what is the size, weight and power consumption of the on-board hardware required to download GPS data? And how does that compare to the size, weight and power consumption of a transponder?
I'm currently on a weeks glider course. The gliders we fly all have a single lead-acid battery, weighing about one kilogram. This powers the mode-S transponder (which is not properly used here
and left in ACS mode - the equivalent of ALT - all day long, even when on the ground), the radio (which also doesn't see much use
), Flarm (the glider variant of TCAS, purely based on GPS position broadcasts) and the vario. Battery capacity is somewhere around 12 hours at least.
So we're not talking about excessive weights to carry transponders and the like, even in an unpowered aircraft.