The Horizon is a horizontal plane, passing through the Observer. Antenna Elevation is measured upward from the horizon. For various reasons, the minimum usable elevation may be greater than zero (nearby buildings or terrain), or less than zero (observer on a hilltop).
Additional displacement of the apparent horizon may be caused by atmospheric refraction or bending. For visible light, the lower atmosphere (Troposphere) has the biggest effect on refraction. This effect depends upon pressure, temperature, and relative humidity, in the visible wavelength band. For radio signals, the Ionosphere or Troposphere may have bigger effects, the former affecting principally MF and HF waves, and the latter VHF and microwaves.
Refraction is the reason why sunrise and sunset appear to occur earlier and later than predicted by WinOrbit.
Some satellite programs include such a refraction correction for satellites near the horizon. Because, for radio propagation, these effects are very unpredictable, WinOrbit does not include any corrections to the geometric line-of-sight calculation.