Why does ‘g’ need to be multiplied by -1 in the NED coordinate system?

[WongRZ]

Dear author, In #10 Issue, you mentioned that " The accel in the body frame = c_n_to_b * f_NED = c_n_to_b * [0;0;-1]g ". My question is, why does g need to be multiplied by -1 in the NED coordinate system? Isn't the direction of gravity consistent with that of D? Thanks a lot.

[dxg-aceinna]

Hi WongRZ,

Sorry for the late reply because I haven't been working on this project for a while. I hope you have found the answer. But if not, please let me explain.

First, the accelerometer measures NON-GRAVITATIONAL acceleration: f = a - g f is the accelerometer measurement vector, a is acceleration we can observe, and g is gravity vector.

When the unit is not moving, a = [0; 0; 0]. On Earth, in the NED frame, g = [0; 0; 1]g.

So, f = -g = [0; 0; -1]g.

[WongRZ]

Thanks a lot for your reply. I found the answer, which is consistent with your reply. And the simulation data works in my code now. This is a very convenient simulation project. Thank you for your efforts.

[dxg-aceinna]

Thank you for your feedback.