drift

Benchmark: Measurements taken to quantify drift over a set time.

Setup: The DVL was mounted to a Deep Trekker PIVOT ROV which was submerged in a water tank. The water tank dimensions were 210x110cm. A camera was mounted 2 meters above the ROV, filming and documenting any movements of the ROV.

Performing the test: The ROV was set in “station keep” mode. Altitude was ~40 cm. The DVL was usd to provide station keeping for 30 minutes.

Results: The variation between starting position and end position was measured to be less than 1cm.

Test C: Station keeping in a strong current

Benchmark: Visual observation of ability to maintain position while in strong current.

Navigation with DVL Only?

A Water Linked DVL has the ability to operate as a stand-alone navigation system providing Dead Reckoning Navigation of your underwater, or on water, vehicle. This is the process of calculating the vehicle's position by applying speed, time and direction 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 can be combined with other navigation sensors which reduce this effect.

Underwater GPS integration (not currently available)

To remove the issue of long-term errors (position drift), it was intended to integrate the DVL with our Underwater GPS G2 system. With the pending launch of our new UGPS, it is now intended to offer this integration as an advanced feature with this new UGPS. The intention is that there will be auto-discovery between the two systems and by integrating the two systems you will be left with an amazing long-term deployment navigation solution.

DVL vs Underwater GPS

These two systems are both used for underwater navigation. They both however have different strengths and weaknesses although if you combine them on one vehicle, you can more or less eliminate the weaknesses of the stand-alone systems.

	DVL	Underwater GPS
Requires initial setup	NO	YES
Limited range	NO	YES
Requires topside communication	NO	YES
High resolution position (millimetre variations)	YES	NO
Position will drift over time	YES	NO

Available DVL products

DVL A50

World’s smallest DVL with 0.05–50m altitude.

DVL A100

Compact but high-accuracy DVL with 0.05–100m altitude.

DVL A125

World’s smallest long range DVL with 0.05–125m altitude.

DVL A250

Compact, long-range DVL with 0.30–250m altitude.

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