service, but a number of problems greatly delayed their entry. Added as they became available, this produced a profusion of different Marks, detailed below. Late in April 1942, during a test flight of V9977, the prototype unit was shown to Flight Lieutenant E. Dickie, a navigator. Dickie pointed out that navigational charts were always produced with north at the top, while the PPI display of H2S had the top of the display representing whatever direction the aircraft was flying. He suggested that this would cause significant problems during navigation. This had not been considered before because H2S had been developed as a bombing aid. Now that it was also used as an important navigation aid, this was a major issue. This led to a crash program at EMI to modify the prototype sets with a system to correct this problem. This was solved with the introduction of a selsyn (or "servo") connected to the aircraft's gyrocompass, whose output modified the scan rotation. A further addition produced a bright line on the display indicating the direction of travel. A later modification allowed the heading indicator display to be manually controlled by the operator. This was used in concert with the Mark XIV bomb sight to accurately correct for any wind blowing the aircraft off the bomb line. The indicator was set to an initial angle provided by the bomb aimer, and from then the navigator could see any residual drift on his display and call out corrections to the pilot, and to the bomb aimer who would update his settings in the bombsight. This basic idea was later expanded to allow the navigator's measurements to be automatically sent back to the bombsight, meaning the bomb aimer no longer had to do this during the approach. Since the other settings, like altitude and airspeed, were already automatically fed in from the aircraft instruments, this left only the selection of the elevation of the target over sea level to be set manually, which could be done before the mission. The other problem was that when the aircraft rolled, the signal hit the ground only on the lower side of the aircraft, filling one side of the display with a solid signal while the other side was blank. This was particularly annoying because it was during the last minute of the approach to the target that the navigator would be giving course corrections to