Re: epoch

Alan Pickup (
Tue, 11 Mar 1997 19:28:42 +0000

In message <>, Anthony
Beresford <> writes
>At 10:53 AM 3/11/97 +0200, Alphonse Pouplier wrote:
>>I have a question:
>>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.

In fact, 0.00000001 day is 0.000864 second.

>>I presume those TLE's are made from several observations.
>>Is the given epoch the one of those observation?
>>My pleasure, avec plaisir,  Alphonse
>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.
>Tony Beresford

With a few exceptions, tles are for a point close to (but not exactly
coincident with) a northbound equator crossing. I doubt if the object is
under active observation at that position - rather I imagine the tle is
an extrapolation from its observed motion at a later time on that rev.
However, to get reasonable mean motion and drag terms, observations
prior to the epoch must (usually) be included in the derivation of the
tle. Of course someone (are you there Mike?) might suggest that some
tles are "predicted", being based entirely on observations prior to the
epoch. Does anyone have the answer?

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