  Using the Earth as a neutrino converter, tau neutrino fluxes from
astrophysical point sources can be detected by tau-lepton-induced extensive air
showers (EASs). Both muon neutrino and tau neutrino induced upward-going EAS
signals can be detected by terrestrial, sub-orbital and satellite-based
instruments. The sensitivity of these neutrino telescopes can be evaluated with
the nuSpaceSim package, which includes the nuPyProp simulation package. The
nuPyProp package propagates neutrinos ($\nu_\mu$, $\nu_\tau$) through the Earth
to produce the corresponding charged leptons (muons and tau-leptons). We use
nuPyProp to quantify the uncertainties from Earth density models, tau
depolarization effects and photo-nuclear electromagnetic energy loss models in
the charged lepton exit probabilities and their spectra. The largest
uncertainties come from electromagnetic energy loss modeling, with as much as a
20-50% difference between the models. We compare nuPyProp results with other
simulation package results.
