In 2012 a comparison campaign between raytracing packages was carried out be several institutions. It is described in:

V. Nafisi et al., "Comparison of Ray-Tracing Packages for Troposphere Delays," 
in IEEE Transactions on Geoscience and Remote Sensing, vol. 50, no. 2, pp. 469-481, Feb. 2012, doi: 10.1109/TGRS.2011.2160952.

The NWM data can be found in this directory.