Mxsmanic wrote:
"Grumman-581" writes:

And high enough that the sky was dark even when the sun was out -- at least
from the photos that I've seen published...

Yes, but the ANS could recognize stars even from the taxiway in broad
daylight. I still don't know how it managed that.

Jupiter, Saturn, and Venus are visible to the naked eye in broad
daylight at sea level, if you know exactly where to look. I've
seen them all, at times when I knew their approximate position
relative to the moon. You must look in exactly the right place --
an error of a half degree or so puts them out of the central field
of view of your eyes and renders them invisible against the
glare of daylight.

The brighter stars are just outside the ability of most people's
naked eyes to see in broad daylight, but a small pair of
binoculars will pick a few out nicely. Again, proper aim is
crucial. Only a handful of the brightest stars will typically be
visible at any given time, but you don't need very many for
navigation.

It sounds like the optics of the SR-71's nav system are at
least as good as a standard issue Mk 1 pair of eyeballs
coupled with a 7x35 pair of binoculars. It may be that, for
rapid startup, the system needs some hint of the location
and time of day to figure out where to begin its search for
stars, but I'm just guessing on that point.