The Horizon is a horizontal plane, passing through the
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
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.