Originally Posted by fisherman
Nords would be better to answer but you have to know where you are exactly to program a missle flight as I understand it.
IIRC (and I'm reaching way into the memory deep freeze here), a transit pass (as it was called over two decades ago) would let you fix your position to within a half mile. A GPS fix (of at least three satellites) would let you fix your position to a quarter-mile.
We used to joke that a fourth GPS satellite would also tell you how much antenna you had sticking out of the water.
Once you had that fix plotted on the chart (yeah, I know kiddies, we dinosaurs used paper charts back in them days) then you'd go deep and use your onboard inertial-navigation system to dead-reckon your movement. It was pretty accurate for 12-24 hours but depending on how fast you raced around you had to assume that it was degrading in accuracy by roughly a tenth of a mile every hour. On an attack submarine you'd be at PD twice a day (at least, hopefully several times a watch) and you'd pick up a fix as part of that routine so you were usually pretty close. On a 1980s boomer you'd come up once a month
or so, but I'll get back to that in a few paragraphs.
So however you got it, once that fix was in the system you'd drive around underwater while the SINS (and the quartermaster) would dead-reckon your position. That position would start with an uncertainty circle around it of a quarter-mile (or half-mile) radius and would eventually grow to several miles. This was no problem in the middle of the Pacific Ocean but it could get a little hair-raising in certain areas near certain land masses with much shallower "charted" soundings where you would rather not be even going active on the fathometer, let alone waving your antenna around.
At some point you had to decide whether you'd rather run aground or risk the antenna exposure for a satellite fix. The transit pass would take agonizing minutes using a huge antenna (big radar cross-section) with much watchstander bickering and finger-pointing. The GPS fix (from a tiny receiver on top of the periscope) would be onboard before I'd even gotten enough scope high enough out of the water to make a safety sweep. Once or twice we'd get
a fix before I even had the scope out of the water.
It wasn't considered essential to have a tight GPS fix for a TOMAHAWK missile launch, but it sure helped you push the limits of the max-fuel range. Once the TLAM hit its initial waypoint enroute the target, it started getting its own fixes-- either from land contours or from GPS satellites. Those missiles have gotten way smarter over the years by at least two generations of technology from the ones I used, so that last sentence may no longer be relevant.
Ballistic missiles were another concern but the solution was to use bigger warheads and shorter ranges until the nav tech caught up. The early POLARIS model was named for its technology allowing it to get a celestial fix enroute the target. I don't remember how POSEIDONs fixed their position but we had a lot more fuel & warhead to work with and we stayed in the North Atlantic most of the time. The TRIDENT missiles use a much-more-accurate ring-laser inertial navigator, but a submarine position fix before launch could give it that much more fuel margin to guarantee a solid hit inside the error circle.
The real limiting factor on coming to PD wasn't navigation but communications. Back in the 1980s and early '90s (the "good ol' days") a hotshot UHF satellite transceiver could send/receive at 2400 bits per second. (Not bytes!) Using one of those with an antenna mast you'd come up to PD at least twice a day to pick up radio traffic so that the shore comms center could load you up with more, and radio traffic was the high point of the day because if you got ahead of the official traffic then the shore comms guys might slip in a few sports scores & familygrams.
Boomers could also stream a buoyant VLF communications wire (several thousand feet long) and a buoy with its own little HF wire. Then you'd poke around at 3-4 knots and 400 feet of depth, holding your breath and not making any sudden moves, while your radio techs would try to coax a decent signal out of the wires floating behind you on the surface. (You'd keep this up for 70 or 80 days, 24/7.) You don't get much of a bit rate at VLF frequencies but you could clear that same broadcast in an hour or two instead of a few seconds. That meant you didn't need to come up to PD, so you didn't. Boomer crews didn't get much practice at coming to PD so they sucked at it and didn't really want to do it anyway.
Luckily boomers are also able to use a couple of alternate methods to fix their position while submerged. I'm not sure whether or not those are still classified, but when you're poking around the ocean at 3-4 knots then one of those fixes will be "good enough" for a few weeks.