ELI5. I kinda get how a rocket might track on the heat of a helicopter or jet. But how does the tracked vehicle know that it is being tracked? I assume this is real life and not just movies when a big read INCOMING MISSILE warning shows up.
In: Technology
This depends on when in time you are.
Say that you’re an American pilot in the Vietnam War:
There’s no way for you to know that an infrared missile has been fired at you unless you see it. The good news on that end is that infrared missiles at this time have a very, very short range so chances are you’re in a dogfight with the plane that launched the missile.
Even better news is that infrared missiles at this time had a very low hit probability and needed to have constant line of site to your engine – and these drawbacks meant that they were used relatively rarely.
At this time there are two very distinct types of Radar – longwave Radar that is used to search for targets and shortwave Radar that is used to track targets that have already been located. Your plane has two indicator lights – one for search Radar and one for Tracking radar.
If the search light is on them it means that *someone* is pointing a search Radar at your plane. If the tracking light is on it means that *someone* is pointing a tracking Radar at your plane. Neither of those lights necessarily means that the Radar being pointed at your plane is enemy Radar, nor does it mean that a missile is actively tracking you. As with an infrared missile, you have to visually see the missile launched and even then, you don’t necessarily know that its going after *you* until it gets close.
The good thing here is that ground based AA missiles are the size of a small plane, leave a very noticeable smoke trail, and have to be shot from a site ahead of you (otherwise you’ll outrun the missile), and have very limited maneuverability. If the weather conditions are clear this makes it easy to see and avoid the missile. If its cloudy then you still get the benefit of missiles having a very low hit probability at this time.
This is the situation that existed for American pilots through the mid 2000’s.
Modernly most radar consists of a phased array, which uses the same “type” of radar for search and tracking. Modern missile warning systems try to distinguish whether a phased array is in search or track mode based on how frequently the aircraft is being pinged by the radar, but this isn’t terrifically reliable. Really all you can tell from the Radar emissions is whether someone is scanning the sky with Radar or not.
Depending on the plane you’re in there are now systems that can detect missile launches on the ground nearby, either with short range Doppler Radar or by looking for the heat from the missile’s engine. Doppler systems can predict where the missile is going, and so can alert you that you, or someone very close to you, is the target of the missile. Systems that look for engine heat can just tell you that a missile has been launched.
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