Darth Lars said:
There is one addition in the trailer that I find a little bit silly, though. That is that the Echo Base gun turret shoots after the Snowspeeders after they have passed it.

The gun turret fires very close to where you would extrapolate the snowspeeders to fly.

The snowspeeders have yet some time to reach the AT-AT walkers. So, the AT-AT's would still be quite far from the gun turret. Right?

The turrets may have a longer range than the guns on the speeders, Luke only concludes that the armour is too strong for them to penetrate with blaster fire after the first few runs.

So the turrets firing isn't silly from the Rebels point of view as they hope it's clearing a path for the Speeders to attack.