  The atmospheric secondary antiproton flux is studied for detection altitudes
extending from sea level up to about 3 earth radii, by means of a 3-dimensional
Monte-Carlo simulation, successfully applied previously on other satellite and
balloon data. The calculated antiproton flux at mountain altitude is found in
fair agreement with the recent BESS measurements. The flux at balloon altitude
is also in agreement with calculations performed in previous studies and used
for the analysis of balloon data. The flux at sea level is found to be
significant. The antineutron flux is also evaluated. The antiproton flux is
prospectively explored at large distance from earth up to around 2 10$^4$ km.
The results are discussed in the context of the forthcoming measurements by
large acceptance experiments.
