I want to convert the epoch time of a TLE to UTC. At this moment I am less interested in explanations and history - I just want to know quantitatively, with certainty, how to convert.
I'm confused because I see references to both UT and JD in NASA and Celestrak sites as show below. Since Julian Date currently leads UT by approximately 68.184 seconds (as well as a 0.5D offset), I am stuck.
The epoch time in the TLE is used in at least two ways. First - it's the time when the prediction from the TLE is most accurate. Second - (I think) it is the moment in time corresponding to the mean anomaly value in line 2 of the TLE, which is 66.7566 degrees in this example:
TLE = """ 1 25544U 98067A 16031.25992506 .00006019 00000-0 97324-4 0 9994 2 25544 51.6430 25.8646 0006733 62.5910 66.7566 15.54344726983528 """
my question: what is the exact relationship between the epoch time in a TLE and the corresponding time in UTC?
The current difference can also be shown with Skyfield:
from skyfield.api import JulianDate jd = JulianDate(utc=(2016,1,1)) print jd.tt, "days" print jd.tt % 1, "day fraction" print ((jd.tt % 1) - 0.5) * 24. * 3600, "seconds"
2457388.50079 days 0.500789166894 day fraction 68.1840196252 seconds
this NASA page calls the fraction "Julian Day Fraction":
but this Celestrak page states that
"Note that the epoch day starts at UT midnight (not noon) and that all times are measured mean solar rather than sidereal time units (the answer to our third question)."