Posted the following in the thread in the main forum before I saw this thread. Apologies for repretition but maybe everyone is not reading the main forum. Also apologies for repetition of the device info from previous posts in this thread.
A good place to read about and get in contact with windsurfers using GPS is http://www.seabreeze.com.au/forums/Windsurfing/Gps/
The organisation that approves GPS devices for speed measurements and records is GPS Speedsurfing http://www.gps-speedsurfing.com/default.aspx
A good device to look at is Locosys Genie GT 31 http://www.locosystech.com/product.php?zln=en&id=30. This is the most commonly used device for measuring speed for windsurfers.
This guy has developed a custom firmware specialised on windsurfing for the device http://mtbest.net/setting-up-navi-GT31.htm where I am sure you can draw some conclusions on required features and algoritms etc.
Thanks for valueable information.
We have read and plan to design specified alogirthm to cut in this market.
There is thread to discuss this topic, let's have deep discussion over there.