2021-10-02, 11:19 PM
I've been reading the HORIZONS documentation (and of course, I had to try the telnet interface, it's been 20+ years since I last used telnet with a public system and it felt nice, like a trip to my youth) and have a question.
I get that the ephemeris is calculated by successive integrations, with the initial conditions given by the osculating elements. What I didn't find in the documentation so far is how often the osculating elements are themselves calculated. They must be as correct as possible, otherwise they'd be useless as a base for integration. The closest thing in the documentation is the section at https://ssd.jpl.nasa.gov/horizons/manual.html#limits but it doesn't address the frequency of updates to the osculating elements. BTW, I loved the warning in capitals: "IF YOUR CAREER OR SPACECRAFT DEPENDS ON A NON-LUNAR NATURAL SATELLITE OR SMALL-BODY EPHEMERIS, CONTACT JPL BEFORE USING IT. YOU MUST HAVE ADDITIONAL INFORMATION TO CORRECTLY UNDERSTAND EPHEMERIS LIMITATIONS AND UNCERTAINTIES.".
I get that the ephemeris is calculated by successive integrations, with the initial conditions given by the osculating elements. What I didn't find in the documentation so far is how often the osculating elements are themselves calculated. They must be as correct as possible, otherwise they'd be useless as a base for integration. The closest thing in the documentation is the section at https://ssd.jpl.nasa.gov/horizons/manual.html#limits but it doesn't address the frequency of updates to the osculating elements. BTW, I loved the warning in capitals: "IF YOUR CAREER OR SPACECRAFT DEPENDS ON A NON-LUNAR NATURAL SATELLITE OR SMALL-BODY EPHEMERIS, CONTACT JPL BEFORE USING IT. YOU MUST HAVE ADDITIONAL INFORMATION TO CORRECTLY UNDERSTAND EPHEMERIS LIMITATIONS AND UNCERTAINTIES.".