VOR is considered to be what type of comparison system?

VOR, or VHF Omnidirectional Range, is classified as a phase comparison system. This system operates by transmitting a radio signal that varies in phase. The ground station emits two signals: a reference signal that is constant, and a variable signal that changes as it rotates around the station. The aircraft's receiver can determine its position in relation to the VOR station by comparing the phase of the received variable signal to the constant reference signal.

This phase comparison allows for accurate directional information to be provided to the aircraft, enabling pilots to navigate along specific radials from the VOR station. By interpreting the angular difference between the two signals, pilots can ascertain their bearing to or from the station, making VOR an essential tool for navigation in aviation.

In contrast, other types of systems mentioned would function differently: Time-based systems measure the delay of a signal arriving at a receiver, frequency-based systems analyze the frequency of received signals, and distance-based systems often rely on the measurement of signal strength or propagation time to determine distance. Each approach has its own application in navigation and communication, but for VOR, phase comparison is the defining characteristic of its operational mechanics.