Dear Mr.Harris,

    Thank you for a very interesting letter.

>  First, as some may know, inertial navigation depends on the use of very 
> precise gyros to stabilise a platform about all three axes.   
    Were they using all air bearings and quartz controlled synchronous 
motors, please?
    Was the angle sensing LVDT, LCDT, optical fringe or what?
    
> Mounted on the platform are two very precise calibrated accelerometers
>  which measure the two components of the horizontal acceleration. 
    What principle was used for measuring accelerations and were they low 
range absolute units, rather than 'AC' / piezo etc. types, please? 

> The outputs of these accelerometers is doubly
>  integrated, and used to correct the vertical orientation of the platform by
>  precessing the gyros.  Back in the 50's, such a platform could be 
stabilised 
> to something like a minute per hour or less. 
    It sounds impressive. Do you know if there are any inertial platforms 
going cheap on the used equipment market? Getting one might be a good start 
to developing a gradiometer!

    Regards,

    Chris Chapman
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