Inertial Navigation ???

From:         Ed McBride <emcbride@wybron.com>
Date:         09 Mar 2000 10:41:45 -0800
Organization: Wybron, Inc.
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Please note that my areas of expertise are mechanical engineering,
engineering mechanics, and some classical physics.  I have essentially
no background in aeronautical, electrical and a bunch of other stuff.

I am a consulting engineer in Colorado Springs with a client who wants
to track a few objects using on-board accelerometers and gyroscopes
that communicate with a central processor. The objects are confined to
an area of roughly 25 meters square, and a height of no more than 5
meters above ground.  It is necessary to know the location of each
object within about 0.1 m, over a time period of a half hour or so.
The objects can move at speeds up to 10 m/s, and can experience
accelerations up to 2-3 g (20-30 m/s^2).  The objects can also rotate
at angular velocieies up to a few rev/s. (10-15 rad/s).  The cost of
the system cannot exceed something like $2,000 per object.

My investigations indicate that there at least two major problems:

1.  Accelerometers have output errors that will, through random walk,
lead to large position errors in a relatively short time.

2.  In order to track using accelerometers, it is necessary to know
orientation very accurately.  I can't find anything that even comes
close.

But I keep hearing that people take off in an airplane, fly across
country, and know where they are within a foot or so.  The only
explanation I can come up with is that these airplanes must be
"re-zeroing" rather often.

I obviously have about a zillion questions.  Can anyone please either
explain to me what's going on, or point me to a reference that will
help me figure things out?

TIA, Ed McBride, P.E.