<p>Current research focuses on the line-of-sight rate estimation for a pitch-yaw gimbaled imaging infrared seeker. The line-of-sight rate estimation problem presents a highly nonlinear dynamic that is influenced by various error sources, such as measurement noise, significant coupling of vehicle channels, time-delayed seeker detector measurements, and unknown target maneuvers. To address these error sources, a new two-step approach is proposed. In the first step, an adaptive discrete-time super-twisting observer with an arbitrary predefined convergence time is introduced to estimate the line-of-sight rate in presence of all errors except for time delay. The proposed estimator has a predefined convergence time that ensures line-of-sight rate converges before the guidance system is activated. In the second step, a delay compensator based on the super-twisting observer method is presented to compensate for seeker measurements delay. To enhance estimation accuracy, nonlinear kinematic equations of the seeker model are applied and roll coupling effects are taken into account. The proposed approach significantly enhances the vehicle performance, especially in engagement with maneuvering targets, even in presence of measurement noise, delay, and disturbances. This is demonstrated via various simulations involving software in the loop test bed and comparison with previously presented algorithms.</p>