Re: Epoch
Bjoern Gimle@tt-tech.se (Gimle@tt-tech.se")
Wed, 12 Mar 1997 09:51:54 +0100
Alphonse Pouplier wrote :
>In the 2d line (21-32) of the "two-lines" TLE's the reference day
>(epoch) is given with 8 decimals, thus to .000001 of a second.
No, 86400 sec/day is about 10**5, ie. 10**-3 or millisecond,
not microsecond, precision remains.
And Tony Beresford answered :
>Alphonse, the epoch is the time of an equator crossing. If you
>look at the TLe's of objects in low eccentricity orbits you
>will see that the sum of the mean anomaly and the argument
>of perigee is always very close to 360 degrees.
By definition, this is not a requirement, but NASA elsets are
quite close. This is handy for rough computations, but prediction
programs work just fine with any epoch (AOP+MA not close to 360).
NASA elsets for geostationary, and some high orbit research
satellites, often have an epoch at midnight UT. Amateur elsets
generated by PELCOR (or FITELEM?) can have any combination,
depending on the initial orbit used.
VEC2TLE, when used to convert XYZ ("state vector") to elset,
defaults to using the vector time as Epoch. When used to apply a
rocket impulse to change a TLE set, it defaults to the time of the
change. In both cases, VEC2TLE has an option to re-Epoch at
Ascending Node Time (with a slight loss of accuracy, according to
the documentation)
In my decay analyses, I previously fitted a function to Epoch
times, but found that the small (random?) deviations from true
node times made the extrapolation unsafe, and switched to using MM
(and ndot)
I have now written a program to compute equator crossing times
from an entire elset file. Below, three runs are tabulated: actual
NASA elsets, Alan Pickup's and my runs with his SatEvo program.
The deviations from elsets' equator crossing times are included.
NASA Epochs appear to have a spread of about one second, close to
zero. SatEvo elset had a very high consistency - all Epochs were
2.29-2.30 seconds before Ascending Node ! These two observations
combined "prove" that these are real effects of the sources, not
program bugs.
OIG Epochs -secs OIG node times +secs Alan's mails +secs My mail
97069.17347493 .288 97069.17347159 -4.56 97069.17341873 5.18
97069.17353152
97069.11293470
97069.11300967
97069.05237822 -1.11 97069.05239118 -3.12 97069.05235497 1.41
97069.05240755
97068.99173053 -.569 97068.99173712 -2.37 97068.99170961 .832
97068.99174676
97068.99172988 -.133 97068.99173143
97068.93101299
97068.93103885
97068.87027388
97068.87029128
97068.80949816
97068.80950913
97068.74869236 -.525 97068.74869844 -.715 97068.74869016 -.185
97068.74869629
97068.68785316
97068.68785570
97068.62698098 -.760 97068.62698978 -.001
97068.62698976
97067.28366537 -.683 97067.28367328 97068.50525876
97067.22246012 -.862 97067.22247010 97068.01728639
97067.22246007 -.592 97067.22246693 97067.52839209
97067.16124962 -.585 97067.16125640
97066.91630664 -.464 97066.91631202
97064.58359546 .260 97064.58359244
97064.27605468 -.424 97064.27605959
97064.21452423 -.580 97064.21453095
97064.21452362 -.582 97064.21453036
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