Joseph Kern wrote:
>What exactly is an Epoch date?
Tony Beresford answered your question exactly,
but you are probably still wondering why the epoch
date of Keps should be of concern to a satellite
observer.
The epoch date tells the satellite observer the
age of a set of TLE (Two-Line-Elements). It is
always best to use the freshest elements possible;
however, depending on the satellite and its orbit,
the observer can judge "how fresh" the Keps need
to be.
A one (1) day old set of Keps for the shuttle
could be half a world off, if for instance the
shuttle has been firing its rockets.
A one (1) year old set of Keps, if it's the only
set you have, (as pointed out to me by Alan Pickup)
is useful for some information; however, don't
expect it to give you an accurate prediction.
So there is no hard set rule regarding how fresh
a set of Keps needs to be other than "the fresher,
the better." By knowing the epoch date of a set
of Keps, the observer can judge the likely accuracy
of any predictions made with that set.
I have been posting on SeeSat-L an "Epoch Date Age
Analysis" of some popular files used by satellite
observers. The EDAA lists the file and a breakdown
of the epoch dates. Here is an excerpt from the post
I made last Friday, 21-June-2002 (three days ago).
****************************************
KEPTLE.ZIP - Updated Tuesdays & Fridays.
http://sattrackhouston.com/keptle.zip
Today's Epoch Date 320
One (1) Day Old 241
Two (2) Days Old 391
Three (3) Days Old 86
Four (4) Days Old 125
Five (5) Days Old 106
Six (6) Days Old 132
Seven (7) Days Old 59
Older Than One Week 2898
---
4358
*****************************************
It is not important how many Kep sets bear
today's epoch date. The main fault with
this file is that there are too many Kep sets
older than one week. I don't think I have
offended anyone because this is my file.<g>
To learn how to read the epoch date of a
Kep set:
http://sattrackhouston.com/epochexp.htm
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