A broadcast-evaluated satellite state at one epoch.
Position is ITRF/IGS-realization ECEF, in meters (frame and unit fixed in the
field names). clock_s is the satellite clock offset in seconds, the
broadcast clock-polynomial total including the relativistic eccentricity term
and the broadcast group delay. The sign convention matches Orbis.GNSS.SP3:
a positive clock_s means the satellite clock is ahead of system time, so
the geometric range correction is range + c * clock_s.