I keep running into a few red grates although they aren't
red on the map like some of the doors.  A few are under
water & others are in odd locations. I'd like to know what
I'm supposed to use in order to open those things. Any
ideas?  
 
Or am I supposed to move ahead & the story line &
get something else to open them later?  I'm at the 
start of the game on my way to the helipad & notice
all of these little grates that don't open to bullets or
grenades, so I'm confused as to how to get in there
when there isn't anything else I can use to knock
open the red grates (red when highlighted).
 
Thanks,
LLT

You use the missile launcher to open the red gates.  You will get it eventually by just progressing through the game