An SP3 satellite state at one epoch.
Position is ITRF/IGS-realization ECEF, in meters (frame and unit are fixed
in the field names per the spec's frames-in-the-type-system rule). clock_s
is the satellite clock offset in seconds, or nil when the product carries
no clock estimate for that satellite/epoch.
Exact parsed records may also carry velocity_m_s, clock_rate_s_s, and
the four SP3 status flags. Interpolated states leave velocity and clock-rate
as nil and all flags as false.