gravitydir

Create an identity quaternion and get the corresponding gravity direction vector. Verify that the gravity direction vector is [0 0 1] as expected.

quat0 = ones(1,"quaternion");
dir1 = gravitydir(quat0)

dir1 = 1×3

     0     0     1

Next, create a quaternion that corresponds to an x-rotation of 90 degrees. Obtain the gravity direction vector.

angles = [0 0 90];
quat1 = quaternion(angles,"eulerd","ZYX","frame");
dir2 = gravitydir(quat1)

dir2 = 1×3

         0    1.0000    0.0000

You can visually verify this result by using the poseplot function. From the figure, you can see that the gravitational acceleration is along the y-axis.

poseplot(quat1)

Change the reference frame to the east-north-up (ENU) frame and get the gravity direction vector.

dir3 = gravitydir(quat1,"ENU")

dir3 = 1×3

         0   -1.0000   -0.0000

Finally, create a vector of random quaternions and obtain the corresponding gravity direction vectors.

rng(2023) % For repeatable results
quats = randrot(10,1);
D = gravitydir(quats)

D = 10×3

   -0.2859   -0.8615   -0.4197
   -0.7971   -0.3204    0.5119
    0.3598    0.7139   -0.6007
    0.3622    0.6720    0.6460
    0.1749    0.4020    0.8988
    0.4627    0.3655   -0.8077
    0.3134    0.8613    0.4000
    0.8776    0.4267   -0.2185
   -0.9924   -0.1226    0.0137
    0.4086   -0.4099    0.8155