Abstract: Since the angle and angular rate measurement of the existing helicopter-borne stabilized sight system is very accurate, a real-time target passive ranging method is put forward based on these measurements. The mathematical model for locating fixed ground target by helicopter-borne stabilized sight system was established, a rotary variable differential transformer was used to measure target angle and an inertial gyroscope was used to measure target angle variation rate. The measurement accuracy is better than 0.1mrad/s, which is ten times accurate than before. The stability and precision of ranging measurement system are improved by extend Kalman filter (EKF). The simulation and experiment results indicate, it can make real-time target range estimation, which is suitable for the real time target position of attack helicopter.