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In this paper simultaneous estimation of states and disturbance for seeker system of a missile using sliding mode observer (SMO) has been proposed. The modeling of the plant is done by classical method and the complete seeker system has been simulated for generation of spiral scan loop with control input provided by a simple state feedback controller. An extended state observer using sliding modes is designed to estimate the system states and the total lumped disturbances acting on the system. The estimated disturbance takes into account the disturbance due to cross coupling in the gimbal system and externally added sinusoidal disturbance in input channel. Simulations verify that the output estimation error is made zero in finite time due to sliding modes and the estimated disturbance is in close agreement with actual disturbance.