To calculate SNR one needs the return signal strength and the instrument noise. There is no 'noise ping' in the instruments that can be used with Storm. What Storm does is to assume a noise floor as given below. 

Aquadopp 25 counts

Aquadopp Profiler 400kHz 33 counts
Aquadopp Profiler 600kHz 33 counts
Aquadopp Profiler 1MHz 25 counts
Aquadopp Profiler 2MHz 18 counts

AWAC 600kHz 33 counts
AWAC 1MHz 20 counts

Continental 190 kHZ 45 counts
Continental 470 kHZ 45 counts

To get a rule of thumb cut-off at 3dB you need to find the noise floor of your instrument and compensate for this if it deviates from the hard-coded values too much.

We realize this is a bit cumbersome, but SNR is not giving the full picture either. Needed SNR to obtain good current measurements have decreased during the refining of these instruments through the years. Different conditions may require different SNR to give good readings.

A good check of the validity of the data is to check that the vertical velocity is close to zero. Current profiles that show large current variations vertically or vertical structures should be flagged suspicious.  The ones that are most doubtful are those that are physically unrealistic (e.g. 1 m/s changes over a few meters). The measurements in a profile that are far from the mean are best removed.

Example of SNR compensating in Storm/Surge: Your Aquadopp Profiler has a noise floor of 25 counts. Storm assumes 18 counts as noise floor. You will then need to add (25-18) counts*0.43dB/counts=3 dB. If you like to use the 3dB rule of thumb threshold, you actually need to input 6dB

The noise floor of your instrument can be found in different ways

• It is stated in the final test check list that came with the instrument. 
• The amplitude profile will converge to the noise floor. 
• A bench test with the instrument out of water would also give the noise floor.
• Contact Nortek

The db/count relation may also vary a bit between instruments, but 0.43 is an OK estimate.