The base detection and attack mechanics are a bit complicated, but that's how it always have been.
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Not sure how dark ones and ratmen do it - if they use similar vessels or a different mechanic (besides - realistically, how would a radar detect a squad of soldiers on foot?).
It might be complicated or abit late to suggest this but the Dark Ones base assaults, and ive never had to deal with one personally, I would believe should be using a Comm Wave to simulate the reference to teleporter invasions from Doom.
Another idea ive had for the ratmen foot patrols is they should In Fact have a UFO type. Its just not the usual kind of ufo ("All Terrain Vehicle"). Since they go across the land and even cross the water at about speed 100 it implies they've got their hands on some handy but not that impressive teck. Id imagine they would have a couple large hovercraft things with defensive stations abit like a ufo but without a roof to it, and no salvageable engine. Two of them side by side or something like that. This would be intimidating enough to locals and they'd cough up whatever info the marauders want.
Id like to collect the loot from them out in the field without letting them near my base. In fact id like to collect loot from the Entire Band of marauders. There should be a way to forcibly land on them without killing them. Maybe give them 15 hp instead of 1 so that a hit from an autocannon can lock them down for a combat. I forget whether they ever stop to "land" with a green x so that you could get a ground mission with them.
The inspiration comes from the Splogorth guy and the maiden slaves protecting him on his hovercraft from the game RIFTS. Its a searchable image, splugorth slaver barge.