Principles of Operation
Acoustic Doppler instruments operate by transmitting a short acoustic pulse from each of its multiple transducers. The pulse is transmitted at a known frequency and propagates through the water along a beam. The pulse then reflects from particulate matter suspended in the water and back to the transducer. The water, or particles (assumed to have same velocity as background water mass) speed relative to the transducer causes a change in the received frequency. The change in frequency is known as a Doppler shift (Wikipedia description). Measuring the Doppler shift provides a means of directly estimating the water velocity relative to the transducer.
Illustration of a pulse transmitted by a single transducer and then reflected acoustic energy back to the transducer from particulate matter in the water. The illustration also shows a Doppler shift, as the pulse is compressed to a higher frequency upon return.
At this point the water velocity measurement is a vector velocity in the direction of the transducer beam. This means that in order to characterize the three dimensional flow characteristics, there has to be a minimum of three beams. The three vector components, which we refer to as beam coordinates can then be converted to an XYZ coordinate system (that of the platform or boat) by knowing the beam orientation. Furthermore, the flow can be presented in Earth coordinates by using tilt and compass sensors. This is necessary to report boat velocity as the two components -- forward speed and leeway. As an aside, one should note that the DVL used by Puma’s Mar Mostro is a four beam sensor; it is over-determined and means that three of the four beams may be used.
A more detailed description is found in the following tutorial for the technically inclined.
At a first glance, this may seem complex, but it is a well-established technology and methodology in the marine science field. More details about the use of the technology for marine science applications can be found on current profilers and velocimeters.
It is worth noting that the principle in which acoustic Doppler sensors operate is different from travel time acoustic sensors, which had a short existence a number of years ago in the sailing community. Acoustic travel time sensors estimate speed by measuring the time of travel for an acoustic pulse over a known distance. The travel time method has its benefits in controlled settings, however it never took hold due several technical challenges presented by sailboats.
