How are Two Line Elements (TLEs) obtained?

Question

This question is a follow up of another question I have asked recently.

For the sake of experiment I propagated the ISS's position (using SGP4) from epoch of first TLE to the epoch of next released TLE. The difference in time is a little over a day. The resulting position comes incredibly close to the one in the epoch of the second TLE. The difference in position vectors is in the range of meters. So, as strange as it may sound, instead of being happy about it, I am a bit concerned.

That result makes me wonder — is it possible that the propagation in the time span of a little over a day can be so accurate? Or is it due to the second TLE being produced from a series of previous TLEs? What is the most likely reason for my results? And what is the usual way of producing TLEs on the Celestrak website?

Answer 1

From my experience, most of the time the elements don't jump more than a few km at most over a 24 hour time span. There are some rare instances where there are dramatic jumps for no reason, but things in space just don't move around that much without a reason.

As for the meter accuracy, that seems a bit high. There are a few things that might be why that is the case:

1. The ISS is really large, and close. Between the two, it is much easier to determine it's exact location, which helps in making more accurate predictions.
2. The ISS being manned is of more interest than most objects, it might have more updates as a result.

I would test something like a booster or dead satellite, and see if you get the same result. But it does seem possible that the result you have is reasonable.