(I changed the previous, more frivolous, title of this post, which suddenly seemed highly inappropriate to me when I learned that a national tragedy had occurred) Since I failed to find the thread on boat speed of a few weeks ago -- where I made the statement that the accuracy of GPS SOG readings is only based on serial position measurements and that the error of the speed reading is therefore strongly correlated with the error of the position reading -- allow me to bring this topic up here. In short; I was wrong and I do apologize to the group for providing potentially misleading information. The fact is that GPS receivers also use so-called Doppler measurements which provide direct information of the speed vector of the vessel in the direction of each GPS satellite transmitter (i.e. satellite) used. So, the error of the speed reading (and in principle also the heading indication) does not necessarily correlate entirely with the error of the position information. However, in order to translate vessel speed relative to the GPS satellites into speed over ground the GPS receiver needs to know the speed vectors of each satellite relative to the coordinate system (map datum) used, e.g. WGS 84, etc. Since the speed of a satellite changes over time, e.g. due to friction with the residual atmosphere up there, it would appear that GPS systems will need to update the satellite speed information regularly in order to stay accurate. To what extent, the satellite speed updating is integrated with the satellite position update which needs to happen every time we switch on our GPS system, I was unable to figure out. Does anyone on this board happen to have information on the relative ability of modern GPS models and brands to update it's Doppler database? Have fun, Flying Dutchman