> In the lab, Bortolozzo and his coworkers have been able to detect speeds as slow as 20 millionths of a billionth of a meter per second (20 femtometers per second) with a measurement lasting only one second.

I wonder how difficult it is to perform this on a glacier from a moving plane? Measuring relative velocities is impressive, but then how do you know how fast the plane is moving?

Shoot the rocks beside the glacier?

A really big cold angry fist?