  The atmospheric secondary proton flux is studied for altitudes extending from
sea level up to the top of atmosphere by means of a 3-dimensional Monte-Carlo
simulation procedure successfully used previously to account for flux
measurements of protons, light nuclei, and electrons-positrons below the
geomagnetic cutoff (satellite data), and of muons and antiprotons (balloon
data). The calculated flux are compared with the experimental measurements from
sea level uo to high float ballon altitudes. The agreement between data and
simulation results are very good at all altitudes, including the lowest ones,
where the calculations become extremely sensitive to the proton production
cross section. The results are discussed in this context. The calculations are
extended to the study of quasi trapped particles above the atmosphere to about
5 Earth radii, for prospective purpose.
