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conan said..
What is doppler?
Conan this deserves an answer.
Basically the earlier GPSs calculated speed from the positional data, this lacks accuracy because the position is low definition it's only accurate to meters.
So later units use doppler, this is the same principle of radar speed guns, as you move relative to a frequency source, the frequency changes, it drops as you go away from it and increases as you approach. The faster you do this the more the effect. A common example is a police or ambulance siren, the note changes as the vehicle passes you, and changes from approaching to departing.
That's why there's the d and t options on the site, it denotes whether you're using an old legacy unit that can overstate your numbers by 2 knots or so, or a newer unit that is potentially more accurate.
So for the GT31 you should select the "d" option on your posts. (unless your software only calculates in trackpoints, but then you should be using better software).
The only possible exception is distance, some of the software uses the trackpoint info and others the doppler data to do this.
OH YES, big congrats on that NM!
Must admit I thought you must have got the gust of the day to get a NM so close to the 2s, and from everybody else's comments about that day's sailing you obviously did.