  Future space-based experiments, such as OWL and JEM-EUSO, view large
atmospheric and terrestrial neutrino targets. With energy thresholds slightly
above 10^19 eV for observing airshowers via air fluorescence, the potential for
observing the cosmogenic neutrino flux associated with the GZK effect is
limited. However, the forward Cherenkov signal associated with the airshower
can be observed at much lower energies. A simulation was developed to determine
the Cherenkov signal strength and spatial extent at low-Earth orbit for
upward-moving airshowers. A model of tau neutrino interactions in the Earth was
employed to determine the event rate of interactions that yielded a tau lepton
which would induce an upward-moving airshower observable by a space-based
instrument. The effect of neutrino attenuation by the Earth forces the viewing
of the Earth's limb to observe the nu_tau-induced Cherenkov airshower signal at
above the OWL Cherenkov energy threshold of ~10^16.5 eV for limb-viewed events.
Furthermore, the neutrino attenuation limits the effective terrestrial neutrino
target area to ~3x10^5 km^2 at 10^17 eV, for an orbit of 1000 km and an
instrumental full Field-of-View of 45 degrees. This translates into an
observable cosmogenic neutrino event rate of ~1/year based upon two different
models of the cosmogenic neutrino flux, assuming neutrino oscillations and a
10% duty cycle for observation.
