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"With all the technology at our fingertips today, the very idea of
celestial navigation remains a romantic and even a bit bewildering one.
It harkens back to a time when mankind had a much richer connection with
the heavens above. But the truth is the task of navigating by the stars
became increasingly automated and miniaturized as a necessity of the
Cold War. The advent of the Global Position System (GPS) and the
diminished threat of all-out nuclear armageddon left advanced celestial
navigation capabilities for a small number of weapon systems and the
history books. But now, with the possibility of having to fight in a
combat environment where GPS and all its benefits are denied, updated
automated 'astro-nav' capabilities may represent one of the best and
most proven ways of overcoming this challenge."


https://www.twz.com/17207/sr-71s-r2-d2-could-be-the-key-to-winning-future-fights-in-gps-denied-environments

The article mentions a circular search pattern and a catalog of 64 stars
in the SR-71 instrument. However, the airplane flight manual says the
search pattern is rectangular and the catalog (revised annually) has 61
stars. The B-2 system also has 61 stars in its catalog. I never did
learn the identities of the stars. Since Northrop built the B-2, it's
not surprising they chose a variant of the astro inertial instrument
they supplied to the SR-71.

However, the B-2 device looks different. It does have the self-test
"artificial star" mentioned in the SR-71 manual. It's a small collimator
within the case at about horizon level. While observing real stars the
telescope never goes that low. As I recall, its field of regard is a
cone within 45° of the zenith.

https://www.sr-71.org/blackbird/manual/4/

The SR-71 manual says, "If either GMT or Julian Day are in error replace
the chronometer and restart the alignment. If chronometer replacement is
not available, fill DAY and TIME as accurately as possible. Time error
greater than 2 seconds may result in star tracking but with erroneous
updating."

In theory, if the aircraft is parked at accurately known coordinates and
the time is approximately correct, the system could perform a time sight
to "hack" its clock. But I've never heard of such a capability.

Speaking of parking spots, when I arrived in the world of B-52
maintenance in 1980 it was sufficient to initialize the ASB-9A lat/lon
mechanical counters (similar to automotive odometers) to N47 57 W097 24,
wherever the aircraft was parked. (That's from memory and I'm not
absolutely sure of those numbers.) A few years later the B-52 was
updated with a pair of SPN/GEANS inertial platforms. One set of
coordinates for the whole ramp wouldn't do. Each parking spot had its
own coordinates, and there was even a survey mark cemented in the floor
of our maintenance shop. The INS test set was positioned over this mark.
I would have liked to watch the surveyors set the mark, but I was on
temporary duty at another base going to school on the new system.

--
Paul Hirose
sofajpl.com

   

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