If I were you, I'd make a ground map, place trees, high res textures, etc. as opposed to a sphere.
It would then appear that you're in the upper atmosphere over a region.
Don't want sub-space? You can use no-warp as an option and have ships come in from the distance.
Want sub-space? Then put in the plot that the enemies have precise coordinates on where to deploy ships, and give the player a way to kill that. (say a red alert mission before in low orbit where you nail an AWACs)
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AceStaff member FreeSpace Watch
http://freespace.volitionwatch.com/