Unit conversion of ut1_to_tt in cirs_to_itrs(), itrs_to_cirs(), tod_to_itrs(), and itrs_to_tod()

[hannorein]

I think there is another error related to the time calculation, specifically on this line in cirs_to_itrs. ut1_to_tt should not be subtracted from jd_tt_low.

[attipaci]

Hi @hannorein,

Thanks for finding the bug. The line you pointed to is definitely not right, although for a slightly different reason than what you noted. Notice, that cirs_to_itrs() takes a TT-based JD as its input, but then needs to call cel2ter() with a UT1-based JD argument, which is why ut1_to_tt1 must be subtracted. However, there was indeed a bug, since ut1_to_tt is in seconds, whereas JD is in days. So correctly, it should be:

   cel2ter(jd_tt_high, jd_tt_low - ut1_to_tt / DAY, ...);

[attipaci]

The issue affects itrs_to_cirs(), tod_to_itrs(), and itrs_to_tod() also. I committed and pushed a different fix in the commit 63449f6. I hope that resolves it sufficiently, but let me know if you think there is more to be done.

[hannorein]

I noticed the units don’t work out. But your are right about the UT vs TT

[attipaci]

Given the severity of this bug, I also added the fix directly to the main branch.