6.2 Ground-Based Navigation Aids & Standard Service Volumes

6.2 Ground-Based Navigation Aids & Standard Service Volumes

Quick Summary: Terrestrial radio navigation aids—including VOR, VORTAC, TACAN, DME, and the Instrument Landing System (ILS)—form the foundational conventional infrastructure of the National Airspace System. Even in an era dominated by satellite-based area navigation, aircraft dispatchers must maintain complete technical mastery over ground navaid operating frequencies, Standard Service Volumes (SSVs), slant-range geometry errors, and the FAA VOR Minimum Operational Network (MON). Navaid outages dictate route viability, approach authorizations, and statutory alternate airport minimum calculations under 14 CFR Part 121 and Operations Specifications C055.

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1. Terrestrial Radio Navigation Principles: VOR, VORTAC & TACAN

VOR Operating Fundamentals and Frequency Spectrum

The Very High Frequency Omnidirectional Range (VOR) is the bedrock ground-based radio navigation aid in civil aviation. VOR stations operate within the Very High Frequency (VHF) band between 108.00 MHz and 117.95 MHz, utilizing a 50 kHz channel spacing. The lower portion of this spectrum is shared with Instrument Landing System (ILS) localizers in accordance with a strict frequency allocation rule:

• 108.00 to 111.85 MHz: Even tenths and even tenths plus 50 kHz (e.g., 108.00, 108.05, 108.20, 108.25...) are allocated to VOR facilities. Odd tenths and odd tenths plus 50 kHz (e.g., 108.10, 108.15, 108.30, 108.35...) are reserved exclusively for ILS localizer frequencies.
• 112.00 to 117.95 MHz: All frequencies are allocated to VOR stations.

Signal Architecture: Reference Phase vs. Variable Phase

A VOR ground station broadcasts two separate 30 Hz radio frequency signals that create an infinite number of radial bearings, conventionally organized into 360 distinct magnetic radials radiating outward from the facility like spokes on a wheel:

1. Reference Phase Signal: Radiated omnidirectionally in all azimuths as a frequency-modulated (FM) 30 Hz signal imposed on a 9960 Hz subcarrier.
2. Variable Phase Signal: Radiated by a specialized directional phased antenna array rotating electronically in a clockwise circular pattern at 30 revolutions per second, creating a space-modulated 30 Hz amplitude-modulated (AM) signal.

The aircraft VOR receiver detects both signals and measures the electronic phase angle difference between the reference and variable waveforms. The ground transmitter is calibrated such that at Magnetic North (360°), the variable phase signal is precisely in phase with the reference phase signal (0° phase differential). As the receiving aircraft moves clockwise around the station:

• Due East (090° magnetic): The variable signal lags the reference signal by 90°.
• Due South (180° magnetic): The variable signal lags by 180°.
• Due West (270° magnetic): The variable signal lags by 270°.

Because VOR radials are defined as radiating outward from the station, an aircraft flying directly toward a VOR tracking an inbound magnetic course of 090° is actually tracking inbound along the 270° radial.

VOR System Derivatives: VORTAC and TACAN

• TACAN (Tactical Air Navigation): A military ultra-high frequency (UHF) pulsed navigation aid operating between 960 MHz and 1215 MHz. It provides military aircraft with continuous bearing (azimuth) and distance information.
• VORTAC: A unified facility consisting of a collocated civil VOR and military TACAN. A VORTAC broadcasts two separate azimuth signals—VHF azimuth for civil aircraft and UHF azimuth for military aircraft—while providing unified Distance Measuring Equipment (DME) to both civil and military users on paired UHF channels.

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2. Distance Measuring Equipment (DME) Mechanics & Slant-Range Geometry

UHF Interrogation and Transponder Response

Distance Measuring Equipment operates in the UHF frequency band from 960 MHz to 1215 MHz. When an airborne pilot or FMS tunes a VHF VOR or localizer frequency paired with DME, the aircraft's airborne interrogator automatically pairs to the corresponding UHF channel:

