Before there were radars, this was how they listened to their enemies
It’s probably not surprising to hear that there used to be a time when people did not have radars at their disposal. As a result, armies could not rely on them in the midst of a battle. Still, that did not stop armies or governments from coming up with other clever replacements.
When armies needed to know where their rivals were, they went for listening devices and acoustic mirrors to detect the characteristic noise from the unidentified aircraft engines. These devices were operated by special soldiers that had prior training to identify the foreign sounds and pinpoint the direction from which they came.