This is unreal and beyond cool. Instead of measuring seismic tremors, it measures, indirectly, the displacement of the ocean surface. Far from the shore, tsunami waves are very long and very low - so they would appear as the whole surface of the ocean going up and down.

That motion pushes air upwards, resulting in a wave reaching high up through the atmosphere, eventually hitting (!) the ionosphere. I didn't even know acoustic waves would propagate through ionised gas!

Finally, this ionosphere disturbance affects GPS signal reception, and can be measured via ground receiver stations.

The upside of this is that it measures, indirectly, motion of the sea, i.e. actual tsunami activity, rather than monitoring directly the potential causes thereof.

It is crazy to me that it works though!