Here are some facts:
- modern monopulse radar missiles (AIM-7/120, R-77/27) can easily filter out chaff, because chaff is simply "noise" to them. In other words, chaff is like another form of ground clutter
- since chaff instantly slows down to zero airspeed, monopulse missiles have no problem filtering them out - it's like filtering out ground clutter
- chaff should only work on the beam, because that is when the target basically "blends" into the noise
- so, missiles don't actually lock onto chaff, it just loses the target in the noise presented by the chaff.
That's the basics.