omg coolest idea for a while! each satellite should have a different function but be broken into two types:
non-geostationary satellites: dont take long to build because they pass across the map every x mins at x km/h and so dont have as much power as the geostationary satellites.
geostationary satellites: take longer to build because they have the ability to and fuel to (after being launched) move into a geostationary orbit (remain in same place).
here are a few good functions:
-large los
-total los (or very very large)
-large radar
-total radar (or very very large)
-ion cannon/particle beam/orbital laser thingo (very large damage range, extreme long recharge time and instant attack when target given)
-nukes/emps (same as regular nuke silo)
-omg!!!111 some kind of funky tractor thingo that could pull the selected unit into the air and drop it once it reaches a certain height, or even better fling it in selected direction (hehehe throw an attacking peewee into the sea

)
there would be problems with being able to have massive los and radar and also be able to attack with a particle beam instantle, so maybe the "satellite making factory" could have a limit to satellites that can be built, or the types that can be built.
for countermeasures you could have special emps that destroy them (not literally, just cause them to not function), and also knock them a little, so they slowly start to spin off course, or crash into the ground.
EDIT: this isnt really just a satellite defence but it would be cool (and has no doubt been suggested before). shields...

...electric shields... these would only appear when attacked (as ripples) and would absorb the energy of the attack. ie. when attacked by a nuke a certain amount of energy would be subtracted from energy. this means that for the shield to be of any use you would have to have a large amount of energy production. this means that a large amount of berthas firing at the same time at the shield means that a few shots could get through. also the shield could act as they do on dune: the shield only blocks objects/energy moving at a certain (high) speed, so it cannot block out actual units. that would be so awesome!!!