Most of us would have to acknowledge that the world of avionics is a closed book to us, but a glimpse into it is provided by an excerpt from an airborne electronics manual reported in the journal of the Institute of Scientific & Technical Communications (and the fact that the folks there have to spend their time dreaming up stuff like this no doubt explains why GEC always gets so dreadfully cross at criticism of Marconi Avionics): The internal guidance system uses deviations to generate corrective commands to fly the aircraft from a position where it is to a position where it isn’t; the aircraft arrives at the position where it wasn’t, thus the position where it was is the position where it isn’t; in the event that the position where it is now is not the same as the position where it originally wasn’t, the system will acquire a variation, and adds helpfully, variations are caused by external factors and discussion of these factors is beyond the scope of this simple explanation.
