Pulse-to-pulse coherent Doppler sonar is an autonomous and powerful tool for velocity measurement in laboratory and field applications. However, coherent Doppler sonar has velocity ambiguity that limits measureable velocity. Some techniques have been developed to extend the ambiguity velocity, but they are not yet enough to eliminate it completely. On the other hand, conventional Doppler sonar, while it does not have velocity ambiguity, has larger velocity error than coherent Doppler sonar. In our research, we combine conventional and coherent Doppler sonars in a method to provide accurate and precise velocity without ambiguity. Three kinds of sonar are introduced: conventional, coherent and combined Doppler. Theoretical errors of conventional and coherent Doppler sonars are explained, and the measurement error of combined Doppler sonar is obtained theoretically. From results based on a simulation, it is clear that under certain conditions, the combined Doppler sonar can provide accurate and precise velocities without velocity ambiguity.