One sampled satellite state at one epoch in the unified ephemeris table.
satellite_id is the canonical RINEX token (e.g. "G01"); epoch is the
NaiveDateTime in GPST the cell was evaluated at. When status is :ok,
x_m/y_m/z_m are the ITRF/IGS ECEF position in meters and clock_s is the
satellite clock offset in seconds (positive = satellite clock ahead of system
time, see Orbis.GNSS.Ephemeris). When status is :no_ephemeris the source
had no valid ephemeris at that epoch and the four value fields are nil — a
gap is never extrapolated.