Pitot tubes
Pitot tubes can be clogged by dust, water or ice cristals because air flows inside from their inlets to their drain holes.
I imagine a tube w/o a drain hole, fed from the aircraft by a variable presssure air pump, controlled to deliver a fixed air flow. The internal pressure is a function of the outside total pressure at the tube inlet, therefore the air speed can be measured.
Side benefit: a simple test on the ground can detect clogging by insects or scotch tape.