Filtering of base motion disturbance from the sight line rate is necessary for homing guidance of missiles. The present work proposes using a noninvasive seeker filter based on the disturbance observer concept to extract the target sight line rate signal from the raw signal corrupted with base motion disturbance. It is shown that the disturbance observer-based filter in favorable (nominal) condition can totally eliminate the platform motion from the raw sight line rate signal. The performance of the filter for off-nominal condition is also derived and exemplified. Simulation results demonstrate improved performance of the proposed filter compared to previously reported schemes.