>         Actually, to nitpick a little, gravity CAN be distinguished fro=
m
> acceleration. The reason is gravity changes with altitude, the farther=20
> you
> are from Earth's surface the less it's effects. Therefore, on a rocket
> traveling away from the earth the force of gravity at the tip will be
> slightly less then at the tail. Acceleration OTOH is consistent along t=
he
> whole rocket. So it is theoretically possible to distinguish the two=20
> (using
> two sensors you can find the difference in the force on the rocket, usi=
ng
> the rockets length and altitude you can figure out the force of gravity=
=20
> on
> it, and using that you could theoretically null the effects of gravity =
on
> your number).
>
>         Now, for reality, this effect is so small it takes VERY=20
> sensitive and VERY
> large instruments to detect. So it's pointless in the sort of app we'd=20
> all
> consider, nevertheless the difference IS there. I'm just in a nitpickin=
g
> mood at the moment, forgive me! :) TTYL

Well, given that according to NPL

g =3D 9.780 318 4 (1 + Asin2 L - Bsin2 2L) - 3.086=D710-6 H

where=09

A       =3D 0.005 302 4
B       =3D 0.000 005 9
L       =3D     latitude=09
H       =3D     height in metres above sea level

Since this is obviously a linear approximation to another more complex=20
functions only intended for small values of H i will assume the rocket=20
mentioned is an Saturn V (363 ft/110m high) just on the point of lift off=
=20
 from Cape Canaveral (Lat: 28.4675 , Long :-80.5664) and that the two=20
accelerometers are at the extreme ends of the vehicle.

At the bottom of the rocket

g =3D 9.780 318 4 * 1.000190644 - 0
   =3D 9.872200115 ms^-2

at the top of the rocket

g =3D 9.780 318 4 * 1.000190644 - 110*3.086=D710-6
   =3D 9.871860655 ms^-2

a difference of 339.4602057 =D7 10^-6   ms^-2

Of course, as the earths gravity drops as you move further away this=20
figure will become smaller, so at it's maximum the difference is 0.00344 =
%

The rocket is accelerating at around 60 ms^-2 so this represents 0.000573=
=20
% of the overall signal meaning to meven discern it's presence you need a=
=20
Signal to Noise ratio for the entire system better then 112dB !

Sorry I likewise had an attack of gratuitious accuracy!

Alex Rice

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