identified. As the entire Dowding system of air control relied on ground direction, some solution to locating their own fighters was needed. The expedient solution to this was the use of huff-duff stations to tune in on the fighter's radios. Every Sector Control, in charge of a selection of fighter squadrons, was equipped with a huff-duff receiver, along with two other sub-stations located at distant points, about away. These stations would listen for broadcasts from the fighters, compare the angles to triangulate their location, and then relay that information to the control rooms. Comparing the positions of the enemy reported by the ROC and the fighters from the huff-duff systems, the Sector Commanders could easily direct the fighters to intercept the enemy. To aid in this process, a system known as "pip-squeak" was installed on some of the fighters, at least two per section (with up to four sections per squadron). Pip-squeak automatically sent out a steady tone for 14 seconds every minute, offering ample time for the huff-duff operators to track the signal. It had the drawback of tying up the aircraft's radio while broadcasting its DF signal. The need for DF sets was so acute that the Air Ministry initially was unable to supply the numbers requested by Hugh Dowding, commander of RAF Fighter Command. In simulated battles during 1938 the system was demonstrated to be so useful that the Ministry responded by providing Bellini-Tosi systems with the promise that CRT versions would replace them as soon as possible. This could be accomplished in the field, simply by connecting the existing antennas to a new receiver set. By 1940 these were in place at all 29 Fighter Command "sectors", and were a major part of the system that won the battle. Battle of the Atlantic Along with sonar ("ASDIC"), intelligence from breaking German codes, and radar, "Huff-Duff" was a valuable part of the Allies' armoury in detecting German U-boats and commerce raiders during the Battle of the Atlantic. The Kriegsmarine knew that radio direction finders could be used to locate its ships at sea when those ships transmitted messages. Consequently, they developed a system that turned routine messages into short-length messages. The resulting "kurzsignale" was then encoded with the Enigma machine (for security) and transmitted quickly. An experienced radio operator might take about 20 seconds to transmit a typical message. Had the UK been using B-T systems, the