if you say so but SDI came first and then it's research was applied latter to anti satellite warfare. So if you shoot a orbiting satellite traveling around 6000-7000 miles per hour, why couldn't it shoot a missile out of the sky?
You find some pictures of how the system was to work here
https://commons.wikimedia.org/wiki/C...:Space_weapons
There is also a chart on how high a EMP wave can travel upwards
http://royal.pingdom.com/2010/03/23/...now-about-gps/