What we are doing is we’re scattering radio signals, we’re bouncig radio signals off free electrons. Electrons are very small, so the signal we get back is very weak. That’s why it wasn’t thought that sort of thing could be measured until after the second world war. For example here if you look at the signal, you will just see noise. Much of the noise comes from the milky way, the radio stars, and the receiver itself, but a very small fraction of it is scatter from the ionosphere. It just looks like noise. But if you repeat this experiment a hundred times and average over many seconds or signals you can actually subtract a signal from these electrons. 

— Michael Rietveld, senior researcher at Tromsø EISCAT in discussion with HH and AM, 2019, EISCAT facility in Ramfjordmoen (see Radar)

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