Q. What is the source for the surface pressure (PSurfStd) in the standard
L2 product, since you do not retrieve this quantity.
A. The source of PSurfStd is indeed NCEP-GFS, interpolated in time from
two different forecasts from the same model run. If the observation
is 3-6 hours after a model run then it is interpolated from the 3 & 6
hour forecasts. It the observation is 6-9 hours after the model run
(which is 0-3 hours after the next model run) we use the 6 & 9 hour
forecasts.
Of course, it is also corrected for the average of the topography in
the FOV.