I am not sure if this is a bug or it's implemented this way, but you can snipe the relay on invasion missions without any soldier nearby to provide squadsight.
It happend to me 3 times already (3 separate missions).
How to reproduce.
- On an invasion mission take a sniper (doesn't matter if it's DFA of Snapshot).
- Make sure you have line of sight to the relay.
- Once the first RNF message goes red, you can target the relay (I am not able to do so before the first RNF drop is incomming).
I am not sure if it's designed this way or it's a bug. I would understand if it's a design choice - since the mission becomes the hardest mission in the game without being able to snipe the relay early.