Have you ever flown a DJI drone? If you have, then you’ll know that if you take your hands off the controls, the drone will stop and hold it’s position exactly where you left it until you again actuate the control to make it move-on. A Doppler Velocity Log (DVL) has the potential to do just that for an underwater ROV.
What is a Doppler Velocity Log (DVL)?
For those operating beneath the surface, the process of estimating the subsea position has a distinct challenge not found in terrestrial navigation since GPS navigation does not function underwater.
The Doppler effect (or Doppler shift) is the apparent change in the frequency of a sound, caused by relative motion between the source of the wave and the observer.
An example of a Doppler shift is the pitch difference heard when a car playing its horn passes an observer or a plane or a train passes us by. Compared to the emitted frequency, the received frequency is higher during the approach, identical at the instant of passing by and lower during the recession.
DVL uses Doppler shifting of acoustic signals to compute velocity relative to the seafloor. DVL is a crucial part of subsea navigation by offering an accurate estimate of velocity referenced to the earth.
How does DVL work?
DVL transmits a pulse with a minimum of three acoustic beams, each pointing in a slightly different direction. The reflected pulses are used to produce estimates of velocity in several axes of the DVL platform coordinate system, which are combined with estimates of DVL platform orientation from an internal Inertial Measurement Unit (IMU). The velocity estimates are integrated into position estimates in the IMU’s coordinate system.
Most Doppler velocity logs use a Janus configuration, named after the Roman god who looks both forward and backward, where a transducer pointing ahead measures speed. A sensor pointing astern is used for checking accuracy, and sensors looking abeam measure athwartship speed. Typically, the beams are only 2 to 30 degrees off vertical referenced from a level DVL platform.
So with a DVL integrated with the INS and the ROV’s control software, it is now possible to stabilize the platform, e.g. to drift from currents, and hold position relative to the sea floor just like those aerial drones do.
Who else uses DVL?
In recent years, navigation for autonomous underwater vehicles technology has progressed significantly in capability. Navigation has remained one of the main hurdles limiting the full potential of AUVs. Doppler Velocity Logs have become a reliable solution increasingly for tackling AUVs navigation requirements.
Applications requiring subsea navigation can vary from diver-held guidance systems to larger autonomous undersea vehicles conducting high-accuracy seafloor studies over vast distances.
Due to large size and high cost, the use of DVLs have been historically limited to larger and very expensive vehicles where the cost could be justified. The upcoming Cerulean Sonar offering will be small enough to deploy on a low cost commercial grade ROVs like the BlueROV2 and will set a new benchmark for low cost. Watch for Q2 2020 launch!