October 1944 to March 1945.. Intelligence reports The codename " 76"—"Flak target apparatus" helped to hide the nature of the device, and some time passed before references to FZG 76 were linked to the V-83 pilotless aircraft (an experimental V-1) that had crashed on Bornholm in the Baltic and to reports from agents of a flying bomb capable of being used against London. Importantly, the Luxembourgish Resistance, as well as the Polish Home Army intelligence contributed information on V-1 construction and a place of development (Peenemünde). Initially, British experts were sceptical of the V-1 because they had considered only solid-fuel rockets, which could not attain the stated range of . However, they later considered other types of engine, and by the time German scientists had achieved the needed accuracy to deploy the V-1 as a weapon, British intelligence had a very accurate assessment of it. Countermeasures in England Anti-aircraft guns The British defence against German long-range weapons was known by the codename Crossbow with Operation Diver covering countermeasures to the V-1. Anti-aircraft guns of the Royal Artillery and RAF Regiment redeployed in several movements: first in mid-June 1944 from positions on the North Downs to the south coast of England, then a cordon closing the Thames Estuary to attacks from the east. In September 1944, a new linear defence line was formed on the coast of East Anglia, and finally in December there was a further layout along the Lincolnshire–Yorkshire coast. The deployments were prompted by changes to the approach tracks of the V-1 as launch sites were overrun by the Allies' advance. On the first night of sustained bombardment, the anti-aircraft crews around Croydon were jubilant—suddenly they were downing unprecedented numbers of German bombers; most of their targets burst into flames and fell when their engines cut out. There was great disappointment when the truth was announced. Anti-aircraft gunners soon found that such small fast-moving targets were, in fact, very difficult to hit. The cruising altitude of the V-1, between meant that anti-aircraft guns could not traverse fast enough to hit the missile. The altitude and speed were more than the rate of traverse of the standard British QF 3.7-inch mobile gun could cope with. The static version of the QF 3.7-inch, designed for use on a permanent, concrete platform, had a faster traverse. The cost and delay of installing new permanent platforms for the guns was fortunately found