Measuring instruments and methods for current 

Current measuring instruments which measure the speed and direction of the current are acoustic with a measuring principle based on the Doppler shift. If an audio source moves in relation to a receiver, the sound frequency at the receiver is shifted in relation to the frequency of the sound emitted. This shift is called the Doppler shift and measuring instruments which work in this way, are called the ADCP (Acoustic Doppler Current Profiler).

The calculation of current data
A small portion of the audio signal, which the ADCP sends out to the water, will return to the ADCP when it is reflected on the small particles and plankton, which are in the water. The returned audio signal will be Doppler shifted, the shift size  reflects the particles and therefore the water velocity past the ADCP.

Current measuring instruments do not only  measure  the current close to the sensor, but also the current throughout the water column, in other words, the current profile.

Current measuring instruments location
Some of the current measuring instruments are lying on the seabed and send sound signals upwards. The measurements are transmitted into a fixed light close by, from which data are continuously transmitted into a central database.

Other current measuring instruments  are mounted on so-called oceanography buoys. They send sound signals down to the bottom. The data is sent on via the buoy’s antenna.