Abstract: The origin of cosmic rays and neutrinos with ultra-high energy ( > 10^18eV) remains mysterious for more than half a century after the first observation. Radio detection of air showers induced by them is considered the technique for next-generation experiments with its low cost and unprecedented sensitivity. More and more experiments (GRAND, BEACON, TRINITY) attempt to detect upward-going air showers, but there is no reliable simulation tool for them. In this work, we have prepared a new version of CORSIKA to meet the requirements of these radio experiments. Its performance has been studied along with a newly adapted version of ZHAireS. Additionally, we have found two smoking guns to prove the discovery of a fundamental radio emission mechanism, the geosynchrotron emission in very inclined and upward-going air showers. Via simulations, we have studied the dependence of the geosynchrotron effect on air density, magnetic field, and other parameters. Results obtained by varying these parameters lead to a compact interpretation of the previous work, which will be confirmed or refined by real data.