Check this out........its the size that GPS receivers have been recently reduced to
So you would think something less bulky & tougher would be developed, screen/digit readability would be the limiting factor I would think.
A GPS needs to be mounted on the board for 2 reasons, 1, they pickup arm movement, so can give eratic speeds, 2, if mounted on the board in view, your paddle stroke can be optimised almost instantly.
I beleive, unless anything has changed, any altitude function is useless, as the GPS signal is inacurate in the vertical....thats why aircraft cant use it?
A great feature for sup use would be VMG, (velocity made good) to a way point. then you could see how straight & fast a board realy goes.