Figure 11. An Aquadopp Acoustic Profiler (AP). The AP emits pulses of energy that are reflected from scatterers suspended in the water. The return pulse has a change in frequency called a Doppler shift. Because Doppler shift is proportional to the component of water flow along the beam, trigonometry is used to convert the returned acoustic signal into eastward, northward, and vertical components of water flow. (Photograph courtesy of Nortek.)