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> *From: *David Pike
> *Date: *2024 Aug 3, 06:39 -0700
>
> 1.Are/were the astro/inertial systems in the SR71 and the B2 only
> practical for flights above the tropopause, or are these systems able to
> receive starlight through cloud.

Altitude doesn't matter as long as the sky is clear, or at least there
are enough holes in the clouds to see some stars. Sunlight is not a
problem. I have had the opportunity to run the B-2 AINS on the ground in
daylight. It is interesting to switch to system to pure inertial mode
and watch the coordinates drift away from the GPS position. Then select
stellar inertial mode and watch the astro tracker pull the position back
to the correct values. (Back in my day the accuracy was classified, so I
won't give any numbers.)

But after a couple such experiences, it's boring. You don't need an
"eye" or "touch" with a sextant or any understanding of tables
incomprehensible to the ordinary person. You merely press the correct
buttons and the device performs to its maximum capacity. A sextant is
fun. The B-2 AINS is not. Of course it wasn't built for fun. But to any
navigation enthusiast who has wished for a chance to operate such a
device, I can say you haven't missed much.


> 2.It’s interesting that you talk about reference positions.What about
> initial heading?I would have thought that in an otherwise perfect system
> the positional error due to an initial position error would increase
> relatively slowly whereas the positional error due to an initial azimuth
> error would build up very quickly.
On the old ASB-9A system in the B-52, heading came from the MD-1 astro
compass (most accurate) or the AJN-8 (?) AHRS. I don't know if the
latter operated by gyrocompassing or a flux valve. Those systems were
maintained by a separate group of people and I never operated them.

The maintenance activity at a SAC bomb wing of B-52s and KC-135s was
massive. For instance, I was in a squadron which did nothing but work on
avionics. It occupied one entire hallway in a big building beside the
flightline. Doors along the hall opened into rooms occupied by the
Bamb/Nav shop (where I worked), ECM (radar warning & jamming), Fire
Control (tail gun), Doppler, etc. Everyone was highly specialized. Each
shop was a little world of its own, with its own boss, toolboxes and
test equipment. We worked only on our portion of the plane. If a ground
test required a tie-in to some other system, a guy from that shop would
power it up for us.

So I don't know much about heading systems in the ASB-9A. For most
ground tests we could use the "emergency control" on the bomb-nav
system. It was a panel which allowed you to bypass the normal airspeed,
altitude, heading etc. inputs and insert arbitrary values.

Later, when the ASB-9A was replaced by the OAS (offensive avionics
system) the B-52 got dual SPN/GEANS inertial nav systems. That fell
within the responsibility of my shop, so we had our own heading source.
I heard a navigator mention one benfit of its extreme heading accuracy.
Most targets were not visible on radar, so they were attacked via an
"offset aim point" whose north and east distances from the target were
known. However, offset bombing is sensitive to any error in heading, so
special techniques were used to minimize it. That became unnecessary
when SPN/GEANS came in. As I previously mentioned, a disadvantage for us
in maintenance was that we needed a list of accurate coordinates for
each parking spot. But we didn't need to initialize the system with even
an approximate heading.

--
Paul Hirose
sofajpl.com

   

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