Silvaire, what kind of "GPS data" would you want to download while airborne? If you mean a GPS receiver supplying raw GPS data to whatever more complex device to interpret and display it, the receiver will draw a max of 200 mA, amounting to +/- 2,5 Watts on a 12V system (which I think hefty already). A transponder will draw a lot more, though. Generally, transmitting takes much more electricity than receiving.