U.S. Geological Survey Open-File Report 2007-1194
Figure 14. A SonTek pulse-coherent acoustic Doppler profiler (PCADP). The PCADP measures water flow from the Doppler shift that occurs when scatterers carried by the water reflect paired acoustic pulses along three acoustic beams. PCADP measurements of water flow in along-beam regions as small as 1.6 centimeters offer increased spatial resolution over single acoustic pulse instruments (like ADCPs).