mention distance between the CTD and the SUNA window

[soerenthomsen]

@orensDef writes:

"I would recommend to mention the distance between the CTD and the SUNA window. Indeed, T/S data are often used in post-processing. When crossing strong gradients, a vertical lag between both instruments can leads to spikes in computed nitrate concentrations (e.g. erroneous salinity compensation)."

[soerenthomsen]

to keep the comments below each side organised please continue the discussion here:

@gkrahmann answered:

"Interesting aspect. I will add that. For Slocums I expect this to be relatively small. There is mostly a vertical distance of 15 cm. Which is about 1 second. CTD is typically recording only every 4 seconds and we run our SUNAs with something like 5 to 15 seconds sampling time. So there will be larger errors due to the required interpolation anyway since steep gradients are not fully resolved. Have you ever tried to calculate how large the problem is ? Meaning creating artifical data with a say 0.5 PSU over 1 m gradient and then shift that in time by 1 to 5 seconds while applying the TS correction to one and the same spectral data..."

[gkrahmann]

I have added some sentences and created a PR