Nortek instruments use the acoustic Doppler principle and measure current speed and direction by transmitting high-frequency sound waves into the water column, from below or above the measurement area of interest. The instrument can either measure at one level (Current Meters) or at several levels through the water column (Acoustic Doppler Current Profilers, ADCPs).
The Doppler current sensors estimates the water velocity indirectly by measuring the velocity of the particles that are moving with the water. The instrument measures velocities along its individual beams by calculating the Doppler shift of the returned sound signal that is reflected from the moving particles in the water. The distance to the measurement volume is defined by the two-way travel time of the transmit pulse. The speed of sound is used to convert the Doppler shift to velocity, and a transformation matrix defined by the orientation of the individual beams transforms the beam velocity estimates to Cartesian 3D velocities in a XYZ coordinate system relative to the instrument head.
| \( V = D \times C \times T \) | (1) | |||
| \(D\) | - | doppler shift (ratio) | ||
| \( C\) | - | speed of sound \( [\frac{m}{s}]\) | ||
| \(T\) | - | transformation matrix | ||
The built-in compass and attitude sensor can further transform the XYZ coordinates to Earth referenced coordinates, East-North-Up (ENU). In the following articles each of the components in equation 1 is described in detail.
Updated