Since A13 the chance for orbital traders to arrive has been dramatically reduced. I've seen 3-4 in a 3 year playthrough. I know incident base chance has been lowered from around 20something to 3 (I think?). I've now brought it up to 25 but there's still fewer of them than before.
What I'm wondering is what does the base chance in incidentsdefs refer to? Is it a chance something happens on a given month / season? If so, it would mean any figure would have to be 3 times higher than originally to compensate for the fact there are 3 times fewer seasons than there are months.