From J. Clarke:
says...
From J. Clarke:
snip
Just a reminder, GPS was originally not for targeting, it was for
positioning the launcher accurately. With three hundred 400KT warheads
close counts.

I'd be interested to know where you got that idea from. If all that
was needed was precise coordinates for launching mobile systems, you
could simply hire teams of surveyors, at little more than minimum
wage, to paint a bunch of 'X's on the ground at potential launch
sites. This is a *far cheaper* solution than a multi-billion dollar
satellite constellation, especially if you wait til the Glidden paint
buckets go on sale.

Good luck painting Xs in the middle of the ocean.

Bingo! We're in total agreement there. Fixing nuke sub positions was a HUGE reason behind the funding of Transit, then Timation, which got amalgamated into DNSS. To identify the single most important reason behind the Navy push, SLBM's would be it, I would say. And strictly speaking, sat-nav was not for getting an accurate fix of the launch position of the subs. It was for getting that fix into the INS (which then integrated accelerations until such time that the launches were commanded).
The Air Force funded MOSAIC as their approach to giving good nav updates for their mobile ICBM launch system, but again, that's a much easier problem to solve.

Now it is also important to note that the Navy requirement to solve SLBM nav was *VERY DIFFERENT* from how GPS came to be defined. The Navy needed 2-D positioning for slow dynamics vehicles. Transit gave them that. But nearly useless to nuke bomber aircraft.

The reason why GPS was developed and implemented was very
straightforward:
Inertial Nav Systems (INS) are inherently prone to errors that can run away in a huge way. ALL THREE legs of the nuke triad depended on INS - the bombers, sub-launched and land-based missiles.

Sorry, but neither ICBMs nor manned bombers DEPEND on INS. Manned
bombers have it and use it as one of their aids to navigation, but they
don't require it to deliver the payload to the target. That's done by
radar or by visual means.

ICBMs are just that, BALLISTIC. There is no terminal guidance. Once
the engine shuts off it is going to go where it is going to go. You
seem to be conflating nuclear armed ICBMs with conventional cruise
missiles.

For the boost phase, all ICBMs have elaborate guidance & control systems. Knowing this, it can be said that the word 'Ballistic' in the term ICBM is a total misnomer. Or only applies in the limited sense of the final flight phase when no terminal guidance is attempted.

In the industry, there are weapons that are called "guided missiles", and there are these IC"B"M's called "ballistic missiles. All have guidance systems. What the primary distinction is from a GN&C standpoint is that the latter do not do any _Homing_. They are launched against fixed targets, whereas "guided missiles" are capable of striking moving targets.

So the more accurate term instead of ICBM would be ICNHM (InterContinental Non-Homing Missiles). Why did they end up getting called IC"B"Ms? Maybe it was because their role was an extension of artillery, a completely ballistic weapon.

But even the V-2 had elaborate guidance engineered into the rocket. Since these pre-dated INS, they used radio nav. And GPS comes full-circle with a return to radio nav, except this time the transmitters are based on-orbit instead of on-shore.

Where GPS was needed was to provide an accurate launch position for the
submarine force.

Most definitely. The Navy's Transit system was not decommissioned until after GPS was declared operational.

INS's measure acceleration, so you have to go through two integrations
to get position, and that's only after you've initialized it to an
accurate position by some other means - taken a fix. After feeding
the INS an accurate fix, errors can still go wildly out to lunch.

So what?

This pesky problem is what drove funding for the Air Force's 621B program.

GPS measures position (and velocity) directly, so the entire nav
problem is *solved*. This is the reason why DoD knew that what GPS
offered was worth megabucks, in order to get those megatons on target
reliably and precisely.

Land based missiles can get those megatons on target reliably and
precisely without GPS.

The problem of INS drift (and full-platform-tilt runaway) was a much greater concern for the bomber leg of the triad.

GPS offered a way to cover nav for the full trajectory of those nukes
as the warheads found their way to their targets, not merely fixing
launch coordinates (a much easier problem to solve).

Except that GPS has never been used for terminal guidance of ballistic
nuclear warheads. Peacekeeper was able to achieve 40 meter CEP while
GPS was still in the process of launching experimental satellites.

Since the reentry vehicles have no propulsion system or control surfaces
there is little that a navigation system can do for them.

Sorry, but it was well known when GPS was first deployed was to allow
FBM submarines to determine their launch position. This information has
since been diluted.

In years past on this forum, it has been explained how GPS was a convergence of the needs of the Air Force as well as the Navy in solving the nav problem. Of course, nav is an across-the-board issue that impacts civilian as well as military. And widespread benefits were planned for at the early stages of the JPO formulation of how the system would work.
But as for Congress coughing up the bucks, the basic analysis was that the nuke triad consisted of:

2x AF
1x Navy

And the Air Force ended up with the strong leadership position over GPS that continues through to today. Yes, SLBMs were one of the key drivers. But the Navy had their solution from slow-dynamic 2-d fixes from the Transit system.

The primary driver behind 3-d positioning for fast-dynamic vehicles was the Air Force. Otherwise "GPS" could easily have just been an improved Transit.