[KenWittlief]

I guess I should have been more specific. In an aircraft its impossible to measure your ground speed directly. GPS also reports ground speed, but that is measured relative to satellites in orbit, with known positions and velocities - its not measured relative to the ground. If the satellites fail (or if your VOR points on the ground fail) then the aircraft cannot know its ground speed.

There is no instrument you can put in a aircraft that can directly measure the speed vector of the groud under the plane by itself.

By contrast, a car measure ground speed directly, by the rotation of its wheels.