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I am a junior aerospace engineer and what I am trying to do is getting the acceleration component which is perpendicular to the earth - no matter how the plane is oriented or rotated.

If the plane would always be horizontal (for e.g. on a road) I would just read the Z component of sensor readings and that'd be it. But when the plane is in the air it is different because obviously it can rotate in all directions. What I do know about the aircraft in every moment is it's attitude (roll angle, pitch angle and yaw angle - basically the angles around each axis) and accelerations (X,Y and Z axis). What I need now is a mathematical tool that would take this data and return the acceleration towards the earth.

What do you suggest?

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Using this picture as the standard for pitch, row and yaw

enter image description here

and let x be the forward direction, y be the side, and z be vertical, then geometrically, it looks like the true vertical acceleartion is $$a_{z}\left[\cos(Pitch)+\cos(Row)\right]+a_{x}\sin(Pitch)+a_{y}\sin(Roll)$$


The funny thing is iphone sensors can provide exactly the same readings. Then MIT came up with a method to mash this data together with the GPS reading, and by doing some kind of noise cancelling, it can actually increase the accuracy of the overall data. I would think as flight engineers you guys would have something much more advanced. I would be very interested to know if there are any algorithms you guys use to reduce the noise. I'm trying to build an Inertial Guidance System on the iphone.

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Formulae of course :) –  Primož Kralj Sep 24 '12 at 12:32
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