1. The aircraft interrogator transmits a stream of paired pulse interrogations to the ground transponder at a randomized pulse repetition frequency (PRF).
2. The ground transponder receives the pulse pairs, delays them by an exact internal calibration interval of 50 microseconds (50 µs), and re-transmits reply pulse pairs on a frequency separated by 63 MHz.
3. The airborne receiver locks onto its own unique randomized pulse signature, measures total round-trip elapsed time ($t_{\text{total}}$), subtracts the 50 µs ground station delay, and calculates line-of-sight distance using the speed of light ($c \approx 161{,}875\text{ NM/s}$ or $300{,}000\text{ km/s}$):

$$\text{Slant Distance (NM)} = \frac{c \times (t_{\text{total}} - 50\;\mu\text{s})}{2}$$

The Slant-Range Distance Paradox

DME does not measure horizontal ground distance; it measures slant-range distance—the direct hypotenuse connecting the airborne antenna to the ground station antenna:

$$\text{Slant Distance} = \sqrt{(\text{Ground Distance})^2 + (\text{Altitude Above Station})^2}$$

Where altitude above the station antenna is expressed in nautical miles (1 NM ≈ 6,076.1 feet). This physical reality produces significant geometric error when an aircraft operates at high altitude close to the station:

• Directly Overhead the Station: If an aircraft flies directly over a VOR/DME station at 6,000 feet AGL (1.0 NM vertical height), the horizontal ground distance is 0.0 NM, but the cockpit DME indicator displays 1.0 NM.
• At FL360 (36,000 feet or ≈ 5.92 NM above the station) and 6 NM ground distance, the DME reads $\sqrt{6^2 + 5.92^2} \approx 8.43\text{ NM}$—an error exceeding 40%.

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3. FAA Standard Service Volumes (SSV): Legacy vs. Expanded SSV 2.0

The FAA's SSV identifies protected signal strength, course quality, and interference conditions for off-route navigation. Terrain or obstacles can block the signal; NOTAMs and published restrictions still apply. Published routes or procedures can use approved extended service volumes. Heights in this table are above transmitter height (ATH), not MSL flight levels or height above the aircraft's local terrain.

Read the current AIM figures for the lower edges and exact boundaries. VOR, DME, and TACAN components at one site need not share a volume. A receiver displaying distance from DME does not prove a VOR azimuth signal is usable. Published procedure qualification, facility restrictions, and current status must be checked separately.

4. VOR Minimum Operational Network

The VOR MON is a conventional-navigation reversion system for a GNSS disruption, particularly for aircraft unable to continue RNAV using DME/DME. FAA describes nearly continuous VOR reception at 5,000 feet AGL outside the Western U.S. Mountainous Area; it retains the existing route/navaid structure in that mountainous area. The MON concept also places a conventional-approach MON airport within 100 NM of positions in CONUS. That design objective does not guarantee that a particular airport, approach, or signal is usable at the actual time or under every weather and terrain condition.

A MON airport's retained ILS or VOR approach supports landing without relying on GPS, and the intended options do not require ADF or DME when radar is unavailable. Consult current charts, Chart Supplement, NOTAMs, equipment status, weather, and ATC coordination. An airplane with approved DME/DME RNAV may have a different reversion option. Fuel, performance, traffic, and conventional-navigation proficiency still constrain the response.

The dispatcher assesses the actual outage area and timing, available route/approach capabilities, and aircraft approvals. Off-route conventional planning uses usable facilities and applicable service volumes; it is not restricted by a blanket rule that every facility must be a designated MON-retained VOR. A retained navaid's existence is not sufficient evidence of a safe diversion.

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5. Instrument Landing System (ILS) Architecture & Signal Geometry

The Instrument Landing System is the premier terrestrial precision approach system, providing lateral course and vertical path guidance to approaching aircraft:

Localizer (LOC) Component

• Frequency Band: Operates on 40 assigned VHF channels between 108.10 MHz and 111.95 MHz, utilizing odd-tenth decimals and odd tenths plus 50 kHz.
• Antenna Location: Sited on the runway centerline extended, between 1,000 and 2,000 feet beyond the departure end of the instrument runway.
• Modulation Mechanics: The localizer broadcasts two directional amplitude-modulated audio lobes:
  • A 90 Hz audio modulation lobe on the pilot's left side of the runway approach course.
  • A 150 Hz audio modulation lobe on the pilot's right side.
  • Along the exact runway centerline, the two audio signals have equal depth of modulation (Difference in Depth of Modulation, DDM = 0).
• Angular Width & Threshold Calibration: Localizers are adjusted to provide an angular course width between 3° and 6° (typically 5°), calibrated to produce a standardized linear lateral course width of 700 feet at the runway landing threshold (350 feet full-scale deflection either side).
• Service Volume: Standard localizer service coverage provides guaranteed guidance up to 4,500 feet above the antenna out to 18 NM within 10° of centerline, and out to 10 NM within 35° of centerline.

