Panzer I it still performed much useful service against entrenched infantry and other "soft" targets, which were unable to respond even against thin armor, and who were highly vulnerable to machine gun fire. The small, vulnerable light tank, along with its somewhat more powerful successor the Panzer II, would soon be surpassed as a front-line armoured combat vehicle by more powerful German tanks, such as the Panzer III, and later the Panzer IV, Panzer V, and Panzer VI; nevertheless, the Panzer I's contribution to the early victories of Nazi Germany during World War II was significant. Later in that war the turrets
AI Mk. IV radar looking up from the display at the last minute, and the system was used only experimentally. By this time the microwave units had started to arrive in small numbers, so Mk. V production was repeatedly delayed pending their arrival, and eventually cancelled. Starting in the summer of 1942 the TRE development team began experimenting with systems to project the display onto the windscreen, and by October had combined this with an image of the existing GGS Mk. II gyro gunsight to produce a true head-up display known as the "Automatic Pilot's Indicator", or API. A single example was fitted to