The VOR encodes azimuth (direction from the station) as the phase relationship between a reference signal and a variable signal. The omni-directional signal contains a modulated continuous wave (MCW) 7 wpm Morse code station identifier, and usually contains an amplitude modulated (AM) voice channel. The conventional 30 Hz reference signal is frequency modulated (FM) on a 9,960 Hz subcarrier. The variable amplitude modulated (AM) signal is conventionally derived from the lighthouse-like rotation of a directional antenna array 30 times per second. Although older antennas were mechanically rotated, current installations scan electronically to achieve an equivalent result with no moving parts. When the signal is received in the aircraft, the two 30 Hz signals are detected and then compared to determine the phase angle between them. The phase angle by which the AM signal lags the FM subcarrier signal is equal to the direction from the station to the aircraft, in degrees from local magnetic north at the time of installation, and is called the radial. The Magnetic Variation changes over time so the radial may be a few degrees off from the present magnetic variation. VOR stations have to be flight inspected and the azimuth is adjusted to account for magnetic variation.