Originally Posted by bocastephen

...You don't even need a GPS - an iPhone with a compass (or heck, just a compass), and a simple formula of 'distance=speedxtime', which can be flipped around to solve the missing variable. It's not hard to look up the average ground speed for your flight (and even view historical routes), then you can solve for time and distance, and use the compass to give a decent estimation of your position and remaining time/route of flight...