Why should you need to define ground speed in an aircraft anyway? You should be moving through the air (if you aren't, you've got bigger problems than this), and the air tends to move relative to the ground. You can calculate an approximate 'ground speed' using normal navigation techniques, and figure out if you will probably arrive at where you are trying to get to before you run out of fuel, the bar closes, or whatever priority you choose to set. Since you never really know exactly what the air is going to do relative to the ground (butterflies in Amazon rainforests, amongst other things, stop you knowing for sure), you can only ever rely on approximations. This doesn't prevent aircraft from being useful - in fact, I'm not sure having an exact definition of what 'ground speed' consists of is of any significance at all as far as getting from one point on the ground to another without colliding with somewhere in between is concerned...