Also wouldn't it only work for aircraft that are flying away from the launcher? IR & light signatures are much weaker from the front. At best I think this guidance system would only be economical for ground-based launchers, as the cost of aircraft and their limited payloads mean you want the most effective weapons onboard, not the cheapest.

Annoyingly, I can't find any information online about such a simple guidance system. The earliest homing missile fielded by the Soviets was the K-13[1], which used technology reversed-engineered from the AIM-9 Sidewinder[2]. Later systems seem to be improvements upon that technology, not simplifications.

1. https://en.wikipedia.org/wiki/K-13_(missile)

2. https://en.wikipedia.org/wiki/AIM-9_Sidewinder