[chmod775]

I don't think I could explain it to a small child, given that I don't have a great understanding of it myself. But here's what I could scrape together based on a linked paper[1]:

You use GPS receivers to detect ionospheric disturbances. Ionosphere, coming from the word "ionized", means it consists of charged particles, positive or negative. (Missile) exhaust is mainly neutral molecules, creating a "hole". These ionospheric holes can be detected through the Faraday Effect[2]. By measuring the Faraday rotation of radio signals (like GPS), you can detect these holes. I think this is similar to how polarized light 3d cinema systems work, except it's the radio spectrum instead of light.

[1] https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/201...

[2] https://en.wikipedia.org/wiki/Faraday_effect