Airborne DF has existed for over 50 years. We had the facility on the old HS Argosy to home in on a buddy using a UHF frequency. He transmitted (a sprung loaded 1000 kcs tone switch was also supplied for this purpose) and the homing aircraft followed an instrument just like a zero reader which gave a fly left/right and a fly up/down indication.
The equipment was known as "Violet Picture".
Needless to say, we could find an ELT (we called them SARBE) with great accuracy. I can remember doing an exercise at Thorney Island. The ELT was inside a hangar and we were even able to tell them which corner of the hangar it was in!