Modern acoustic Doppler current profilers (ADCPs) can measure the complex movements of currents in oceans, rivers and lakes with great accuracy. But to take those measurements, you need to attach your device to something – and in the middle of a river or at sea, away from the shore, the easiest place to put it may be on the underside of a vessel.
What else do I need for vessel-mounted current measurements?
You need to know where you are and how the instruments are oriented. So, you need a global navigation satellite system (GNSS), such as GPS, for positioning, an electronic compass and a pitch and roll sensor. Many vessels have these systems on board and you can also get combined units for all signals.
Knowing the speed at which sound travels through the water you are measuring is also important, as that affects the ADCP’s accuracy. The speed of sound is affected by the salinity and temperature of the water, so you will need those measurements.
You will also need accurate time-keeping. You need to know how the timing of your measurements relates to changes in the water level or the tide. In addition, you must know what the time lag is between the ADCP and any auxiliary sensors such as the GNSS – modern systems can close the gap to milliseconds or even less.
What will the current measurement data look like?
There will be a lot of data – and it may well be very colourful! Typically, you will view your data as current vectors (arrows) on a map or in Google Earth.
You will be able to look at the distribution of current speeds as a function of along-track distance and depth below the water surface. There is also the option to display along-shore currents in a graph etc. Finally, you can use a series of these graphs over time to create an animation.
Others may need to access your data too – perhaps your boss wants to analyse it for a model he or she is developing, or a port authority may need a current map. So, you have to be able to deliver the data in a format which is accessible to other staff, such as geographic information system (GIS) mapping.
What’s the end-result for users?
The best ADCPs on the market provide great data. Both the bottom track data and the measured currents are accurate, as well as robust.
The best ADCPs combine this with a modern GNSS sensor, which also gives you heading, pitch and roll and – on top of that – has the most accurate timing option, known as Precision Time Protocol (PTP).
In addition to an ADCP, it would also be advantageous to add an altimeter, distance measurement on the vertical beam and a super-resolution echo sounder for echo profiling.
Combining all that with a mounting bracket, a connection box and a computer you will have a coherent, comprehensive and easy-to-use package.
Can I operate vessel-mounted current measurement technology myself?
Yes, you can. The best systems are coherent and quite easy to use.
The acquisition software is designed to automatically communicate with the ADCP and the GNSS. The software will store all data, continuously provide updates on the project and will ask you to make a decision, if one is needed.
You can transfer the data into a flexible data processing package with just a few mouse clicks.
What’s next for vessel-mounted current measurement?
The potential to use unmanned vessels, or even autonomous vehicles, offers the possibility of lower cost measurements and the opportunity to harvest even more data.
Improvements in data processing techniques should make analysis easier and further improve data quality. Ever-increasing computer power should allow more tasks that once had to be done on shore to be carried out on the vessel.
More sophisticated use of the internet should allow more people, whether on the vessel or onshore, to monitor measurements in real time.
Measuring currents using acoustic Doppler equipment is already highly accurate and much easier to do than a few years’ back. Expect it to get faster, simpler and even more accurate in the future.
Instruments in use
Vessel-mounted current profiler
Signature VM Coastal
The Signature VM package delivers vessel-mounted AD2CP capabilities based on present-day technology