As an ADA soldier for 13 years I can tell you you will be engaged without blinking at 30K feet 40 kilometers especially if you are high speed and head on, at 20K were gonna punch two missiles off at you because that's considered too close. If you are fast enough the system will engage automatically as you would be seen as a possible HARM or TBM threat. ECM is almost useless against a phased array radar that has final lock on. Ask the US coalition pilots flying in the last IRAQ war who got shot down by PATRIOTS (F-16/F-18). (They were flying outside the return flight corridors, improper IFF codes, and above the speed for the corridor).
The FAS site has some good info which is fairly accurate as far as distance and capabilities.
http://www.fas.org/spp/starwars/program/patriot.htm