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Subject: Re: accuracy of automatic celestial navigation
From: Paul Hirose (paulhirose@XXX.XXX)
Date: Mon Dec 09 2002 - 02:07:34 EST
George Huxtable wrote:
>
> Not so difficult to define the direction of a star to that precision,
> perhaps, but what worries me more are the errors in establishing the
> direction of the vertical reference. An aircraft is different from a
> spacecraft in that it has some unavoidable degree of buffeting from the
> atmosphere that it's flying in, small changes in engine output, tiny
> movements of control surfaces, movement of the crew, together with
> gravitational anomalies from the ground it's passing over.
In the case of the B-2 AINS (astro-inertial navigation system) the
star tracker is inside the same case as an inertial nav system. The
inertial and stellar hardware are tightly integrated with each other.
So the star tracker's short term pitch, roll, and heading
stabilization comes from the inertial platform.
Earth's lumpy gravitational field is indeed a factor in inertial
navigation; it has to be stored in some sort of internal map or table.
Through some magic not clear to me (I worked in maintenance, not
engineering!) the AINS feeds the measured directions of stars back to
the inertial portion to correct its errors. Giving the otherwise blind
inertial system a look at the outside world is enormously helpful. As
long as it's got a clear view of the sky the AINS will remain within
[classified] meters of the correct position, worldwide and practically
independent of mission duration. It doesn't drift away over time like
a normal INS.
In obscured skies the star tracker keeps trying unless you deselect
stellar mode. It can shoot through a hole in the clouds. It lingers on
a body only long enough to measure its position, then goes looking for
others in its field of view (anything within 45 deg of the zenith).
Initially the scope is placed on the star's predicted az/el. It
searches outward in a "square spiral" pattern until it locks on or
concludes the star is obscured. There are 61 stars in its catalog;
unfortunately I don't know their identities. The 57 classic
navigational stars plus 4?
Another thing I never learned is how the AINS knows the offset between
UT1 and UTC. An ATTU (airborne time transfer unit) was installed in
the plane shortly before each flight to provide accurate time. Between
flights we kept the ATTU connected to a rack-mounted rubidium time
standard which in turn was synched to radio station WWV. Distance from
WWV was set on thumbwheels to correct for propagation time. But as far
as I know the whole rack was on UTC. I never saw any DUT1 readout
provision or any other hint of UT1. My guess is that the offset was
supplied as part of the computer-generated mission plan uploaded
before flight.
Physically, the AINS is fairly compact. It would fit in the passenger
seat of a small car and weighs roughly as much as a man. Using it to
navigate the car would take some doing, though. It needs a powerful
flow of chilled air into its cooling duct, and gulps a fair amount of
400 Hz 115 VAC 3-phase and 28 VDC power.
In operation the system was quite reliable. I remember only a few
times when the AINS had to changed out. Good thing, since it was
removed with a crane. The location is easily visible in an overhead
view of a B-2: the star tracker window looks like a round dark hole
about the size of a dinner plate between the cockpit and the left hand
engine air intake.
I got to run it outdoors a time or two. It was started like a normal
INS. You inserted the lat, lon, and elevation, then selected an align
mode. There were several available. It depended on how much of a hurry
you were in; of course the best nav performance was attained by using
the most time consuming align mode.
(It's not enough to simply know the airplane is parked at such and
such coords when you align the navs. What *part* of the airplane? In
the B-2 the reference point is the nose landing gear; our parking
spots at Edwards AFB all had little brass survey marks set in the
concrete at the spot where the nose tires would go. I bet you won't
learn trivia like that from Discovery Channel!)
When alignment was done, you saw a NAV READY message on your little
screen in the cockpit. At that point the system was frozen to your
starting coordinates, waiting for you to select a nav mode. Again
there were choices, basically boiling down to what kind of assistance
the inertial platform would receive. For example, you had pure
inertial, stellar inertial, GPS inertial, etc.
Being an enthusiast I liked to select pure inertial and watch the
coordinates drift away from the correct values. Then I switched to
stellar inertial and watched the star tracker pull it back to the
right spot. Worked as advertised - in broad daylight!
Although it was fascinating to do celestial at a superhuman level, I
quickly found it boring. All it required of me was to push the correct
buttons. There was no pride from mastering tables of mysterious
figures, no "eye" or "touch" to develop, no satisfaction at seeing
LOPs meet nicely. I found the pinnacle of high tech celestial
navigation barren of personal satisfaction, and after a couple visits
was satisfied that I had seen enough.
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