A dual-mode, semi-active, laser-based and passive image-based seeker for projectiles, missiles, and other ordnance that persecute targets by detecting and tracking energy scattered from targets. The disclosed embodiments use a single digital imager having a single focal plane array sensor to sense data in both the image-based and laser-based modes of operation. A shuttering technique allows the relatively low frame-rate of the digital imager to detect, decode and localize in the imager's field-of-view a known pulse repetition frequency (PRF) from a known designator in the presence of ambient light and other confusing target designators, each having a different PRF.