Dear Tom Schmitt,

>  About fifteen years ago I went to a non-classified briefing about a land
>  version of a gravity gradiometer. The Defense Mapping Agency had it.
 
    Were any details published?
 
>  It was in a Winabago of some sort 

    What is that, please? 

>  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 sort
>  of auto-correlation or FFT was done to get the direction and magnitude of
>  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 all 
mounted on the same frame?
    Were the two accelerometers diametrically opposed and on the same radius? 
Were they sensitive to forces in a radial direction or perpendicular to the 
disk? What sort of accelerometers were used? 
    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 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 horizontal 
plane sounds difficult enough. Doing it in the vertical plane would seem to 
be much more difficult. Doing it while the system is in motion sounds very 
dificult.
  
    Regards,

    Chris Chapman
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