Cruise missile Marsey

The Marsey knows where she is at all times. She knows this because she knows where she isn't. By subtracting where she is from where she isn't, or where she isn’t from where she is (whichever is greater), she obtains a difference, or deviation. The guidance subsystem uses deviations to generate corrective commands to drive the Marsey from a position where she is to a position where she isn't, and arriving at a position where she wasn't, she now is.