# Does the Zenith have anything to do with the read-out of an IRU accelerometer?

Let's say that I have an accelerometer as part of an inertial reference system. Will the position of the Zenith with-respect-to the current position (in terms of rotation) of my accelerometer, have any impact on the accelerometer readings? Is there any kind of relationship at all?

Or, in other words, do we ever speak of accelerometers as part of an IRU... in terms of relationship with the Zenith?

• @BowlOfRed That is exactly what I mean. Let me edit this question, hopefully things will be a little more clear. – Snoop Sep 14 '16 at 18:20
• I'm not sure what you're asking here, can you clarify what you mean by the "position of the zenith"? Are you using some specific point of reference? – Pondlife Sep 15 '16 at 23:13

Good question.

Another name for the "position of the Zenith" would be the gravity vector. Which for an aircraft is always down (shortest distance to reunite with terra firma).

Note that it is necessary to subtract acceleration due to gravity from the vertical channel before performing the integration.—An introduction to inertial navigation

So the answer is yes, the acceleration due to the gravity vector is corrected.

Check the PDF for a detailed description and equations.

• This helps, thanks. – Snoop Sep 14 '16 at 18:51
• shortest distance to reunite with terra firma Even when flying next to a ridge? – Steve Sep 14 '16 at 19:42
• @Steve you'll hit the ridge, and fall down... longer distance ;) – ymb1 Sep 14 '16 at 19:43
• It should also be said that because the aircraft speed relative to Earth is bounded, the acceleration due to gravity is the long-term average of the measured value. And it's the only way to find it. – Jan Hudec Sep 16 '16 at 18:58