The profiling instruments are able to measure velocities at different distances from the transducers by time-gating the received acoustic signal. The sound wave travels with the speed of sound through the water column and as the signal hits the scattering particles part of the acoustic energy is reflected back to the transducer, while the rest of the energy continues further into the water column and is reflected at a later point in time. By measuring the time it takes for the energy to travel two ways one can know the location of the reflection point. With scaling by the speed of sound in water, the duration can be expressed as a distance (meters) from the instrument and each measurement can be assigned to different cells. How the position of each cell is defined is described in the next sections.
The profiling range and spatial resolution is primarily a function of the acoustic frequency. Lower frequency instruments have longer range than instruments using higher frequency; on the other hand, the latter has better spatial resolution. The maximum ranges for specific instruments can be found in the respective technical specifications. Be aware that these are nominal values. In addition to the instrument frequency, the profiling range is also dependent on the scattering conditions and the cell size. As described in the Doppler Effect section we depend on particles to scatter the sound signal to be able to measure velocities with the Doppler Principle. The concentration of scattering materials and the strength of the scattering return from the water will influence the profiling range; more particles reflect more sound and hence give a better range. There is usually more biological activity close to the surface, so generally an upward looking current profiler is likely to get more range than an instrument pointing downward.
There are two parameters that can be used as a criteria for determining the actual range of the instrument, in addition to the limit when the pulse hits a physical boundary or the surface/bottom, amplitude and correlation.
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