only system known to the Germans at the time, determining the location of such a transmission would have required considerable luck. With huff-duff, these messages were more than long enough to easily measure. At first, the UK's detection system consisted of a number of shore stations in the British Isles and North Atlantic, which would coordinate their interceptions to determine locations. The distances involved in locating U-boats in the Atlantic from shore-based DF stations were so great, and DF accuracy was relatively inefficient, so the fixes were not particularly accurate. In 1944 a new strategy was developed by Naval Intelligence where localized groups of five shore-based DF stations were built so the bearings from each of the five stations could be averaged to gain a more reliable bearing. Four such groups were set up in Britain: at Ford End in Essex, Anstruther in Fife, Bower in the Scottish Highlands and Goonhavern in Cornwall. It was intended that other groups would be set up in Iceland, Nova Scotia and Jamaica. Simple averaging was found to be ineffective, and statistical methods were later used. Operators were also asked to grade the reliability of their readings so that poor and variable ones were given less weight than those that appeared stable and well-defined. Several of these DF groups continued into the 1970s as part of the Composite Signals Organisation. Land-based systems were used because there were severe technical problems operating on ships, mainly due to the effects of the superstructure on the wavefront of arriving radio signals. However, these problems were overcome under the technical leadership of the Polish engineer Wacław Struszyński, working at the Admiralty Signal Establishment. As ships were equipped, a complex measurement series was carried out to determine these effects, and cards were supplied to the operators to show the required corrections at various frequencies. By 1942, the availability of cathode ray tubes improved and was no longer a limit on the number of huff-duff sets that could be produced. At the same time, improved sets were introduced that included continuously motor-driven tuning, to scan the likely frequencies and sound an automatic alarm when any transmissions were detected. Operators could then rapidly fine-tune the signal before it disappeared. These sets were installed on convoy escorts, enabling them to get fixes on U-boats transmitting from over the horizon, beyond the range of radar. This allowed hunter-killer ships and aircraft to be