Most people haven't.
Countermeasures are things carried into space on a ballistic missile to confuse the defense.
They might include coated balloons, chaff, electronic jamming, dummy warheads and other devices to cause an interceptor to miss the real warhead. Supported by the New York Times and much of the national media, this view was accepted by many congressional Democrats. All 45 Senate Democrats voted last month to require testing the NMD interceptor against more stressful countermeasures. That amendment was defeated because it was not necessary. The NMD program is conducting flight tests against countermeasures, and the plan always has been to make those tests increasingly difficult.