APRIL 30-- The basic designation adopted was a two letter combination of the three letters A (Air), S (Surface), U (Underwater), in which the first letter indicated the origin of the missile and the second letter its objective; followed by the letter M for missile. Thus a surface-to-air missile was designated SAM. This basic designation was followed by a model number; odd for Army and even for Navy. For popular names, it was agreed that ASM's would be named for birds of prey, AAM's for other winged creatures, SAM's for mythological terms, and SSM's  for astronomical terms or bodies.