With all the recent discussion regarding GPS accuracy, I have had a bit of a closer look at what Real Speed actually shows when looking at SBN files from the GW60.

I have a few questions
What are the units of HDOP?
What are the units of SDOP?
I assume the Doppler error is in knots, but what exactly does this mean? If my speed is say 30kt and my doppler error is 0.3, What is the level of certainty that the true speed was between 29.7 and 30.3?