Hi Mauricio,

This is a part of the same project as the encoder question was: the
autonomous helicopter project that I've been working on for quite some time.

We are using inertial navigation techniques for position/orientation
sensing. For our first test flights, we are hoping to have an inertial
navigation system that can know the position of the vehicle to within about
1 meter for almost a minute. We have an excellent and expensive inertial
measurement unit but we need to know the magnitude of the gravity vector in
order to subtract it out properly. Hundreds of microG can make a
significant difference in accuracy. There probably are ways to use the IMU
to measure what I need to know accurately enough, but because we are so
close to the IMU's level of inaccuracy, I thought I would try to find out
the actual value.

You can see a photo of the latest incarnation of the vehicle at:

http://www.people.cornell.edu/pages/ebn2/105_0583.JPG

That block in the center is the IMU.

Sean

At 11:48 AM 6/10/2003 -0300, you wrote:
>Sean:
>         Can I ask you, what are you going to do with this information...
>?
>
>Thanks
>
>Mauricio Jancic
>Janso Desarrollos
>(54) 11 4542 3519
>Republica Argentina
>
>
>-----Original Message-----
>From: pic microcontroller discussion list
>[mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Sean H. Breheny
>Sent: Martes, 10 de Junio de 2003 11:43
>To: PICLIST@MITVMA.MIT.EDU
>Subject: Re: [OT]: Gravity
>
>
>Hi Adam,
>
>Thanks, I will give that a try (emailing them).
>
>Actually, I found that same site before. The problem I'm running into is
>that most of this data is given in the format of "height above the
>Geoid". As I understand it, they pick some gravitational equipotential
>surface and call it the Geoid, and then give you the physical height of
>each point on earth above or below this surface. I think this is done
>because one of the applications of gravity data is to know exact
>elevation (or at least, it seems so from what I've read) and also to
>allow easy reference to GPS data (which I believe uses the Geoid as its
>reference, too).
>
>I don't know how to convert from this data format to local gravitational
>acceleration. If I knew what the potential level of the Geoid was then I
>guess I could back out the local g by using the 1/r^2 gravity formula
>and assume that the extra eight above the Geoid was just added to the
>radius of the earth at this latitude and plug that into the r^2 part.
>However, there are multiple opportunities for mistakes and assumptions
>there and so I just kept looking to see if there was a map or dataset
>that gave me the data directly.
>
>Sean
>
>At 07:53 AM 6/10/2003 -0400, you wrote:
> >Did a google
> >http://www.google.com/search?q=earth's%20gravitational%20field
> >
> >and found the website of the two satellites used to measure the
> >gravitational field http://www.csr.utexas.edu/grace/
> >
> >I couldn't find the actual data online in my cursery search, but you
> >can probably find out where to get it by emailing the professors
> >involved in the above link.
> >
> >Since it's NASA, the data should be publicly available.
> >
> >-Adam
> >
> >Sean Breheny wrote:
> >
> >>Hi all,
> >>
> >>Ready for another of my OT questions? :-)
> >>
> >>I'm trying to find a map or other data that can tell me the local
> >>gravitational acceleration if I know the lat/lon of a point on the
> >>earth. In other words, I need to find out the local gravitational
> >>acceleration to better than 1 milliG in Ithaca NY. So far in searching
>
> >>the web, I have not found anything that gives this information (at
> >>least not directly, I found maps of height above the Geoid but I don't
>
> >>know everything that I need to know in order to convert this to simple
>
> >>local gravitational accel).
> >>
> >>Any ideas?
> >>
> >>Thanks,
> >>
> >>Sean
> >>
> >>--
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> >>
> >>
> >>
> >
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