  In this paper we describe a new release of the PRESHOWER program, a tool for
Monte Carlo simulation of propagation of ultra-high energy photons in the
magnetic field of the Earth. The PRESHOWER program is designed to calculate
magnetic pair production and bremsstrahlung and should be used together with
other programs to simulate extensive air showers induced by photons. The main
new features of the PRESHOWER code include a much faster algorithm applied in
the procedures of simulating the processes of gamma conversion and
bremsstrahlung, update of the geomagnetic field model, and a minor correction.
The new simulation procedure increases the flexibility of the code so that it
can also be applied to other magnetic field configurations such as, for
example, encountered in the vicinity of the sun or neutron stars.

Instrumentation and Methods for Astrophysics;High Energy Astrophysical Phenomena