when attack was imminent. This greatly reduced pilot and aircraft fatigue. Very early in the battle, the Luftwaffe made a series of small but effective raids on several stations, including Ventnor, but they were repaired quickly. In the meantime, the operators broadcast radar-like signals from neighbouring stations in order to fool the Germans that coverage continued. The Germans' attacks were sporadic and short-lived. The German High Command apparently never understood the importance of radar to the RAF's efforts, or they would have assigned these stations a much higher priority. Greater disruption was caused by destroying the teletype and landline links of the vulnerable above-ground control huts and the power cables to the masts than by attacking the open latticework towers themselves. To avoid the CH system, the Luftwaffe adopted other tactics. One was to approach the coastline at very low altitude. This had been anticipated and was countered to some degree with a series of shorter-range stations built right on the coast, known as Chain Home Low (CHL). These systems had been intended for naval gun-laying and known as Coastal Defence (CD), but their narrow beams also meant that they could sweep an area much closer to the ground without "seeing" the reflection of the ground or water – known as clutter. Unlike the larger CH systems, the CHL broadcast antenna and receiver had to be rotated; this was done manually on a pedal-crank system by members of the WAAF until the system was motorised in 1941. Ground-Controlled Intercept Systems similar to CH were later adapted with a new display to produce the Ground-Controlled Intercept (GCI) stations in January 1941. In these systems, the antenna was rotated mechanically, followed by the display on the operator's console. That is, instead of a single line across the bottom of the display from left to right, the line was rotated around the screen at the same speed as the antenna was turning. The result was a 2-D display of the air space around the station with the operator in the middle, with all the aircraft appearing as dots in the proper location in space. Called plan position indicators (PPI), these simplified the amount of work needed to track a target on the operator's part. Philo Taylor Farnsworth refined a version of his picture tube (cathode ray tube, or CRT) and called it an "Iatron". It could store an image for milliseconds to minutes