difference between the known (estimated) output of the system and the actual achieved output
NOTE 1 It is denoted by eK
NOTE 2 Usually this is time dependent.
NOTE 3 Sometimes confusingly referred to as 'measurement error', though in fact the concept is more general than direct measurement.
NOTE 4 Depending upon the system, different quantities can be relevant for parameterising the knowledge error, in the same way as for the performance error. A degree of judgement is used to decide which is most appropriate.
NOTE 5 For example: the difference between the actual and the known orientation of a frame can be parameterised using the Euler angles for the frame transformation or the angle between the actual and known orientation of a particular vector within that frame.