That is not true Darryl. The shift is not that quick; it takes a while for the position to swing about significantly while a typical record speed run will be done in under 50 seconds. In that time frame the shifted is neglectable. Also on the such a short 500 mtr stretch the position error is as good as the same at the start and finish so the COVERED distance will be 500 mtr with a very high accuracy. Otherwise the differential GPS methode like you describe can't work, you should have know that as a certified landsurveyer.

Sure the WHOLE course may move about by 5 mtr on the GPS during the hours of all the record attemps but that is pretty unimportant. All you need is a central bouy, a stop and go at the bouy and the assigment to all sailors to hold their max speed for at least 60 seconds. The GPS travelled distance readouts (not momentary speed !) from stop-and-go to the 500 mtr mark after that can then be used to calculate speed with an accuracy of at least 2 decimals.

Simple, easy and can be done by everyone with a GPS with a PC port and tracking ability (garmin 201 Geko = 145 US$)

Wouter