"We have continued in our effort to investigate the use of Numerical Weather Prediction (NWP) models in GPS positioning, in an attempt to obtain better prediction of tropospheric delay. We have expanded our previous work on deriving zenith path delays from the Canadian GEM NWP model to obtain slant path delays, via ray-tracing in the three-dimensional NWP grids. We have developed an algorithm for this purpose. We have tested our approach in the ranging domain, comparing it to slant delays given by the UNB3 model and to zenith hydrostatic delays given by the Saastamoinen model with surface pressure measurements. We have also tested it in the position domain, using real GPS data from the Princess of Acadia Project, in a combination of short- and long-baseline solutions. Even though preliminary, our results indicate that (i) there is no advantage in ray-tracing for elevation angles greater than 30¼, and (ii) NWP is equivalent to UNB3 for decimetre-level positioning. Further comparisons are needed to assess the performance of the GEM NWP model at higher precision levels."