Re: Inertial Navigation ???

From:         "Chuck" <>
Date:         Thu, 9 Mar 2000 20:40:19 -0500
Organization: NASA Dryden
References:   1
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Ed McBride wrote in message ...

>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?

Aircraft do use inertial navigation systems (INS) for navigating. A
INS combines an inertial reference unit that determines current
position along with a navigation computer. They can provide navigation
to anywhere without any external inputs. But they do typically receive
updated position data from systems such as a flight management
computer or GPS.

Accuracy to within a foot with an IRU is not likely. Accuracy to
within 2-4 nm is more like it for a system that uses laser gyros
rather than mechanical gyros to stabilize the accelerometer
platform. A formula that is often used to get a ruff idea of allowable
drift for a given amount of time in operation is the following: 3+3T

T is equal to the amount of time the system was operated in its
navigation mode. If we say for example 3 hours the approximate maximum
allowable drift would be 12 nm.

Hope that answers some of your questions,