The following explanation, in simplified terms, is typical of all GPS navigation systems.
The GPS navigation system knows where it is at all times.
It knows this because it knows where it isnít.
By subtracting where it isnít from where it is or, where is from where it isnít (whichever is greater), it obtains a difference of deviation.
The GPS navigation system uses deviation to generate corrective commands to drive the system from a position where it is to a position where it isnít, arriving at the position where it isnít, it is now.
Consequently, the position where it is, is now the position where it wasnít and it follows that the position where it wasnít is the position where it isnít.
In the event the position where it is now isnít the position where it wasnít, the GPS navigation system acquires a variation (variations are caused by celestial factors and discussion of these factors is not within the scope of this discussion).
The variation is the difference between where the GPS is and where the GPS wasnít.
If the variation is considered to be a significant factor, it too may be corrected by the GPS navigation system, however, the GPS navigation system must know which where it wasnít to determine which where it isnít.
The GPS Navigation System Thought Process :
Because a variation has modified some of the data that the GPS navigation has obtained, it is not sure where it is.
However, it is sure where it isnít (within reason), and it knows where it was but may not know where it wasnít.
It now subtracts where it should be from where it might not have been and differentiates this with the algebraic sum of the deviation and the variation of the last known position where it wasnít and the position where it isnít to produce an error to which it compensates for all external factors and thus provides accurate navigation information to guide you along your way.
Yup, that is exactly how mine works!
So that's the reason she says "RECALCULATING!!!"
(to find out where it should be but isn't, and wasn't before, even tho you thought it was, but it wasn't!).