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 and the relativistic term, without the broadcast
group delay (GPS and QZSS TGD, Galileo BGD, BeiDou TGD1, CNAV TGD less ISC),
as RTKLIB satposs returns it. That delay is a single-frequency term, which
the single-frequency positioning models apply to the pseudorange. The sign convention matches Sidereon.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.