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Doppler Velocity Log (DVL) GuideÂ
A Guide to Doppler Velocity Logs (DVL)
Exploring the underwater world and mapping its features efficiently and accurately requires robust navigation tools. This is where Vehicles or equipment equipped with Doppler Velocity Logs (DVL) come in. This guide provides an overview of these systems, their applications, and key considerations for use with ROVs, USV's and AUV's.
Understanding Doppler Velocity Logs (DVL)
A DVL is an advanced sonar system that calculates the velocity of a vehicle relative to the seabed or water column by harnessing the Doppler Effect. It emits sound waves, then measures the change in frequency or pitch of the returned echo, called the Doppler shift.
Why is a DVL Important?
Without GPS signals underwater, accurate navigation is challenging. Traditional methods like Dead Reckoning tend to accumulate errors over time. However, real-time data from DVLs provide a reliable solution. They offer high-resolution information about the ROV's speed (DVL).
For example, DVL can operate in the bottom-track mode, using the reflected signals from the bottom-mounted DVL to determine velocity. When the ROV ascends and the seabed gets out of the DVL's range, it switches to water-track mode, using the echo from particles suspended in the water column.
Integrating DVL with ROVs
When integrating a DVL with an ROV, consider:
1. Size and Weight: Select devices that are lightweight and compact.
2. Frequency: DVLs work on different frequencies. Higher frequency units provide more detailed data but have a shorter range. For instance, a 38 kHz unit can provide valuable information over greater distances but with lower resolution.
3. Interface and Power Requirement: The devices should be compatible with the ROV's control system and power capabilities.
4. Data Quality: Good quality data increases navigation accuracy and the effectiveness of ROV operations.
Remember that DVLs perform best in environments with enough particulates in the water or a rugged seabed for effective signal reflection. Their performance may be affected in very clear water or a smooth seabed.
Integrating a DVL with an ROV significantly enhances navigational accuracy, improving safety and efficiency of underwater operations.
By understanding these devices, users can harness their full potential for tasks such as mapping the ocean floor or inspecting underwater structures. As we continue to explore our oceans, technologies like the DVL remain integral to our efforts.
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.
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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.
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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.
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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
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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.
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Test B: Station keeping drift
Benchmark: Measurements taken to quantify drift over a set time.
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Setup: The DVL was mounted to an 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.
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Test C: Station keeping in a strong current
Benchmark: Visual observation of ability to maintain position while in strong current.
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Setup: The DVL was mounted to an 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.
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Test D: Reliable navigation
Benchmark: To establish a benchmark for distance covered an RTK enabled GPS system with 7 mm horizontal accuracy was used.
Setup: Setup: The test was carried out on a boat with the DVL mounted on the bottom of a pole and the RTK GPS system mounted on top of the pole.
Performing the test: The boat was driven 295 meters in a straight line at a speed of 2 knots (~1 meter per second). The DVL altitude from the seabed varied between 4 and 9 meters over the 295 meters route.
Results: The distance measured by the RTK GPS system as 29,569.1 cm and the distance measured by the DVL-A50 was 29,530.6 cm. This 38.5cm difference translates to a distance error of 0.13%.
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 will be possible to integrate the DVL with our Underwater GPS. This will be offered as a software upgrade and will feature auto-discovery between the two systems. By integrating the two systems you will be left with an amazing long-term deployment navigation solution.
