Was going through the weps pack and was looking up various SAM's and compared them to their real world counterparts.
SA-9
In game
IR Guided with optical back up
Real World
IR Guided with optical back up
SA-15
In game
Radio Command with optical back up
Real World
Semi Active Homing Radar with optical back up
SA-8
In game
Radio Command with optical back up
Real World
Semi Active Homing Radar with optical back up
SA-6
In game
Semi Active Homing Radar with optical back up
Real World
Semi Active Homing Radar with optical back up
SA-3
In game
Semi Active Homing Radar
Real World
Semi Active Homing Radar
SA-13
In game
IR Guided with optical back up
Real World
IR Guided with optical back up
SA-19
In game
Semi Active Homing Radar with optical back up
Real World
Semi Active Homing Radar with optical back up
SA-7
In game
IR Guided with optical back up
Real World
IR Guided
I think this might help answer why sometimes chaff, ECM and or stealth might not work. I mean if it has optical back up, unless you cloak, you are a viable target.