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Subject: Re: accuracy of automatic celestial navigation
From: Paul Hirose (paulhirose@XXX.XXX)
Date: Mon Dec 09 2002 - 14:42:05 EST
George Huxtable wrote:
>
> down to affordable levels. And on a rough sea-surface, the disturbing
> accelerations are orders of magnitude greater than a high-altitude aircraft
> has to contend with.
I don't remember the price tag on the B-2 AINS, but it must be
staggering. The thing is an optical, mechanical, and electronic
marvel. We never opened it - if there was a malfunction it went back
to the factory to be repaired in a "clean room".
An INS can handle motion far more violent than a seagoing vessel will
experience. For many years fighter and attack aircraft have carried
INS. The older ones, at least, were not "strap-down" systems, but
mounted the gyro and accelerometer platform on gimbals. But since the
platform maintains a fixed orientation in space as the plane gyrates
around it, inertia is your friend. The gimbal torque motors need only
compensate for the friction in the gimbal bearings, and of course this
is reduced to a very low level.
> In the days before GPS, how could the instaneous position in flight be
> independently determined sufficiently accurately, for testing purposes? Is
> Loran accurate enough, and fast-responding enough? The aircraft will
> presumably pass through 15 metres in less than 30 milliseconds, so it's a
> demanding requirement.
A tracking instrument on the ground called a cinetheodolite is the
old-tech method. You often see these in documentaries about the space
program. The operator manually follows the target as a motion picture
camera records the scene. Azimuth and elevation readings are optically
superimposed on each frame, along with the time. With simultaneous
film from two or more of these instruments it's possible to
reconstruct the target's flight path.
The "kinetos" I saw at Edwards AFB were so big, they were motorized
and the operator rode the instrument.
http://www.serve.com/mahood/nellis/ttr/sln5.htm
http://www.emitechnologies.com/optped.html
http://www.acq.osd.mil/te/mrtfb/commercial/afftc/time.html
Rodney Myrvaagnes wrote:
>
> I don't understand how the earth's lumpy gravitation could affect
> inertia in a B2. I know it goes pretty fast, but it would have to get
> close to the speed of light for relativistic effects to become
> comparable to the drift of the ANS. Am I missing something?
The 1984 Bowditch explains this better than I can:
"Since an accelerometer cannot distinguish between a kinematic
acceleration and a gravitational acceleration, any uncertainty in the
gravitational environment manifests itself as a system error. in the
case of marine inertial navigation in locally level coordinates, it is
the horizontal components of gravity that cause significant errors.
These are directly due to deflections of the vertical (art. X4), which
are tilts of the actual (plumb bob) vertical vector relative to the
presumed reference vertical."
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