2.2 Terrestrial Radio Navigation: VOR, Localizer, and Glideslope

Terrestrial Radio Navigation: VOR, Localizer, and Glideslope

Terrestrial radio navigation systems form the traditional backbone of the National Airspace System. Certified Aircraft Electronics Technicians must master the RF phase comparison architecture of VHF Omnidirectional Range (VOR) systems, the dual-frequency amplitude-modulated beams of the Instrument Landing System (ILS), and the frequency-selective marker beacon receivers installed on civil and commercial aircraft.

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VHF Omnidirectional Range (VOR) Principles

VOR stations transmit within the VHF band from 108.00 MHz to 117.95 MHz. The 108.00–111.95 MHz segment is shared with ILS localizers; within this shared band, VOR frequencies utilize even tenths (e.g., 108.00, 108.05, 108.20, 108.25 MHz), leaving odd tenths for localizers. From 112.00 to 117.95 MHz, all 50 kHz channels are dedicated to VOR.

      VOR Signal Radiation Scheme
      
          Magnetic North (000°)
                  ^
                  | Variable & Reference
                  | are Exactly In-Phase (Δφ = 0°)
                  |
                  O  <-- VOR Ground Station
                 / \
                /   \
   Radial 270° /     \ Radial 090°
  (Δφ = 270°)         (Δφ = 90°)
                  |
                  v
             Radial 180°
             (Δφ = 180°)

The Two 30 Hz Signals

A VOR ground station radiates two separate 30 Hz signals whose relative electrical phase relationship varies with azimuth:

1. 30 Hz Reference Phase Signal: Radiated omnidirectionally (equally in all directions). To prevent it from interfering with the variable AM signal, the 30 Hz reference tone frequency-modulates a 9,960 Hz subcarrier with a deviation of $\pm 480\ \text{Hz}$. This frequency-modulated subcarrier then amplitude-modulates the main VHF carrier by approximately 30%.
2. 30 Hz Variable Phase Signal: Radiated by a phased directional antenna array that creates a rotating cardioid figure-eight pattern. The pattern rotates electronically clockwise at 30 revolutions per second (1,800 RPM), creating a 30 Hz amplitude modulation on the aircraft's receiver as the beam sweeps past.
3. Phase Relationship at Magnetic North: At the instant the rotating variable beam passes Magnetic North ($000^\circ$), the 30 Hz reference modulation wave is at its positive-going zero crossing. Thus, at Magnetic North, the reference and variable signals are exactly in phase:

$\Delta \phi = \phi_{\text{variable}} - \phi_{\text{reference}} = 0^\circ$

As the receiver moves clockwise around the station to any magnetic bearing (azimuth $\theta$), the variable signal is received later in time. Therefore, the electrical phase angle difference ($\Delta \phi$) directly equals the magnetic radial from the ground station:

$\text{Magnetic Radial} = \Delta \phi$

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Airborne VOR Receiver Architecture and OBS Resolvers

[VHF Antenna] --> [RF/IF Receiver] --> [Audio Filters]
                                        |--> [9,960 Hz FM Subcarrier Det] --> [30 Hz Reference Wave] ---+
                                        |                                                                |
                                        +--> [AM Detector] -------------> [30 Hz Variable Wave] ---+     |
                                                                                                   |     |
[Omni-Bearing Selector (OBS)] ------------------------------------------------------------------> [Resolver Phase Shifter]
                                                                                                         |
[Course Deviation Indicator (CDI)] <-------------------------------- [Phase Detector / Comparator] <----+ 

Within the airborne receiver:

1. The incoming VHF signal is detected, yielding two channels: the 30 Hz variable audio and the 9,960 Hz subcarrier.
2. An FM discriminator extracts the 30 Hz reference sine wave from the subcarrier.
3. The pilot sets a desired course on the Omni-Bearing Selector (OBS) knob, which mechanically rotates the rotor of an internal electromagnetic resolver (or digital rotary encoder).
4. The resolver shifts the phase of the 30 Hz reference signal by an amount equal to the selected course.
5. The phase comparator compares the phase-shifted reference signal against the incoming 30 Hz variable signal.

