Time Standards
One instant, expressed in every time standard that appears in astrodynamics and flight operations. Values update each second from this device’s clock. Where a standard cannot be derived without external data — Earth orientation, a mission clock correlation — the table says so rather than showing a number it cannot stand behind.
| Standard | Value | Definition | Used for |
|---|
These are the independent variables equations of motion are integrated against. GCRF/J2000 is defined at JD 2451545.0 TT — a frame without its time scale is half a specification, which is why this platform’s ephemeris endpoints take jd_tdb_int/jd_tdb_frac split across two floats rather than one absolute Julian Date.
| Standard | Value | Definition | Used for |
|---|
Each constellation runs its own continuous scale, offset from TAI by a fixed integer and never stepped for leap seconds. Products in these scales are expressed in ITRF, an Earth-fixed frame — not the inertial frame an orbit is propagated in.
| Standard | Value | Definition | Used for |
|---|
These track the Earth’s actual rotation rather than an atomic count, and they are what carries a state between an inertial frame and an Earth-fixed one. UT1 is not computable from a clock: it needs DUT1 from IERS Bulletin A, published as an observation. The sidereal values below therefore assume UT1 ≈ UTC, which is correct to within 0.9 s by definition of the leap-second rule — adequate for orientation, not for precise pointing.
| Standard | Value | Definition | Used for |
|---|
A day count is a number of days from a chosen origin. It carries no time scale of its own — JD in UTC and JD in TT differ by 69.184 s today, which is why every value below is labelled with the scale it counts in. All are shown in UTC unless the row says otherwise.
| Count | Value | Origin | Used for |
|---|
| Standard | Value | Definition | Used for |
|---|
- Source — this device’s clock, not a time service. Every value is only as good as your machine’s synchronisation.
- Leap seconds — TAI−UTC is held as a single constant, shown above. The IERS announces changes roughly six months ahead; when one lands, that constant is the entire edit.
- TDB — the standard two-term periodic approximation, good to a few tens of microseconds. TDB−TT is at most about 1.7 ms, so at the resolution displayed here it is very nearly TT.
- Sidereal — IAU 1982 GMST from UT1, with UT1 approximated by UTC. GAST additionally applies the equation of the equinoxes to first order.
- Not shown — anything requiring an observation we do not hold: polar motion, precise DUT1, or a spacecraft clock correlation.