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I used SPICE's pxform at an interval of epochs to determine the transformation from J2000 (inertial) to ITRF93 (Earth body-fixed) frame. Then, I converted these rotation matrices to quaternions with SPICE's m2q function. Lastly, I converted it to the alternative quaternion formulation where the real part is the last element.

I am looking at the result and I'm not sure that it's reasonable since there are a lot of cyclic fluctuations in the quaternion values. Previously, I have done transformations for J2000 to MOON_ME (moon body-fixed) frame and there was not these cycles in the same epoch interval. Is my result reasonable or do I need to rethink how I'm using SPICE for this to make sense?

The kernels I'm using are as follows:

KERNELS_TO_LOAD=(
    'KernelsMoon\PCK\pck00010.tpc',
    'KernelsMoon\PCK\moon_pa_de421_1900-2050.bpc',
    'KernelsMoon\PCK\earth_000101_200729_200507.bpc'
    'KernelsMoon\FK\moon_080317.tf'
    'KernelsMoon\SPK\de430.bsp'
    'KernelsMoon\LSK\naif0012.tls.pc'
    )

See the picture for how the first quaternion element undergoes periodic change over time: Quaternion element 1 (r*sin(phi)

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I am looking at the result and I'm not sure that it's reasonable since there are a lot of cyclic fluctuations in the quaternion values.

1689593 seconds is about 19.6 sidereal days. The graph shows 19 to 20 cycles -- just about what should be expected.

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  • $\begingroup$ That makes perfect sense. And for the J2000 to MOON_ME frame conversion which I just looked now, there's a cycle as well but for about 27 days due to its rotation. $\endgroup$
    – Shen
    Oct 8, 2020 at 17:36

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