Signal-to-noise ratio (SNR) is a normal measure to compare desired signal strength to background noise. For ADCP instruments this is to look at the ratio of received echoes and the instrument-specific noise floor. To maintain good data quality, we recommend as a rule of thumb that the signal strength should be at least 3 dB higher than the noise floor and that data with signal strength lesser than this should be discarded.
To have sufficient signal strength, the water must contain enough scattering materials that can reflect the transmitted signal. When the concentration of particles is low, it can be difficult to make valid velocity measurements. Seasonal variation of scattering materials and diel vertical migration of zooplankton are two typical reasons for low SNR.
During winter, especially in polar regions, the amount of suspended particles decreases. This naturally results in a problem when measuring under these conditions. Diel vertical migration is a movement pattern that zooplankton uses. They are found higher in the water column at night than during the day. This can be seen with diurnal variation in amplitude data, as shown in Figure 1 below. SNR must be considered in both situations to assess the quality of the data. Decreased signal strength means increased standard deviation due to noise.
Figure 1: Diel vertical migration of zooplankton. The signal strength varies daily depending on where zooplankton is located in the water column.
Some measures can be initiated in an attempt to increase the SNR. If you have an issue with low SNR, you should be sure to have the Power level set to the highest (High or 0 dB) as this determines the amount of energy that is sent into the water. This requires higher power consumption but a shorter average interval and lower measurement load may be considered to compensate. Increasing the cell size will provide more data within each cell, but reduces the spatial resolution.
There is no way to improve data suffering from low SNR, it simply does not contain enough information. Rather, it is almost just noise, and should for that reason be discarded. Nortek post-processing software offers easy methods to discard data due to low SNR. A low SNR threshold can be chosen in the processing settings, where 3 dB is set to default. (If you are processing data in Storm or Surge, the following FAQ may be helpful: How does Storm calculate the Signal to Noise ratio (SNR)?)