Earth radius missing in sat_xyz computation in parallax modifier

Hello,

I am working on a parallax correction of the MTG-LI data. Recently, I have realized that my correction for photon travel time is unrealistic and led to wrong results for my matching algorithm with ground-based data. I noticed that maybe the line 158-160 of the script modifiers>parallax.py may have a mistake. Therefore, I start this discussion in case others may have trouble with this as well.

The lines I am unsure about in the script parallax.py are the following:

 158 sat_xyz = np.hstack(lonlat2xyz(sat_lon, sat_lat)) * sat_alt
 159 cth_xyz = np.stack(lonlat2xyz(lon, lat), axis=-1) * EARTH_RADIUS*1e3  # km → m
 160 delta_xyz = cth_xyz - sat_xyz

In my understanding, the variable delta_xyz is a vector coming from the satellite to the parallaxed location of the cloud at the earth surface. But it seems that the origin of the sat_xyz vector is defined at ground level, since it is specified in the code at line 88 that: "This code expects sat_alt and height to be in meter above the Earth's surface." In contrast, the cth_xyz is defined w.r.t. the center of the earth (since we multiply with the earth's radius).

Finally, I get more realistic results for the photon travel time since I have changed line 158 to:

sat_xyz = np.hstack(lonlat2xyz(sat_lon, sat_lat)) * (sat_alt+EARTH_RADIUS)

Would you have comments or other opinion regarding this part of the modifier?

Thank you