In radio navigation, particularly with a VOR (VHF Omnidirectional Range) system, reference and variable signals are two distinct radio waves transmitted simultaneously from a ground station. The phase difference between these two signals, as detected by an aircraft, is crucial for determining the aircraft's exact bearing or "radial" from the station.
Understanding the VOR Signal System
VOR stations are vital for aircraft navigation, providing pilots with a precise radial direction to or from the station. This is achieved through the ingenious interplay of two simultaneously broadcast signals: the reference signal and the variable signal.
The Reference Signal
The reference signal serves as a baseline for comparison. It is a consistent, non-directional signal that is broadcast uniformly in all directions from the VOR station. Think of it as the constant "tick" of a clock that all receivers can hear regardless of their position.
- Key Characteristics:
- Constant: It maintains a stable phase across all directions.
- Non-directional: Its signal phase does not change with azimuth.
- Omnidirectional: Broadcasts equally in all 360 degrees.
The Variable Signal
In contrast to the steady reference signal, the variable signal is highly directional and its phase changes depending on the radial from the VOR station. The VOR station achieves this by rotating the phase of this signal.
- Key Characteristics:
- Rotating: This signal effectively "rotates" its phase, typically at 30 times per second.
- Directional: Its phase changes as it sweeps around the station.
- Phase Shift: The phase of the variable signal at any given point in space is relative to the magnetic bearing from the station.
How They Work Together: Determining a Radial
The brilliance of the VOR system lies in how an aircraft's receiver interprets the interaction between these two signals.
- Reception: An aircraft receives both the constant reference signal and the rotating variable signal.
- Phase Comparison: The VOR receiver in the aircraft measures the phase difference between the reference signal and the variable signal.
- Radial Calculation: Because the reference signal is constant and the variable signal rotates, the time difference (which translates to a phase difference) between when the aircraft receives a specific phase of the reference signal and when it receives the same phase of the variable signal directly corresponds to the aircraft's magnetic bearing from the station.
For example, if an aircraft is directly north of the VOR station (000° radial), it will detect no phase difference (or a specific calibrated difference) between the two signals. If it is directly east (090° radial), it will detect a 90-degree phase shift, and so on. This phase difference directly determines the aircraft's radial from the station.
Reference vs. Variable Signal: A Comparison
To further clarify, here's a direct comparison of the two signals:
| Feature | Reference Signal | Variable Signal |
|---|---|---|
| Nature | Constant, omnidirectional baseline | Rotating, directional |
| Phase | Stays constant regardless of direction | Phase changes with the radial/direction from the station |
| Transmission | Broadcasts uniformly in all directions | Phase rotates, effectively sweeping a directional beam |
| Function | Provides a stable reference for comparison | Provides the directional information by its phase relationship |
| Frequency | Modulates the main VOR carrier (e.g., 9960 Hz) | Modulates a subcarrier that rotates (e.g., 30 Hz rotation) |
| Purpose | Establishes a universal "zero point" | Indicates the precise bearing by its phase relative to the reference |
Practical Application for Pilots
- Pilots tune their VOR receiver to the frequency of a desired VOR station.
- The receiver then processes the reference and variable signals.
- The phase difference is displayed on instruments like the Omni Bearing Indicator (OBI) or Horizontal Situation Indicator (HSI), showing the aircraft's current radial or the deviation from a selected course. This allows pilots to navigate precisely along airways or to specific points.