CDI Needle Deflection and TO/FROM Logic

• On Course: When the aircraft is on the selected radial (or its reciprocal), the phase difference entering the phase detector is $0^\circ$ (or $180^\circ$), producing $0\ \text{V}$ DC output. The Course Deviation Indicator (CDI) needle centers.
• Off Course Deflection: If the aircraft drifts off course, a DC error voltage proportional to the angular displacement drives the CDI D'Arsonval meter movement or PFD pointer. Full-scale needle deflection represents $\pm 10^\circ$ of angular course deviation (each dot on a standard 5-dot CDI equals $2^\circ$).
• TO/FROM Ambiguity Resolver: A secondary phase detector compares the signals in quadrature ($90^\circ$ phase offset). If the phase relationship indicates that flying the selected course will take the aircraft toward the station, the "TO" flag appears. If flying that course leads away from the station, the "FROM" flag displays. When flying directly over the station or perpendicular to the radial, signal cancellation occurs, dropping the indicator into the NAV Warning Flag (the "cone of confusion").

Conventional VOR (CVOR) vs. Doppler VOR (DVOR)

Conventional VOR stations (CVOR) suffer from course scalloping and bending when signals reflect off nearby hills, buildings, or transmission towers. To eliminate this siting vulnerability, modern installations use Doppler VOR (DVOR):

• DVOR reverses the modulation roles: the 30 Hz reference signal is transmitted as an omnidirectional AM wave.
• The 30 Hz variable signal is generated by feeding a 9,960 Hz subcarrier sequentially through a circular ring array of 50 antennas (approx. 44 feet in diameter). The simulated counter-clockwise rotation creates a true Doppler shift of $\pm 480\ \text{Hz}$ at 30 Hz in space.
• Because the Doppler subcarrier occupies a wide spatial aperture, multipath reflections are suppressed by a factor of 10 or greater, producing stable radials without requiring hardware changes in airborne receivers.

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Instrument Landing System (ILS) - Localizer

The localizer provides horizontal lateral guidance along the runway extended centerline. Localizers operate on 40 allocated channels from 108.10 MHz to 111.95 MHz, exclusively on odd tenths and odd tenths plus 50 kHz (e.g., 108.10, 108.15, 108.30, 108.35 MHz).

                         Runway Centerline
                                 |
             90 Hz Lobe          |          150 Hz Lobe
          (Fly Right / Yellow)   |       (Fly Left / Blue)
                  . - ~ ~ - .    |    . - ~ ~ - .
              .               .  |  .               .
            /                   \|/                   \
           (        DDM > 0      |      DDM < 0        )
            \     (90 Hz > 150)  |   (150 Hz > 90)    /
              .               . /|\ .               .
                  ' - ~ ~ - '    |    ' - ~ ~ - '
                                 |
                           DDM = 0 (On Centerline)

Difference in Depth of Modulation (DDM)

The localizer antenna array at the departure end of the runway radiates two overlapping directional amplitude-modulated lobe patterns:

• Left Side of Approach: Modulated with a 90 Hz audio tone (predominantly 90 Hz depth of modulation).
• Right Side of Approach: Modulated with a 150 Hz audio tone (predominantly 150 Hz depth of modulation).

The airborne localizer receiver demodulates the carrier and separates the 90 Hz and 150 Hz audio filters, feeding them into bridge rectifiers. The receiver calculates the Difference in Depth of Modulation (DDM):

$\text{DDM} = \frac{\text{DOM}_{90} - \text{DOM}_{150}}{100}$

• On Centerline: $\text{DOM}_{90} = \text{DOM}_{150} = 20\%$, yielding $\text{DDM} = 0$. The CDI needle centers.
• Left of Centerline: The aircraft receives stronger 90 Hz modulation. The DDM is positive, driving the CDI needle to the right (commanding "Fly Right").
• Right of Centerline: The aircraft receives stronger 150 Hz modulation. The DDM is negative, driving the CDI needle to the left (commanding "Fly Left").

