This is transcribed from an Air Force training video on the topic of missile guidance. The actual sound track is here.
A similar text was published at http://afmissileers.com/newsletters/NL1997/Dec97.pdf (page 5) where it was attributed to the Ground Launched Cruise Missile (GLCM).
In the event that the position that it is in is not the position that it wasn't, the system has acquired a variation, the variation being the difference between where the missile is, and where it wasn't. If variation is considered to be a significant factor, it too may be corrected by the GEA. However, the missile must also know where it was.
The missile guidance computer scenario works as follows. Because a variation has modified some of the information the missile has obtained, it is not sure just where it is. However, it is sure where it isn't, within reason, and it knows where it was. It now subtracts where it should be from where it wasn't, or vice-versa, and by differentiating this from the algebraic sum of where it shouldn't be, and where it was, it is able to obtain the deviation and its variation, which is called error.
Perhaps related, Rockwell created this brief video of a research project to build a Retroencabulator. The video is at https://www.youtube.com/watch?v=RXJKdh1KZ0w. Though uncredited in the video, this is actually based upon earlier work by JH Quick in a paper entitled Turboencabulator which can be read here.
In fact there is a Wikipedia entry, http://en.wikipedia.org/wiki/Turboencabulator which gives a much better history, and has a link to another excellent video, from Chrysler.