  A considerable fraction of incident high-energy photons from astrophysical
transients such as Gamma-Ray Bursts (GRBs) is Compton scattered by the Earth's
atmosphere. These photons, sometimes referred to as the "reflection component",
contribute to the signal detected by space-borne X-ray/$\gamma$-ray
instruments. The effectiveness and reliability of source parameters such as
position, flux, spectra, and polarization, inferred by these instruments are
therefore highly dependent on the accurate estimation of this scattered
component. Current missions use dedicated response matrices to account for
these effects. However, these databases are not readily adaptable for other
missions, including many upcoming transient search and gravitational wave
high-energy Electromagnetic counterpart detectors. Furthermore, possible
systematic effects in these complex simulations have not been thoroughly
examined and verified in the literature. We are in the process of investigation
of the effect with detailed Monte Carlo simulations in GEANT4 for a Low Earth
Orbit (LEO) X-ray detector. Here, we discuss the outcome of our simulation in
form of the Atmospheric Response Matrix (ARM) and its implications of any
systematic errors in the determination of source spectral characteristics. We
intend to apply our results in data processing and analysis for AstroSat-CZTI
observation of such sources in near future. Our simulation output and source
codes will be made publicly available for use by a large number of upcoming
high energy transient missions, as well as for scrutiny and systematic
comparisons with other missions
