Abstract: When correlating a reference representation with current representations the scales of which are changing, errors occur. In order to reduce the errors, a normal current representation is stored in a store (7), and a scaled current representation is stored in a store (8). Both representations are correlated in a correlator (11) with a normal reference representation stored in a store (9). The reference representation is updated by a control processor unit 5 when the correlation of it with the current representation reaches a peak. Instead of using two current representations a single current representation could be correlated with normal and scaled reference representations.