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The missile knows where it is at all times because it knows where it isn't. By subtracting where it is from where it isn't (or vice versa), it obtains a deviation. The guidance system uses this deviation to generate commands to move the missile from where it is to where it isn't. Upon arriving at the position where it wasn't, it now is. Thus, the position where it is is now the position it wasn't, and the position it was is now the position it isn't. If the current position is not the one it wasn't, a variation has occurred. This variation—being the difference between where the missile is and where it wasn't—may be corrected by the system. The missile must also know where it was. Due to variation, it may not be sure where it is, but it is sure where it isn't, and knows where it was. By subtracting where it should be from where it wasn't (or vice versa) and comparing it to where it shouldn't be and where it was, it derives a deviation and variation—called "air."