Since in practice the line-of-sight(LOS)rate is difficult for a pursuer to measure accurately,the robust nonlinear H∞observer is applied to estimate it using LOS azimuths without any priori information of target maneuvers or its estimate in the presence of measurement noise.A threedimensional guidance law is implemented by replacing the LOS rate with their estimates using only relative range,speed,and LOS azimuths.The stability analyses and performed simulations results show that the presented approach is effective. Since in practice the line-of-sight (LOS) rate is difficult for a pursuer to measure accurately, the robust nonlinear H∞observer is applied to estimate it using LOS azimuths without any priori information of target maneuvers or its estimate in the presence of measurement noise. A threedimensional guidance law is implemented by replacing the LOS rate with their estimates using only relative range, speed, and LOS azimuth. The stability analyzes and performed simulations results show that the presented approach is effective.