How exactly do epochs fit into the computational propogation of data of NASA's Jet Propulsion Laboratory and what methods do they use to calculate the properties of those epochs in the first place?

Here's some reports I've found which suggest methods such as radio tracking by spacecraft, astronomical observations and radar ranging:

Relativistic aspects of the JPL ephemeris

The Planetary and Lunar Ephemeris DE 421

  • $\begingroup$ There are many additional relevant links and references to look at here and also here. $\endgroup$
    – uhoh
    Apr 8, 2018 at 22:45
  • $\begingroup$ and perhaps something more about TDB here and here and here as well. $\endgroup$
    – uhoh
    Apr 8, 2018 at 22:58


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