Wasn't there reporting of a dual radar and infrared seeker on the PL-15?
If terminal targeting is done via infrared, there wouldn't be any radar emissions for the Rafale to detect.
That is another scenario.
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And if this was a maximum range launch, then the second rocket motor pulse would have occurred mid-phase in a ballistic trajectory. So the missile rocket motor would not be generating an infrared signature when it reaches the target
Not sure about the IR seeker. I haven't heard of it. Seems too much to fit in a separate seekerhead when you are constrained by space and needing as much space as you can for the radar.
The PL-15's main advantage over all other AAMs is the AESA radar. This is the pacing threat that kicked off the AIM-260.
AESA theoritically can operate in LPI (low probability of intercept) mode which reduces radiation while still maintain solution for firing. This description is for aircraft radar. But in theory, a missile's AESA can do the same.