In target tracking system, the converted measurement method is usually used to convert spherical measurements to a Cartesian frame of reference. The performance of the conventional converted measurement degrades for large crossrange errors. A prediction-conditioned unbiased conversion is presented. The unbiased conversion is used and the covariance of the converted measurements is approximated by the Kalman prediction and unscented transform. Simulation results show that the converted measurement Kalman filter proposed is more accurate and credible.