In my view, this screams out for expert opinion and it's not me, but I'll provide you my two cents' worth.
For RADARSAT, currently what is needed is at least three satellites and I believe there are plans for only one, or perhaps two. RADARSAT does a great job of tracking ships. I'm almost 99% sure it does nothing about the air-launched or submarine-launched cruise missile threat, ergo you need some kind of enhanced northern warning system direct-line-of-sight radars. It's going to end up to be a balance of the two, but that's the limit of my competence.