hcs_$reboot writes: Airplanes use currently the (un)famous pitot tubes to get an estimate of their current speed. Unfortunately the tubes may suffer from the very low temperatures at high altitudes, and send wrong data to the captain — that may lead to a crash in the worst case scenario. Australian engineers offer a brand new technology to calculate the plane speed: two lasers beams are emitted in opposite direction, the difference in frequency gives the speed, thanks to the Doppler effect. Link to Original Source
We warn the reader in advance that the proof presented here depends on a
clever but highly unmotivated trick.
-- Howard Anton, "Elementary Linear Algebra"