How is a base as target picked?

[7Saturn]

I have played some games with OpenXcom, and of course experienced many base attacks. But one thing remains totally unclear: On what grounds is a specific base being picked as a target? The question whether a base attack is generated, is one of shot downs, that I know. And that there's always a random element involved. No shot downs would be the only thing, to prevent base attacks 100%. (Right?) But I had two completely opposite situations, which beg the question, how a base is picked:

One saved game, where I exclusively used Avengers from two bases (all the other bases didn't even have hangars, so cannot be considered as some sort of attacking base). I shot down a lot of UFOs from those two bases and in the beginning had also some attacks. But after some time, not one single base attack was generated any more. I even removed all mind shields from those two bases, but simply nothing happened any more.

Then on the other hand I just had started a new game, where I managed to place many radar bases (only having a hyperwave decoder and a mind shield) around the globe. One in particular got snatched lousy three days after I placed it, so not even the facilities were ready, only the access lift.

So how does it come, that in the one saved game, I don't even get base attacks generated any more, and another, where an almost mint condition base is attacked? What's involved in the selection?

[Hobbes]

Book of Retaliations, chapter 1, versicle 1
* Starting on the first month, each shootdown of a UFO can trigger a retaliation mission by the aliens, with the chance being determined by difficulty level, at Easy being 4%, then increasing each difficulty level by 4%, until it reaches 20% on Superhuman
* The chance of the Retaliation taking place in the region of the shootdown is determined by the formula (50 - 6 * getDifficultyCoefficient). If this fails the aliens will assign the Retaliation to a likely point of origin of the interceptor, based on its approach vector
* After The Martian Solution is researched OR depending the difficulty level (March 2000 on Easy or November 1999 on Superhuman): 1 Alien Retaliation mission is added, in addition to the above. It will be spawn always on a region already with an XCom base, randomly chosen
* The detection chance for Retaliation missions is given by the formula: (completedFacilities / 6 + 15) / (mindShields + 1)
* There can only be 1 active Retaliation mission per region

[7Saturn]

So the important part here is the paragraph with the Martian solution. It simply cheats itself the knowledge where to look. But then again, how can it be, that I was quite literally shooting down pretty much everything, that can't fire back on me (plasma cannon...) and still got no retaliation missions any more for months?

[Hobbes]

So the important part here is the paragraph with the Martian solution. It simply cheats itself the knowledge where to look.

Even if they spawn in a region with an XCom base, the UFOs still need to detect your base with a flyby. This is the detection radius of all UFOs



When a UFO gets within range of a base, its sensors have a chance every 30 min of detecting the base and this chance is determined by the number of buildings on the base, etc. + actual distance between of and base

But then again, how can it be, that I was quite literally shooting down pretty much everything, that can't fire back on me (plasma cannon...) and still got no retaliation missions any more for months?

If you're shooting everything then that includes the UFOs that are being sent in retaliation missions

[7Saturn]

True. But one thing I noticed is, that sooner or later, battle ships appear, no matter what. And I didn't touch them... They just don't come any more, not even for the other bases.

[Hobbes]

True. But one thing I noticed is, that sooner or later, battle ships appear, no matter what. And I didn't touch them... They just don't come any more, not even for the other bases.

You need to sacrifice virgins at the volcano to please the RNG gods

[KZad Bhat]

Don't even the randomly generated retaliation missions require a precursor mission to "find" the base? In which case, shooting down everything that shows up means taking out the precursor mission before it can be complete, never giving a hand off to the battleship attack stage . . . so you never see the battleship.

Or were those precursors only created for some of the mods?

[7Saturn]

My observation: Smaller UFOs start the search, bigger follow, until battle ships are looking for the base. At this point I'm not really into attacking them, as this will mean most likely damage to the aircrafts. I don't even have a problem with base attacks in general. Only really bad thing is, when they happen far to early or when they happen just at the same time, your whole team of that base is on a mission.... Aside from that, it's OK. But I wondered, why I seem to be getting a lot of them in the early and mid game, but at times almost none in the late game. Probably I just got lucky a lot...

[Hobbes]

Don't even the randomly generated retaliation missions require a precursor mission to "find" the base? In which case, shooting down everything that shows up means taking out the precursor mission before it can be complete, never giving a hand off to the battleship attack stage . . . so you never see the battleship.

That's exactly how Retaliation is designed: the aliens send several UFOs in succession, from Medium Scouts up to Battleships, until one of them detects an XCom base, which triggers the attack by a Battleship. If you shoot down all the UFOs then you prevent any base attacks.

Only really bad thing is, when they happen far to early or when they happen just at the same time, your whole team of that base is on a mission...

That's simply an unfortunate coincidence - but that just means that it's better to always have several reserve soldiers + HWPs in case it happens