Where is the variation of a VOR reading derived from?

The variation of a VOR (VHF Omnidirectional Range) reading is derived from the ground station. VORs operate based on radio signals emitted from a ground-based station, which sends out two signals: a reference phase signal and a variable signal. The difference in phase between these two signals allows the aircraft's receiver to determine its bearing relative to the ground station.

This means that the VOR reading, or the direction to/from the station, is produced based on the specific characteristics and location of the ground station, including its magnetic variation, which is influenced by the Earth's magnetic fields and can differ from true north. Thus, the variation is inherent in how the VOR is set up and operates, rather than being affected by external factors like the aircraft itself, weather conditions, or the pilot's calculations.

In summary, the ground station is responsible for determining the characteristics of the VOR signal, including the magnetic variation, making this option the correct choice.