Squadron had to send two Lysander aircraft in what they termed "a double" if larger parties needed to be picked up. 138 Squadron and other Special Duties units operations No. 138 Squadron's primary mission was the delivery of equipment, and occasionally agents, by parachute. It flew a variety of bomber-type aircraft, often modified with extra fuel tanks and flame-suppressing exhaust shrouds: the Armstrong Whitworth Whitley until November 1942, the Handley Page Halifax and later the Short Stirling. The Stirling could carry a very large load (18 containers) but the aircraft with the longest range was the Halifax, which when based in Italy could reach drop zones as far away as eastern Poland. Stores were usually parachuted in cylindrical containers. The "C" type was long, and when fully loaded could weigh up to . The "H" type was the same size overall but could be broken down into five smaller sections. This made it easier to carry and conceal but it could not be loaded with longer loads such as rifles. Some inert stores such as boots and blankets were "free-dropped" i.e. simply thrown out of the aircraft bundled together without a parachute, often to the hazard of any receiving committee on the ground. Locating and homing equipment Some devices used by SOE were designed specifically to guide aircraft to landing strips and dropping zones. Such sites could be marked by an agent on the ground with bonfires or bicycle lamps, but this required good visibility, as the pilot or navigator of a plane had not only to spot the ground signals, but also to navigate by visible landmarks to correct dead reckoning. Many landings or drops were thwarted by bad weather. To overcome these problems, SOE and Allied airborne forces used the Rebecca/Eureka transponding radar, which enabled a Hudson or larger aircraft to home in on a point on the ground even in thick weather. It was however difficult for agents or resistance fighters to carry or conceal the ground-based "Eureka" transponder equipment. SOE also developed the S-Phone, which allowed a pilot or radio operator aboard an aircraft to communicate by voice with the "reception committee". Sound quality was good enough for voices to be recognisable, so that a mission could be aborted in case of any doubts of an agent's identity. Sea SOE also experienced difficulties with the Royal Navy, who were usually unwilling to allow SOE to use