Yesterday I swam 2400 m, did the GPS soak on beforehand and at the turn point I waited for GPS signal and also at the end before quitting. Thought it was strange that as I was swimming out it read 1014 m at the turn point but 2440 m when I returned. And the speed! Just faster and faster! So I checked with a walk along the shore with the same GFR945 and it measured 700m each way, totalling 1400m. A margin of error of >1000m! What is wrong?