Pure coherent Doppler systems - how far can we push it?

The excellent space-time resolution of pure coherent Doppler system has led many scientists to select this processing technique to solve his or her measurement problem. There is nothing in the acoustic Doppler world that can beat the information content of a pure coherent system. The high vertical and temporal resolution give an instantaneous picture of the flow field with photographic clarity; slow moving lake flow looks like something out of a physics text book and boundary layers come alive as you observe bottom sediments being ejected into the flow. Still, in the transition between the scientific prototype and the commercial implementation we tend to stumble. Instead of being a commonly used approach to oceanographic observations, successful deployments using pure coherent techniques are disappointingly few and far between. When we want to explain why, we quickly point to the inherent ambiguity problem of the pure coherent technique, which can be expressed as a limitation on the product of the profiling range and the unambiguous velocity we can resolve. In other words, the faster the flow, the shorter the profiling range, which effectively limits the technique to low-energy environments. Since high energy environments generally are the most interesting, the push is to operate where ambiguity limitations are exceeded. This has again led to the search for "ambiguity resolution schemes" that allow us to measure phase shifts larger than plusmnpi. Mathematically, this is not a hard problem to solve and methods for solving the ambiguity problem were suggested by Lhermitte as far back as the early 1980s. In the lab, with tow carriages moving back and forth across a quiet body of water, these schemes work well. When measuring in the field, however, these schemes have at times failed miserably, much to the chagrin of both the scientists and the support departments at commercial vendors. This paper provides an overview of the key operational elements of the pulse-to-pulse- coherent system and addresses some recent findings that explains what previously has been unexplained differences between laboratory and field experiments. It also details some of the performance limitations we should realistically expect with pure coherent systems.