A modular femoral prosthesis includes a stem member adapted to be implanted into a medullary canal of a femur. The head and the spring arm extending upwardly from two opposite sides of the top edge of the base portion. The empirical model generates estimate values based on a calculation that uses an input pattern array and the reference data to determine a similarity measure between the input values and reference data. Accordingly, the position of a receiver can be found even in a condition where latest time information cannot be obtained or a condition where a signal transmitted by each satellite is weak such as a condition immediately following activation of the receiver so that a diffusion-code transmission time ti of the satellite cannot be confirmed.