Here is a little background:
This post will focus on the E1 pulse created by the Compton effect.
Due to the magnetic field of the earth, the Electro-magnetic flux is not evenly distributed. Over most of the United States the strongest part of the field (75% or more of the maximum) is shaped like a clown smile.
The problem of altitude
The North Korean satellites are currently orbiting at approximately 300 miles above the earth's surface. If detonated at that altitude the gamma rays would be diluted. Remember that the intensity is proportional to 1/R^2. Doubling the distance reduces the intensity by a factor of four.
Then there is the issue of North Korean nuclear weapon yield. The first time they tested a nuclear weapon that was more than 10kt yield in September of 2016. Some of the earlier weapons might have been slightly more than 10kt based on the range of estimates but we know with certainty that they passed that yield level in September of 2016.
The problem for the military is to identify North Korean satellites that are dropping from 300 miles to less than 60 miles (which should be easy) and to have batteries of SAM stationed on the south shore of Lake Ontario and somewhere around Panama City, Florida. The SAM must have sufficient range to reach out to approximately 100 miles of altitude to ensure they have enough reach.