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I recently noted in the November 22, 2007 issue of EDN (Electronic
Design News - <>), page 19 a brief announcement
regarding a MEMS-based (microelectromechanical-system)
inclinometer apparently capable of accurately measuring the
deviation from vertical by sensing the downward G force.  The
device/s is described as an Analog Devices (<>)
ADIS16209.

All the key elements are obviously available to designers for
construction of an advanced sextant now, even one that
automatically tracks objects day or night.  I suspect a gimballed
arrangement would make the software engineering easier although
it might be avoided at the expense of much more complicated
tracking schemes.  I recently saw optical sensors for similar
systems go for several hundred dollars each on eBay.  The cost of a
'new' sensor like these in small quantities would be much pricer.

Frank J
Rochester, NY

Date sent:          Wed, 28 Nov 2007 22:25:11 -0800
From:               Paul Hirose 
To:                 NavList@fer3.com
Subject:            [NavList 4161] Re: automatic celestial navigation
Send reply to:      NavList@fer3.com

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 From the web page it's not clear to me how much the devices
resemble
the astro-inertial navigation systems I've seen.
http://aa.usno.navy.mil/otherprojects/

However, judging from this USNO paper, "Celestial Augmentation of
Inertial Navigation Systems", the main idea is to eliminate the
mechanical complication and expense of a gimballed star tracker. In
its place will be a fixed, wide angle tracker similar to those
currently used to sense spacecraft orientation.
www.dtic.mil/dticasd/sbir/sbir021/n104.pdf (about 30 k)

Interestingly, that paper says the SR-71 astro-inertial unit had a
catalog of 57 stars. I wonder if those were the same 57 stars listed
in the nautical almanac.

Note the tiny field of view: 6 minutes of arc.

The B-2 AINS (astro-inertial navigation system) uses 61 stars. It
operates day and night. Accuracy is classified, but the statement in
the paper that it renders GPS "virtually superfluous" is food for
thought.

Time comes from a battery powered time transfer unit. The ground
crew
synchronizes this to a time code generator (regulated by a rubidium
oscillator) and installs it in the plane shortly before flight.

Back in my day (1990s) the time code generator itself was
periodically
checked against WWV time ticks from a receiver in the same rack.
A
digital readout showed the amount of error. You had to input
distance
in miles via a row of thumbwheels to account for propagation delay
from the transmitter. Reception was frequently lousy, however.
Nowadays I suspect the shortwave receiver has been replaced by a
GPS
time standard.

The AINS is interesting to play with -- for a short while. After you
try all the functions and look at all the screens, there isn't much
more to do. No expert eye or skilled touch is needed. Just push the
right buttons and the machine does the rest with fantastic accuracy.
That's exactly what you want if the objective is to get the job done.
But if you're looking for fun and a feeling of accomplishment, a
bubble sextant beats the B-2 AINS.

--
I block messages that contain attachments or HTML.




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