I am looking for an official description of the location system called "Darky" used by Allied aircraft over England during the years before GEE came into use. I have run across its mention in a few veteran narratives as a sound based network for location and vectoring of aircraft in low visibility situations attempting to find their destination. Does anyone have a detailed description and a time period for which it was the main tool for helping lost or visibility compromised planes find a suitable landing site?