Looks as if once I am on the defensive I am always on the defensive. But if I scare off his ships and a big fleet above the battle and he doesn't, shouldn't I have the option for ground support instead of him?

(Also, some improvements like the portals should be targetable in space not something that gets randomly destroyed through ground battle...)

yes.

All his troops are already on planet (you may notice, that ships don't carry any manpower once battle is started), they just can't all participate in the battle.

By ground support I mean the orbital bombardment tactic. (can you research other ground tactics)? 

You can't research any, however collection bonuses of Tikanan and Bhagaba unlock unique ground battle tactics and there is also a Stampede tactic which you can use if you've previously hacked a system and supported revolutionaries.

To answer your question: Ground support tactic is available for attacking player, only. And right, it feels somehow irritating, to offer ongoing ground support tactic, even attacking fleet has moved on to another place while ground combat lasts.

It is kinda realistic though.

mamarider wrote:

It is kinda realistic though.

Perhaps I misunderstand, but isn't ground support related to attacking ships/fleets from orbit firing on defending units on  ground?