20MTs at 200 miles up? I can't really take (preparing for) this EMP seriously. There is only 1 country on the planet that has ever had an operational capability to haul a 20MT warhead into space.

Since such a weapon would have to be not just a relatively simple fission device but a fusion device -- and a reasonably heavy one at that -- it's beyond the technical grasp of just about every country but 1 for the foreseeable future. Only Russia makes rockets big enough to move a payload that large into space. Shahab-3? Let alone from a freighter? Please.

The largest weapon the US has ever put on an ICBM was under 5MT. Russia reportedly maintains 30 SS-18's with 20MT warheads so they do have the capability (though most maintain that those missiles are destined for bunker-buster missions). However, if those are launched at us, everything else is coming as well. There isn't much preparation you can do to prepare for that. EMP will be the least of your worries.