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Just what the title states please. This site writes to say

Orbit epoch is the time at which the established orbital elements are true

The site goes on to mention sub-elements -

  • Start Day The satellite's start date that we are interested.
  • Start Month The satellite's start month that we are interested.
  • Start Year The satellite's start year that we are interested.
  • Step Size The interval between calculated output points.
  • Propagate Duration The satellite's time period that we are interested.

I'm less than nouveau at this stuff - my programming background tells me a suitable analogy for 'orbital epoch' may be 'timestamp'.

Is this interpretation of the term correct?

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Please remember that time depends on your reference frame (relativistic effects do matter). –  Deer Hunter Sep 11 '13 at 6:12

2 Answers 2

up vote 3 down vote accepted

The Epoch in the common Two Line Elements (TLEs) is in fact a record of when the TLEs are valid. While some of them only change very slightly with time most of the time (Period, eccentricity, perigee and apogee), at least one of the parameters changes frequently with time (Argument of Perigee). In this case, you have to know when they are valid from, in order to determine their work.

If you take a TLE file, and change only the Epoch, you will change the location of the orbit, and possibly the plane of the orbit. The period, etc will remain the same, however.

Quoting from the TLE definition page on CelesTrak

The epoch defines the time to which all of the time-varying fields in the element set are referenced.

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Somewhat. The term epoch denotes distinctive time at which an era of some significance or distinction has started, so while they are some timestamps relative to an arbitrarily chosen and fixed date of the first epoch, they are not just any timestamps (they have more weight), and in our case also not merely timestamps (they also include two other data), so perhaps a better replacement term could be a milestone, or even a stage.

The problem of using timestamp as a replacement term is that there might be several reference times defined, even for a single project, event, or as is in our case - a mission. For example, you could be using recorded times by the probe, and translate those to a local time. Both would then be timestamps, only their reference time would be different. They might also not mark any time of greater significance for your project, or indeed say anything about whether it's a start, or a beginning of something. Timestamp is also just that, a recording of date and time in a predefined format, relative to a predefined reference time frame, and the term doesn't denote the inclusion of some other descriptive parameters. Timestamps might also be a lot more frequent than epochal events.

And what you called sub-elements of the orbital epoch in your question, those merely define some arbitrary format to record each epoch, and are specific to their use. The point of a static format being, that you can easily retrieve any individual pieces of the information it contains at a later date. You could easily define your own format, and a timestamp would be a part of it, just like the first three parameters in our example. But the last two parameters are possibly already outside the definition of a timestamp, depending on your convention, especially if you'll be using timestamps to also record other events, not just those of greater significance.

So to avoid confusion, I'd suggest using a different substitution term, or even simply an epoch, now that we've hopefully described what it means in a way that's easy enough to understand.

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