In general, amplitude is a typical variable used to describe the characteristics of a wave. It says something about the amount of energy transferred by the wave and can be seen as a measure of how strong or big the wave is. Regarding acoustics, a greater amplitude means a greater intensity and the wave sounds louder (in the audible frequency range). The amplitude, or signal strength, for current measurements, can be defined as "a measure of the magnitude of the acoustic reflection from water". As the amplitude is a measure of the echoes, it is a function of the scattering conditions. The amplitude is given in the dimensionless units decibel (dB) or counts. The relation between the two units may vary slightly between instruments, but a conversion of 0.5 dB/counts is a good estimate.
ADCP instruments emit longitudinal sound waves into the water and measure the echoes. To receive an echo signal it is crucial to have enough scattering material in the water that can reflect the transmitted signal. When we talk about amplitude for current measurements, we usually refer to the amplitude of the echo signal, which is what the instrument registers. However, the transmitted sound pulse also has an amplitude, telling us something about how much energy is emitted from the instrument. The echo signals reflected from scattering materials usually contain far less energy, i.e. lower amplitude, than the transmitted signal, due to reflection, scattering and absorption of the signal as it travels through the profile. This also implies that the strength of the echo signal decreases with distance from the instrument because there is less energy to reflect. This decrease follows the Sonar equation and will look similar to the left amplitude profile in Figure 1. After a gradual decrease in signal strength, the amplitude reaches a constant limit known as the noise floor. At this point, the instrument only measures noise, and the standard deviation becomes large. All instruments have an individual noise floor due to internal electronic noise inside the instrument. The point where the signal strength profile reaches the noise floor value therefore determine the profiling range. Beyond this point, noise is dominating the signal and the data should be discarded. For further details on quality control measures, please see these guidelines.
How far the instrument can measure before the signal strength reaches the noise floor depends on several factors. The type and amount of scattering material play a decisive role. The upper layer of the water column normally contains more scattering materials, which implies that an upside-looking instrument might have a longer range than an instrument measuring from the surface and down. The range is also affected by the instrument frequency, where lower frequencies generally enable longer ranges. Cell size is also a factor when it comes to the maximum profiling range, as the cell size determines the length of the transmit pulse. Bigger cell size means longer transmit pulses and more data points within each cell. A drawback is that increasing the cell size reduces the spatial resolution. Increasing the power level means more energy is emitted, which is useful when a longer range is desired. A consequence of this is higher power consumption and possible shorter deployment. The configured profiling range also depends on chosen blanking distance (start of profile) and the number of cells. A maximum profiling range is given in the technical specifications of every current profiling instrument. Keep in mind that this is a nominal value, with the possibility that the actual values obtained could be longer or shorter.
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