Some types of Distance measuring equipment have accurate speed readings. I've used this method myself, in the past, especilly if you need to fly at accurate constant speeds. Don't fly too close to the transmitter though, the nearer you get the more inaccurate the speed readings become. Best is to fly up a radial to a VOR/DME about 30 miles away. If you want airspeed you have to fly in both directions and take a mean. Both GPS and DME gives the ground speed.