In the 1920’s and 30’s, before RADAR was developed, there were various attempts at developing an acoustic system that literally listen to the sound of approaching planes. An example of this can be seen in King Ottakar’s Scepture.
In the UK, massive concrete acoustic mirrors were built that focused the sound into a microphone, allowing planes to be detected earlier. Now these crumbling structures are to be saved as national landmarks.
Its amazing that technology Herge used as cutting edge science in his early books is now needing preservation orders and listing as a national landmark.