jrrt@hogpd.UUCP (R.MITCHELL) (09/07/84)
I didn't see the initial posting that prompted Troy's answer, but I'd like to chip in a few comments about ELF (Extremely Low Frequency) transmissions as they apply to submarines. I'm by no means an expert on the physics of such things, but I was the Communications Officer for two years on an operational SSBN (ballistic missile sub). Of course, the staments made below are my own and are not necessarily part of official policy. Water severely attenuates all but extremely low frequencies (ELF). As a result, submarines have to surface to get information... Not exactly true; VLF (the major frequency band used for shore->sub communication) can penetrate sea water somewhat. Hence, subs don't have to be surfaced to receive transmissions, they *do* have to be relatively shallow (the exact depths are, naturally, classified). Certainly, ELF has much greater penetration potential than does VLF, so the sub can patrol at a greater, and hence more secure, depth. ELF obviously requires a correspondingly extremely large broadcasting antenna (c = f(lambda)). The curious thing to me in all of this is how the pentagon envisages receiving these waves. Perhaps making extremely long submarines??? Sort of. Just string a *long* wire behind you, make sure it doesn't get snarled in the propeller or surface shipping, and you're all set.