What is the VOR reference signal?
VOR Ground Station is aligned with magnetic North. It emits two signals:


a 360° sweeping variable signal
an omni-directional reference signal


When an aircraft receives those signals, its receiver compares those and a measures the phase difference. This gives a precise radial position of the aircraft which is displayed on its Omni-Bearing Indicator (OBI), Horizontal Situation Indicator (HSI) or a Radio Magnetic Indicator (RMI), or a combination of two different kinds.

This picture shows the two signals:




Blue is 360° sweeping signal
Green is omni-directional reference signal