Once you call in the rocket strikes at the beggining of the mission, move to the right side of the map. There you will find a puddle.










Get in the puddle and wait there for about 6 seconds.

Then get out and move to your left. Soon you will notice another puddle.





Get in the puddle and again, wait there for about 6 seconds

Now, you'll need to move to the final puddle. Move to your left and you'll see the puddle. It's next to a barbed wire barricade.




Stand in the third puddle and wait there until 4 statues come out of the ground.




After that, go pick up the Ray Gun from one of the statues



I have also recorded a video that might help you


Sorry for my bad English.