of a transponder system combined with existing radars to allow accurate bombing or delivery of supplies or troops by parachute, a role that would almost always be carried out by twin-engine aircraft or larger. If this broadcast on the 200 MHz frequency then being used by many British radars, any aircraft with AI or Air-to-Surface Vessel radar could pick it up. To illustrate the concept, Brown gave them a small transponder and told them to hide it anywhere within of Army Co-operation headquarters in Bracknell. One of the TRE aircraft from RAF Christchurch would attempt to find it and fire a smoke signal within of its location. The test was carried out on 28 July 1941, and while they waited for their aircraft to arrive, another aircraft approached the hiding spot and flew around several times before flying off again. The Army suspected that they had sent this aircraft to spy out the location. Just as Brown managed to convince them they were not spies, their own aircraft, a Bristol Blenheim, arrived and fired a smoke signal only 50 yards from the transponder. To be sure, the Army was told to hide it again in another location, this time choosing to place under a tree on the lawn of their headquarters. Their Blenheim once again easily found it. It was later learned that the first aircraft was from the Fighter Interception Unit who saw the odd blip on their radar and decided to investigate. In spite of the successful demonstrations, and enthusiastic support from Barratt and others, no orders for a transponder system were immediately forthcoming. A visit to the airborne headquarters at RAF Ringway would ultimately result in orders for both the UK and US, but those would be some time in the making. SOE A more immediate outcome of the visit to Ringway was an invitation for Brown to meet a secret group known as the Special Operations Executive (SOE) at Whitehall. Brown arrived to find this was not actually their office, and had to prove his identity before being told the real address was quite a distance away. When he finally arrived at Baker Street he was not impressed. SOE then explained the problem they were having dropping supplies to partisans operating across Europe, as far away as Poland. Brown explained that their beacon could be seen as far as under good conditions, but that might drop