Although the satellites on the outer rings will engage attackers and destroy them it is worth having a last line of defense around the central hub just in case some get through.
Multiple Satellites
Don't place multiple satellites on a single ring as trying to fight enemies on multiple fronts can spread you too thinly making it difficult to defend.
Strategy
A good strategy in this game is to only build up the firepower you need and spend the rest of your resources on research.