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!
To me this is a wonderful example of the power of mathematics and science to go well beyond our normal intuition of how the world around us works works (which was the normal way things were done until only a few hundred years ago).
The fact that we are able to measure things globally indirectly and accurately based on our understanding of physics, hypotheses we make, and then apply those experiments is very cool.
Never mind the marvel of GPS that we've just taken for granted for decades...
I wonder if someone theorized that tsunamis were detectable by this method or whether someone trying to debug these disturbances figured out what they were caused by. Not sure which is more impressive.
Is this how it's measured? Just to see if I got it right:
The atmosphere directly above the tsunami will have a different TEC (total electron count) pattern due to the upward acoustic waves created by the tsunami waves. This patch of atmosphere may or may not be in the line of sight of your many GPS receivers, to some satellite. Those for which it is in the line of sight will show a disturbance. Others won't. You can now cross-compare to "triangulate" where the tsunami waves are.
You've got exactly the right idea, except "cross compare" is underselling it :)
Here's a previous thread on this topic[0].
For each (receiver, satellite) pair, you can calculate the TEC along the signal propagation path by comparing the time of flight of two carrier waves (e.g. L1 and L2)[1].
By fusing the data from each line of sight together you can get a rough, real time, 3D (4D) model of the ionosphere. Then, you have a separate problem of identifying ionospheric anomalies in the model and relating them to phenomena like earthquakes.
[0] https://news.ycombinator.com/item?id=42441772 [1] https://news.ycombinator.com/item?id=42471052
As I read the article. The tsunami wave (water) displaces air at the surface and creates a sound wave, and gravity waves, that travels to the upper atmosphere. These waves then interact with electrons in the upper atmosphere.
> and gravity waves, that travels to the upper atmosphere
You spoke correctly. But to further clarify, these are gravity waves, not gravitational waves.