It's not well-known, but the British tracked the Germans down to the unit level using radio transmission triangulation, and plotted their positions across Europ

It's not well-known, but the British tracked the Germans down to the unit level using radio transmission triangulation, and plotted their positions across Europe on giant maps. Even without decrypting signals, they knew a great deal. The Germans were the first army to have radio on every vehicle, sometimes 3 for higher ranks. It was handy on the go, but leaked intelligence like a sieve when stationary for days, and the Germans were overconfident in its secrecy. The British were warned of air raids even before takeoff because German pilots followed a procedure of testing their radio on the ground. The mastermind behind the British effort built a similar system, but global, for the US, after WW2. SR-71 flights initially used radios, but since Russian "trawlers" were tracking the broadcasts, later flights, including multiple refuelings, were done in radio silence.