The trophy system in Ghosts extinction mode nullifies the ranged projectile launched by Scorpion enemies. Here's the quote from the COD wiki:
The Trophy System returns in Call of Duty: Ghosts's multiplayer. It is also available in Extinction mode (where it is simply called "Trophy"), where it destroys the Gas Clouds attack from Scorpions, and it will not be attacked by Aliens.
I've used these quite a few times, they very helpful in keeping an area clear of the scorpion's gas. Since the gas damages almost everyone on your team, a well-placed trophy can benefit the whole team. They come in handy for times such as repairing the drill. Just set the trophy (system) near the drill and it will destroy a few of the gas projectiles before being destroyed. I'm not sure the exact number of projectiles it will destroy, but I've observed it to be around 2-5 projectiles.
Personal experience playing COD Ghosts Extinction and the links in the answer above.