NavList:

Lu Abel, you wrote:
"So I suspect if GPS went down Air Force One wouldn't exactly be back to the 1800s for navigation."

Holy Cow. Did you really think that's what I was saying?! You've been reading my NavList posts for years and years and years, and I am genuinely surprisd by your comment here. You surely didn't think I was talking about Air Force One on an ordinary flight, like this afternoon from Rhode Island to Washington, DC, losing GPS and digging out great-grand-dad's old sextant. Did you??! In fact, it's likely that they have multiple systems running all the time that provide instant backup in the event of "ordinary" GPS interruption, including multiple inertial systems, multiple radars, active fixes from other aircraft, other radio and satellite systems, and automated celestial, too.

Maybe the problem here is that you're not picturing the VC-25A (that's the heavily modified 747 that's usually known as Air Force One) as a military command and control aircraft. It's sometimes portrayed as a glorified air taxi with luxury seating and a nice new color scheme. But that's just the superficial part. The VC-25A is a military command aircraft, very similar to the E-4B (also heavily modified 747s), from which "World War 3" or equivalent can be authorized and commanded nearly as well as from any hardened military bunker on the ground. If it's your job to design the capabilities of such an aircraft, the last resort, the last bastion of presidential authority in a global thermonuclear war, would you really leave out a manual sextant?? As I said previously, I think it's quite likely that they carry one precisely because the survival of presidential authority under severly degraded conditions is so vital. 

Suppose a nuclear war breaks out. You can write off GPS within minutes unless the US is winning from the first minute. As I said in my original post: "Naturally they also have every form of electronic navigation, and I suppose they also have automated celestial systems". So if GPS is out, then you switch to inertial navigation augmented by automated celestial fixes. The war drags on for 36 hours (considered a long time in nuclear wat scenarios), and things get worse. Suppose the main computers go down due to a major power fault, and you're out over the Northern Pacific. By bad luck or bad design, you realize that you can't start the automated celestial system unless the inertial system is initialized. But neither will start without stable power. So how do you get home (any US airfield)? You could call the last B-2 flying and ask for a ride, but if you have the basic tools for manual celestial navigation (and your navigator hasn't been irradiated to incoherence) then you're all set. 

There are few cases where manual celestial navigation still makes any sense at all as a genuine navigation tool. One, at the low end where other electronics are not available, is to serve as cross-check on GPS and related systems which are subject to spoofing and human error. The best backup to a GPS is another GPS and another after that. But if all GPS signals can be spoofed, then you need something else as a sanity check. The other end of the spectrum also makes sense. And that's where people are literally, actually preparing for Armageddon. It's this sort of extreme, wildly unlikely scenario that still reserves another seat for manual celestial navigation. I wonder if they carry a $30 Davis Mk3, too, just in case the periscopic sextant doesn't work when it's pulled out of storage... OK, maybe not.

Frank Reed