ABSTRACT
In order to increase the signal-to-noise ratio of Doppler velocimeters, several authors have proposed to use the emission of a sine wave phase modulated by a pseudorandom binary sequence with an on/off ratio equal to unity. The velocity measurement of the scatterers located in the range cell volume may be perturbed by all the other reflecting clutter targets which are moving in the ultrasonic field. The ratio between the Doppler signal issuing from the range cell and that from the mobile clutter targets will be defined as the signal-to-mobile clutter ratio (SCR)m. We have evaluated the (SCR)m by computing the ambiguity function of the pseudorandom binary sequence as a function of the vessel diameter, the number of phase states and the normalized particle velocity. The results show: for high velocities, the (SCR)m decreases rapidly; an increase of the number of states of the code does not entail a proportional improvement in the (SCR)m because of the simultaneous decrease of the range cell; it is confirmed that pseudorandom flowmeters perform better than pulsed Doppler systems for small, even deep-lying, vessels. However, they do not allow for a precise velocity measurement in the case of vessel diameters larger than three centimeters.
