The base measurement method for the water velocity in the Nerite sensor is based on Pulse-to-Pulse Coherent Doppler. Two ultrasound pulses are sent at certain PRF (pulse repetition frequency). Coherent doppler is an accurate method for fine velocity resolution with the downside of a limited range. Velocity aliasing occurs when the PRF is less than twice the doppler shift.
In order to account for this limited range of the default coherent doppler method a ultrasound backscattering particle tracking method is employed for phase-tracking of the doppler shift. Finer resolution than the tracking method is obtained by further employing the coherent doppler method.
Both methods combined work when sufficient scatter sites are present in the flow of water for a range of at least 0 to -3.5 m/s (towards transducers) and 0 to 0.5 m/s (away from transducer).