PSN-L Email List Message

Subject: Re: Article on gravimeters
From: "Tom Schmitt" tschmitt@..............
Date: Sun, 7 May 2000 16:58:41 -0400

----- Original Message -----
Sent: Sunday, May 07, 2000 1:25 PM
Subject: Re: Article on gravimeters

> Dear Tom Schmitt,
> >  About fifteen years ago I went to a non-classified briefing about a
> >  version of a gravity gradiometer. The Defense Mapping Agency had it.
>     Were any details published?

Not that I know of.  It was a briefing.

> >  It was in a Winabago of some sort
>     What is that, please?

A large recreational vehicle.  A bus like thing with all sorts of gear in

> >  and had three disks that rotated at a fairly low rpm, a few Hz at most.
> >  The disks were less than a meter in diameter.
> >  The disks had two accelerometers on them.  The acceleration as a
> >  function of position of the two accelerometers was recorded and some
> >  of auto-correlation or FFT was done to get the direction and magnitude
> >  the gradient.
>     Did the disks all rotate at the same speed? Were they all the same
> diameter?  Were they orientated at right angles to each other? Were they
> mounted on the same frame?

The all rotated at the same speed.  Each disk had two accelerometers 180
degrees apart.   At any time the acceleration
on them was

                        (1) an omega sqaured r term radially out ward.

                        (2) the component of gravity resolved to the
orientation of the line the two
                             accelerometers were on.

                        (3) any acceleration due to bumps in the road.

The two accelerometrs per disk were necessary to compensate for any bumps.
The difference between
the readings of the two accelerometers gave the gradient of gravity in the
direction the disk was pointing at
that time.  A bump would cause the same acceleration at both accelerometers
and thus would cancel out
when the gradient was measured.

The axeses of rotation were 90 degrees to one another.

They were all mounted to a very stiff platform.
>     Were the two accelerometers diametrically opposed and on the same

Yes above.
> Were they sensitive to forces in a radial direction or perpendicular to
> disk?
Yes above.
What sort of accelerometers were used?

That is the trick I am sure.  Though the briefing was not classified they
did not show us the parts manual!
I suspect these were very special accelerometers.  They had to be sensistive
enough to measure the difference in
g over a distance of one meter.

>     How were the signals and power transferred to the disks?
>     How were the disks driven? Were air bearings used?
>     Was there any automatic gravity alignment system? Naval gyro compasses
> and aircraft artificial horizons have them built in.

I do not remember or was not told.

>     I am trying to understand the principles / practical limitiations of a
> system which can detect the tiny changes in force that need to be measured
> against the noise background of even a large ship. Doing it in the
> plane sounds difficult enough. Doing it in the vertical plane would seem
> be much more difficult. Doing it while the system is in motion sounds very
> dificult.

Honestly, I had trouble believing the thing worked.  The physical principle
is simple but how they got
the system to work I do not know.   I am sure they did a lot of post
collection processing.

They claimed that they could drive the thing at 30 miles an hour and get
good data.

About that time there was at least one sattelite launched at low altitude
that had very good
elevation measurements.  I suspect that that technology replaced the land
based measurements.
However, my satelite geodesy is real rusty ( never was much good ) and that
stuff was not

>     Regards,
>     Chris Chapman
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Larry Cochrane <cochrane@..............>