As you can imagine considerable thought has been put into how to make convincing decoys and to detect them. The reason it's possible at all to do the decoys is

As you can imagine considerable thought has been put into how to make convincing decoys and to detect them. The reason it's possible at all to do the decoys is that they are released in space during a coast phase, so they follow the same trajectory as a high mass real warhead. They can add heating elements to get a similar thermal signature as well, and are designed to have similar radar signatures by having an appropriately metalized surface.