Radio Simulations of High-Altitude Horizontal Air Showers for POEMMA Balloon with Radio (PBR)

High-Altitude Horizontal Air Showers (HAHAs), also known as atmosphere-skimming cosmic ray showers, are produced by primary cosmic rays whose development occurs entirely within the Earth's atmosphere. NASA's Probe of Extreme Multi-Messenger Astrophysics Balloon with Radio (PBR), scheduled for launch in 2027, has as one of its main science goals the radio observation of HAHAs. These showers will be measured in the 50-500 MHz band by two dual-polarized antennas externally triggered by an air-Cherenkov camera. Accurate simulations are essential for predicting and analyzing HAHA signatures. This work presents a comparative study of two leading radio simulation frameworks: CoREAS, the radio extension of CORSIKA, and RASPASS-ZHAireS based on AIRES. We first compare downgoing showers to establish a baseline for expected differences between the two codes and then extend the comparison to HAHA events. The goals of this study are to determine whether statistical agreement exists between HAHA simulations from both frameworks and to inform the selection of the simulation software to be used for future PBR analyses.