Localizer Sensitivity vs. VOR

The localizer course sector is engineered to provide a standardized linear width of 700 feet at the runway threshold, typically resulting in an angular course width between $3^\circ$ and $6^\circ$ ($5^\circ$ nominal). Full-scale deflection represents $\pm 2.5^\circ$, making the localizer four times more sensitive than a standard VOR ($\pm 10^\circ$ full-scale).

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Instrument Landing System (ILS) - Glideslope

The glideslope provides vertical descent guidance down to runway touchdown. Glideslopes operate in the UHF band from 329.15 MHz to 335.00 MHz (40 channels spaced 150 kHz apart).

Automatic Frequency Pairing

The glideslope receiver contains no pilot-accessible frequency tuning controls. Each UHF glideslope channel is paired by ICAO standard directly with a corresponding VHF localizer frequency. When the technician or pilot tunes the VHF navigation head to 109.30 MHz, the internal wiring matrix or ARINC 429 digital tuning word automatically commands the glideslope receiver to 332.00 MHz.

Vertical Guidance and False Glideslope Hazards

The glideslope antenna mast, located approximately 1,000 feet down the runway and 400 feet to the side, radiates two vertically stacked UHF beams:

• Upper Lobe: Amplitude-modulated with 90 Hz audio.
• Lower Lobe: Amplitude-modulated with 150 Hz audio.

The intersection of the two equal modulation lobes ($\text{DDM} = 0$) defines the descent path, nominally aligned at $3.0^\circ$ above the horizontal. The vertical glideslope beam has a total angular thickness of $1.4^\circ$ ($0.7^\circ$ above and below the on-course centerline for full-scale fly-down/fly-up deflection).

        Upper Lobe (90 Hz) --> Above Path: Fly-Down Command
            \ 
             \  Centerline (3.0°): 90 Hz = 150 Hz (DDM = 0)
              \ 
        Lower Lobe (150 Hz) --> Below Path: Fly-Up Command
       ================================================= Ground Level

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75 MHz Marker Beacon Subsystem

Marker beacons operate on a single crystal-controlled frequency of 75.000 MHz. The airborne receiver drives three distinct audio-visual annunciators to indicate along-track position during an approach:

Aircraft Approach Path -->
      |                                      |                     |             [Runway]
      v                                      v                     v                 |
[Outer Marker (OM)]                  [Middle Marker (MM)]  [Inner Marker (IM)]       |
  4-7 NM out                           3,500 ft out          1,000 ft out            |
  Blue Lamp / 400 Hz                   Amber Lamp / 1300 Hz  White Lamp / 3000 Hz    |
  ( - - - - )                          ( . - . - )           ( . . . . )             |

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Avionics Bench Testing and 14 CFR 91.171 Verification

Federal Aviation Regulation 14 CFR 91.171 says no person may operate a civil aircraft under IFR using VOR unless the VOR equipment is maintained under an approved procedure or has been operationally checked within the preceding 30 days. The permissible bearing error is ±4° against an FAA-operated or approved test signal (VOT), a test signal radiated by a certificated repair station, or a designated surface checkpoint; ±6° at an airborne checkpoint; and 4° between two independent VOR receivers in a dual check. The person doing the check records the date, place, and bearing error and signs the aircraft log. Shop and ramp tests then follow the test-set and receiver manufacturers' procedures:

1. VOR Bearing Checks: Using an avionics signal generator (e.g., Aeroflex/Cobham IFR 4000), simulated bearings of $000^\circ, 090^\circ, 180^\circ,$ and $270^\circ$ must yield CDI centering within $\pm 4^\circ$ of course.
2. Localizer Centering and DDM: Injecting $\text{DDM} = 0.000$ should center the localizer needle. Full-scale deflection is typically checked at the standard value of $\text{DDM} = \pm 0.155$.
3. Glideslope Centering and DDM: Injecting $\text{DDM} = 0.000$ at UHF carrier frequency must center the horizontal glidepath pointer. Full-scale glideslope deflection is typically checked at $\text{DDM} = \pm 0.175$.