• A spinning reticle seeker contains a single thermal detector and a spinning patterned disk

- such as the ‘Rising Sun' Reticle.

 
- consisting of half a disk of opaque material and half a disk with alternating transparent and opaque bars.

• If the target hotspot is in the centre of the disk

- meaning that the missile is pointed at the target - the output from the thermal detector is constant.

• If the target hotspot is to one side of the centre 

- the output is modulated by the bars and zero when the opaque semi-circle covers it up.