I've actually advanced severals days just to make sure.
The FC's resolutions aren't too high, they have a resolution of 10 and the alien ships are all larger than 2900 tons. Even when I advance some time and get closer, I still can't target them, even when they are in range of my resolution 1 active sensor. However, once I get almost right on top of them, about a million km away, then I can target them.