If an aircraft’s artificial horizon system fails, its autopilot should still be able to carry on flying level by sensing the contrast between the darker land and the lighter sky, according to a patent application from the Australian government (WO 2004/10236).
The autopilot uses sensors on the sides, front and rear of the plane, some of them tuned to UV and some to green light. The UV sensors discriminate between the land and the sky, but they are dazzled by the sun. So the green sensors measure the sunlight, and this is then subtracted from the UV readings. This…
