Can the epoch time of a TLE be in the future?

Context: ISS TLE from Heavens Above

1 25544U 98067A 18193.60071450 .00016717 00000-0 10270-3 0 9013
2 25544 51.6401 252.0472 0003848 304.4303 55.6485 15.53943096 2461

This gives the date 12 July ~2pm UTC, but the current time is ~9am UTC. Is this a mistake?

  • In general, it can, showing the predicted position. I'm not sure about this specific case; it's sufficient to compute the orbit and position at arbitrary not too distant time (what the site needs), but obviously that's a calculated TLE, not measured, so not as accurate. – SF. Jul 12 at 9:07
  • I thought this question has been answered here before, and that the answer went into some detail about why this is done from time to time, but I can't find any trace of that now. @PearsonArtPhoto does this issue look familliar? – uhoh Jul 12 at 9:43
  • @PennyPlayer thanks for the accept, but why not wait a few days to see if further answers are posted? It's okay to accept right away, but it sometimes discourages others from adding additional, also informative answers. Maybe something I've said is incorrect or needs clarification. – uhoh Jul 12 at 11:36
up vote 0 down vote accepted

I thought this question has been answered here before, and that the answer went into some detail about why this is done from time to time, but I can't find any trace of that now.

The epoch could, technically, potentially, be in the past or the future by quite a lot, as long as the satellite isn't falling too quickly, since the SGP4 algorithm propagation is predictable and deterministic.

In other words, the epoch could be next year, as long as when you run a recent, supported SPG4 propagator it produces a fairly accurate answer now. However, most people interpret the epoch as the time of best accuracy, though it doesn't necessarily have to be.

So I think the answer to the limited question

Can the TLE epoch be listed as in the future?

is yes it could be. Nothing mathematically prevents that from happening as long as at the current epoch (i.e. now) SPG4 returns a fairly accurate answer that hasn't burned up in the Earth's atmosphere yet, and this is because SPG4 is predictable and deterministic.

update: I went to Space-Track.org and downloaded all TLEs issued on day numbers 191 and 192 of 2018. You can see there are many issued with epochs days or (not shown) months in the past, and some stragglers with epochs many days into the future. That the TLEs with epochs in the future are higher altitude is probably meaningful. Plotted with https://pastebin.com/LaxR6p4Y

enter image description here

enter image description here

From Space-Track.Org FAQ

TLEs can contain future epochs.

About 20 satellites are categorized as "multi-day objects" because their period is so large. Consequently, our data provider propagates the epoch into the future based on perigee to enable better tracking by available sensors when the object finally comes back into view.

An example is Object 10370 with a 5683.23 minute period.

This is an explanation, but it wouldn't apply to the ISS.

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