Number of groundstations needed to cover LEO: An optimization approach

This is an interesting number but less so if you're constellation of satellites can talk to each other, then you only need a couple of ground stations.

That said, having lived near the "blue cube"[1] it was hard for them to hit satellites on the horizon because they had to punch through a lot of atmosphere. I would expect that if you were to say, put radar systems around the planet in order to monitor LEO 24/7 for all debris/satellites you'd want to keep them close enough that they were less than 45 degrees off vertical.

[1] http://en.wikipedia.org/wiki/Onizuka_Air_Force_Station