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Subject: Re: Digital Sextant
From: Paul Hirose (paulhirose@XXX.XXX)
Date: Mon Apr 01 2002 - 01:15:49 EST
Hal Mueller wrote:
>
> In the round of SBIR topics that closed back in January, US Navy's
> SPAWAR was looking for proposals for developing an automatic
> celestial navigation system that would be able to image stars in the
> daytime. There was also some work by SPAWAR a few years back to
> adapt space/aviation star trackers to shipboard use.
For many years military airplanes have had star trackers. For example,
the B-52 had (it was deleted when inertial nav was installed) an
astrocompass which would give an accurate heading even in polar
regions. The business end was a prism protruding a little above the
top of the fuselage and protected by a small (5"?) glass dome. You set
the star's SHA and declination into the system via mechanical drum
counters, similar to odometers. The system clock also had to be set to
GHA Aries. The present position required to solve the celestial
triangle came through a tie in to the bomb-nav system. The horizontal
reference was gyro-stabilized.
It was called an astrocompass, but the system actually did a full
sight reduction continuously in real time. Counters displayed
heading to a tenth degree as well as the intercept (to 1 minute? -
can't remember anymore).
My memory is hazy, but I believe there were two sets of SHA/dec
counters and a knob so the navigator could switch back and forth
between stars quickly.
The system could use the sun or stars. Can't remember if it was good
enough to track stars in daylight. There was an "aurora filter" which
could be swung into the optical path via a switch throw.
My specialty was maintaining the bomb-nav system, but the astrocompass
was of similar vintage, so I assume it used the same technology:
vacuum tube analog summing amplifiers, mechanical differentials,
synchros, etc.
Years later, in the B-2 program, I encountered an AINS (astro
inertial navigation system). As the name implies, it married an
inertial platform with a star tracker. They were physically in the
same unit. The star tracker was a little telescope (looked like a
Schmidt Cassegrain) on an alt azimuth mount. Instead of being shaped
like a dome, the optical window was flush with the aircraft skin.
Field of view was a cone centered about the zenith. It could see up to
45 degrees away from the zenith.
If you look at an overhead shot of a B-2, there's a round dark spot
about the size of a dinner plate between the left hand air intake and
the cockpit. That's the AINS window.
This system absolutely would track stars in broad daylight, even under
a hazy sky. I don't think it observed the sun, just stars. It had a
catalog of 61 stars. Unfortunately I never asked an engineer for a
list of those stars.
In the early years of the flight test program, GPS was not yet
integrated into the B-2 avionics, so system time came from an ATTU
(airborne time transfer unit). Setting the time manually into the
plane wouldn't give enough accuracy. Shortly before flight, we'd put
fresh batteries in the ATTU, "hack" it electronically from a lab time
standard, then install the ATTU on the plane.
AINS operation was totally automatic. It went from star to star on its
own schedule, tracking each for less than a minute. This system had
the advantages of an inertial nav system, except that the drift was
bounded by the star tracker. I.e., the nav solution stayed within x
meters of the truth no matter how long the flight, or where in the
world you were. The "x" is classified, but I wouldn't bet on beating
it with a sextant!
On one occasion I was cockpit operator and had enough slack time to
play with the nav system on a B-2 parked outdoors. With the AINS in
"free inertial" mode (star tracker disabled) I could see its position
slowly drift away from the GPS. But when I switched to stellar
inertial mode, it pulled right back in. Very impressive, and this was
in bright sunlight.
I was told this astro inertial platform was nearly the same device the
SR-71 carried. You can see the same round port on top of the '71. I
also heard Northrop's expertise with this type of navigation was an
outgrowth of the old Snark intercontinental cruise missile. They had
to develop astro inertial because a pure inertial system would drift
too far during the missile's flight, much longer in duration than an
ICBM's.
--paulhirose@XXX.XXX (Paul Hirose) B-2 flight test maintenance, 1987 - 97
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