# VOR, CVOR, DVOR, and VOR Frequency Range

> Understand VOR reference and variable signals, VOR frequency range, and differences between CVOR and DVOR.

- **Subject:** 062 — Radio Navigation
- **Section:** 062-02 — Radio Aids
- **Topic code:** 062-02-03
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/radio-navigation/radio-aids/vhf-omnidirectional-radio-range-vor-conventional-vor-cvor-and-doppler-vor-dvor/

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The **VOR** is a radio navigation aid that operates in the VHF band and provides the pilot with the magnetic direction to or from the ground station.

## Operating Principles

*   **Frequencies:** Operates in the VHF band, between **108.00 MHz and 117.975 MHz**.
    *   **108.00 - 111.95 MHz:** Shared with ILS. VOR uses frequencies with an **EVEN first decimal** (e.g., 108.20, 108.45). ILS uses odd decimals.
    *   **112.00 - 117.975 MHz:** Exclusive use for VOR.
*   **Signals:** The station transmits two 30 Hz signals:
    *   **Reference Signal:** Omnidirectional (constant phase in all directions).
    *   **Variable Signal:** Directional (rotating). Its phase changes according to direction.
*   **Radial Determination:** The receiver measures the **phase difference** between both signals. This difference in degrees corresponds to the magnetic radial the aircraft is on.
    *   Example: If the phase difference is 90°, the aircraft is on radial 090.

## VOR Types

![VOR Station (Antenna)](https://upload.wikimedia.org/wikipedia/commons/f/fe/D-VOR_PEK.JPG)

1.  **CVOR (Conventional VOR):**
    *   Reference Signal: Frequency Modulated (FM).
    *   Variable Signal: Amplitude Modulated (AM) by a physical rotating antenna.
    *   Susceptible to site errors (multipath).
2.  **DVOR (Doppler VOR):**
    *   Reference Signal: Amplitude Modulated (AM).
    *   Variable Signal: Frequency Modulated (FM) simulating rotation using an antenna ring (Doppler Effect).
    *   **Advantage:** Much more accurate and less susceptible to site/multipath errors thanks to the FM variable signal.
3.  **TVOR (Terminal VOR):** Low power, short range (25 NM), used at aerodromes.
4.  **VOT (VOR Test Facility):** Emits a test signal with zero phase difference (Radial 180 in all directions). Used on the ground to check equipment (Indication: 180 TO or 360 FROM with needle centered).

## Instruments and Indications

![VOR Indicator (CDI)](https://upload.wikimedia.org/wikipedia/commons/3/32/Vor_indicator.png)

*   **CDI (Course Deviation Indicator):**
    *   **OBS (Omni Bearing Selector):** Allows selecting the desired course.
    *   **Deviation Needle:** Shows angular deviation from the selected course.
    *   **Scale:** Typically 5 dots on each side. **Full Scale Deflection = 10°** (2° per dot).
    *   **TO/FROM Indicator:** Indicates if the selected course leads to (TO) or from (FROM) the station.
        *   **TO:** The selected radial is more than 90° from the current radial.
        *   **FROM:** The selected radial is less than 90° from the current radial.
*   **RMI (Radio Magnetic Indicator):**
    *   **Needle Tail:** Indicates the current **Radial** (QDR).
    *   **Needle Head:** Indicates the magnetic heading to the station (QDM).

## Range and Accuracy

*   **Range:** Limited by **Line-of-Sight**.
    *   Approximate formula: $Distance (NM) = 1.23 \times \sqrt{Height (ft)}$.
*   **Accuracy:** $\pm 5^\circ$ (on airways).
*   **Established:** On approaches, considered established within **half scale deflection (5°)**.
*   **Monitoring:** The monitor shuts down the VOR or removes identification if the bearing error exceeds **1°** or power drops by more than 15%.

## Procedures and Additional Concepts

*   **Radials:** Always magnetic and **FROM** the station. Magnetic variation at the **VOR** location is used.
*   **Cone of Confusion:** Area above the station where no signal is received or it is erratic.
*   **Scalloping:** Signal waviness caused by terrain reflections.
*   **ATIS:** Some VORs transmit airport ATIS information on their frequency.

## Enroute Navigation

![IFR Enroute Chart (VOR Airways)](https://upload.wikimedia.org/wikipedia/commons/d/d6/Holdings_on_fligth_chart_IFR_Enroute_Low_Altitude.png)

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Free study notes from pilotsyllabus.com. Not a substitute for training at an approved ATO.