A pretty good guess is going to be that they'll be on SATCOM on the way in, and while there for comms back to the Pentagon - and I would imagine they'll join in one of the COTHEN networks or even more likely, stay within the confines of the ship's own radio systems. It is, after all, not safe for anyone with all those idiots shooting people. I wouldn't rule out them getting on one of the systems down there, or joining up with SHARES, either. Most of this is defined in the Katrina Wikil
Let's hope that the Guard can get some order down there quickly. 73s Mike