The Locosys Navi GT-31 or BGT-31 (bluetooth) are made for precise GPS speed measurements. Thanks to the doppler measurements stored in the memory or SD-card the accuracy is very good. The device has a low market price and is really compact and strong, it suits many sports (speedsurfing, icesurfing, icesailing, speedskiing, speedbiking, mountainbiking, windsurfing, etc.). For watersports it is adviced to carry the GT-31 in an Aquapack / Box-It.
The battery lasts 60 hours of action, this makes it very good useable on trips without power. I can use it for weeks without recharge.
Navi link & software updater (on CD with device)
GPSAR (gps action replay) (good for adding session on www.gps-speedsurfing.com)
GPSresults (good for adding session on www.gps-speedsurfing.com)
KA72 (free online speedreader)
GT-31 CD with instructions and software for free
How to setup your GT-31
Amaryllo Trip Tracker
Locosys Navi GT-11
GT-32 the new model (still in development)
The new GT-32 will have 2 chips (so measures the same amount of data 2 GT-31 devices) plus 3 accelerometers. All to eliminate an chance of spikes.