Glide Slope (GS) Component

• Frequency Band: Operates in the Ultra High Frequency (UHF) band between 329.15 MHz and 335.00 MHz, paired automatically with the assigned VHF localizer channel.
• Antenna Location: Positioned 250 to 650 feet to the side of the runway centerline, set back 750 to 1,250 feet past the runway threshold.
• Signal Structure: Broadcasts an upper lobe modulated at 90 Hz and a lower lobe modulated at 150 Hz. Along the published glidepath (nominally 3.0° above horizontal), DDM = 0.
• Beam Dimensions & False Slopes: The vertical beam thickness is approximately 1.4° (0.7° above and below the glidepath). Ground reflection physics generate harmonic "false glide slopes" at integer multiples above the true path (typically near 6° and 9°), where the 90 Hz and 150 Hz lobes invert or create steep descent profiles. To prevent capturing false lobes, flight crews must always intercept the glide slope from below at the published glide slope intercept altitude.

Range Guidance: Marker Beacons & Precision DME

• Marker Beacons: Operate on a fixed frequency of 75 MHz across all installations:
  • Outer Marker (OM): Located 4 to 7 NM from the threshold. Modulated at 400 Hz (two dashes per second), activating a blue/cyan cockpit annunciator. Marks the Final Approach Fix (FAF) and glide slope intercept point.
  • Middle Marker (MM): Located ≈ 3,500 feet from the threshold. Modulated at 1,300 Hz (alternate dots and dashes), activating an amber cockpit annunciator. Marks the decision point on standard Category I ILS approaches (≈ 200 feet AGL).
  • Inner Marker (IM): Located ≈ 1,000 feet from the threshold. Modulated at 3,000 Hz (continuous dots), activating a white cockpit annunciator. Marks the Category II decision height (100 feet AGL).
• DME Fix Substitution: Under modern criteria, precision DME colocated with the ILS or GPS fixes frequently replace marker beacons, as authorized on the approach chart header.

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6. Dispatch Planning & Navaid Outage Management

Navaid outages reported via NOTAMs directly influence flight dispatch feasibility, aircraft equipment MEL releases, and destination alternate qualifications:

Alternate Airport Selection Under Operations Specifications C055

When designating an alternate airport under 14 CFR 121.619, dispatchers calculate derived ceiling and visibility minimums using FAA OpSpec C055:

• 1-Nav Facility Rule: For airports with at least one operational navigational facility providing a straight-in non-precision or precision approach, add 400 feet to the published Decision Altitude (DA) or Minimum Descent Altitude (MDA), and add 1 statute mile to the published visibility minimum.
• 2-Nav Facility Rule: For airports with at least two operational navigational facilities, each providing a straight-in approach procedure to different suitable runways, add 200 feet to the higher DA/MDA of the two approaches, and add 1/2 statute mile to the higher visibility minimum.

Current Expanded Service Volumes

Legacy T/L/H classifications remain relevant to some figures and facilities, but FAA also uses VL/VH VOR and DL/DH DME service volumes. Published VOR VL coverage expands from 40 NM at 1,000–5,000 ft above the facility to 70 NM at 5,000–18,000 ft. VH adds its higher-altitude layers. DME volumes have their own shapes, including the lower-altitude parabolic boundary; a VOR/DME or VORTAC does not necessarily give identical volumes for every component. Consult the current Chart User’s Guide, AIM and facility status. Service-volume heights are referenced above the transmitter; obstacles, outages, and published restrictions still matter. MON coverage is the published resilience objective and has geographic limitations, not a guarantee that every mountain valley or worldwide point has a usable signal.

Official References

Source check: 2026-10-12. FAA AIM navigation guidance; FAA aeronautical handbooks and current addenda; Current AC 91-70 oceanic guidance. For operational use, apply current issued authorizations and the assigned aircraft’s approved data.