So experiment proves 26 successiful detections from 60 tries, which means ~43% chance. I've made a check for the base with single large radar (0 is failed to detect, 1 is successifully detected). And then load this save, like, 100 times, and count real %% ratio (after putting saveScumming in the advanced options to ON). If you want to disprove probability ratio, you can make a save, put 'retaliationTarget: true' to all your bases (to make all bases revealed to aliens), and in the alienMissions section set 'spawnCountdown: 1' for the retaliation. Unless there is a hyberwave decoder (it will always get its 100% chance). So roughly each one of two battleships manages to get to the base without being noticed by radars. Since it's very large ship which is flying very high - there's a chance modifier +20%. You can be unlucky and get only ONE scan. First time let say be LARGE (20%) and second time is SMALL radar, thus 10+20 = ~28%.
To be precise - battleship covers large radar range in 57 minutes. Retaliation battleships are flying with speed of 2700knots, and large radar detection distance is 2577 miles, so radar manages to scan almost 2 times. Conventional radars have 10%(small)/20%(large) chance to detect and scan every 30 minutes.
