The investigation of glaciers and ice sheets by means of ice penetrating radar has become one of the most commonly used geophysical techniques in glaciology.Although many different applications utilise internal reflection horizons, assuming that an individual reflector is isochronous, open questions concerningthe different reflection mechanisms remain.We demonstrate successful simulation of ice penetrating radar traces in polar ice by numerical finite-difference time-domain forward modeling.Based on a combined analyses of ice core records, radar surveys, and numerical sensitivity studies, we are able to provide better insights into the origin ofelectromagnetic reflections.By means of high resolution dielectric profiling (DEP) the physical properties of the upper 100 m of an ice core from Dronning Maud Land, Antarctica, aredetermined.As the considered medium is an ice-air composition, the real and imaginary partsof the constituents are subject to complex-valued mixing to form the measuredcomplex dielectric constant $\epsilon$.In contrast to earlier assumptions, correct frequency-scaling of $\epsilon$ requires complex decomposition of the dielectric constant at the 250~kHz DEP frequency to yield the frequency-independent properties, i.e.~density and conductivity,and has to be composed again at the 200 MHz radar frequency.Forcing of the numerical model with differently scaled and altered DEP data setsreveals the role of the initial physical parameters for the formation ofreflection horizons and enlights their isochronic characteristics, an importantapproach for interpreting internal reflection horizons.