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6.2 Ground-Based Navigation Aids & Standard Service Volumes
FAA Aircraft Dispatcher Certificate ADX
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1 Chapter 1: Regulations, Certification & Operational Control
1.1 Part 65 Subpart C: Certification, Eligibility & Currency 1.2 Operational Control & Dispatcher-PIC Joint Responsibility 1.3 Domestic, Flag & Supplemental Operations Framework 1.4 Dispatch Release Elements, Signatures & Recordkeeping 1.5 Regulations, Manuals, Crew Qualifications, and Source Control
2 Chapter 2: Atmospheric Physics, Pressure Systems & Fronts
2.1 Atmospheric Structure, Standard Atmosphere & Altimetry 2.2 Air Masses, Frontal Systems & Frontogenesis 2.3 Atmospheric Stability, Lapse Rates & Cloud Formation 2.4 Earth Motion, Regional Weather, Local Winds, and Water Phase Changes
3 Chapter 3: Aviation Weather Hazards & Severe Weather Avoidance
3.1 Structural and Induction Icing: Types, Accretion & Operations 3.2 Thunderstorms, Squall Lines, Microbursts & Low-Level Windshear 3.3 Clear Air Turbulence (CAT), Mountain Waves & Jet Streams 3.4 Low Ceilings, Fog Types & Visibility Restrictions 3.5 Hurricanes, Typhoons, Tornadoes, and Tropical Dispatch Hazards
4 Chapter 4: Weather Observations, Forecasts & Advisories Decoding
4.1 METAR and SPECI Surface Observations: Decoding & Operations 4.2 Terminal Aerodrome Forecasts (TAF): Groups & Timing 4.3 Pilot Weather Reports (PIREP): Coding, Severity & Urgent Alerts 4.4 In-Flight Advisories: AIRMETs, SIGMETs & Convective SIGMETs 4.5 Weather Collection, NWS Facilities, GFA, and EWINS
5 Chapter 5: Aviation Weather Charts & Upper-Air Analysis
5.1 Surface Analysis & Radar Summary Charts 5.2 Constant Pressure Analysis & Winds Aloft (FB) 5.3 Significant Weather Prognostic Charts (SIGWX) 5.4 Satellite Imagery, Moisture-Stability Analysis, and Forecast Integration
6 Chapter 6: Air Navigation, Airspace Architecture & Route Structure
6.1 National Airspace System: Classes, Rules & Special Use 6.2 Ground-Based Navigation Aids & Standard Service Volumes 6.3 Area Navigation (RNAV), GNSS/GPS, PBN & RNP Fundamentals 6.4 En Route Airways, SIDs, STARs & Chart Interpretation 6.5 Earth Coordinates, UTC, Chart Projections, and Flight Instruments 6.6 North Atlantic, Pacific, RVSM, and Global Navigation Differences
7 Chapter 7: Instrument Approaches, Minimums & Airport Operations
7.1 Precision Approach Systems: ILS Categories & Tolerances 7.2 Non-Precision Approaches, RNAV & Decision Minimums (MDA/DA) 7.3 Takeoff Minimums, Lower-Than-Standard & Takeoff Alternates 7.4 Holding Procedures: Patterns, Speed Limits & Timing
8 Chapter 8: Subsonic Aerodynamics & Jet Aircraft Performance
8.1 Subsonic Jet Aerodynamics: Lift, Drag & Flight Regimes 8.2 High-Altitude Flight: Mach Number, Buffet & Coffin Corner 8.3 Transport Turbine Engines: Thrust, Specific Fuel & Bleed Air
9 Chapter 9: Transport Category Takeoff & Landing Runway Analysis
9.1 Takeoff Performance Speeds & Balanced Field Concept 9.2 Takeoff Climb Segments & Obstacle Clearance Gradients 9.3 Landing Performance: Field Lengths, 60% Rule & Contamination
10 Chapter 10: En Route Performance, Driftdown Profiles & ETOPS Operations
10.1 One/Two-Engine-Inoperative En Route Net Profiles & Driftdown 10.2 Extended-Range Operations (ETOPS): Philosophy & Regulations 10.3 ETOPS En Route Alternates: Adequate vs. Suitable & Critical Fuel
11 Chapter 11: Aircraft Weight & Balance, Center of Gravity, and Loading
11.1 Transport Aircraft Structural Weight Limits: Definitions & Rules 11.2 Center of Gravity Computations: Datum, Arm, Moment & Index 11.3 Mean Aerodynamic Chord (%MAC) Computations & Stability 11.4 Payload-Range Analysis, Zero Fuel Weight Tradeoffs & Loading
12 Chapter 12: Destination Alternate Airport Selection & Weather Rules
12.1 Destination Alternate Selection: 91.169 & Part 121 1-2-3 Rule 12.2 Flag & Supplemental Operations Destination Alternate Rules 12.3 Derived Alternate Minimums: 1-Nav and 2-Nav Facility Rules
13 Chapter 13: Part 121 Fuel Planning, Reserves & En Route Re-Release
13.1 Part 121 Domestic Fuel Requirements: Reserves & Calculations 13.2 Part 121 Flag Fuel Requirements: 10% Reserve & Island Rules 13.3 Supplemental Fuel Requirements & Holding Fuel Management 13.4 Redispatch and En Route Re-release Procedures
14 Chapter 14: Transport Systems, Pressurization & MEL/CDL Procedures
14.1 Transport Jet Auxiliary Power, Electrical, Hydraulic & Pneumatic Systems 14.2 Cabin Pressurization, Rapid Decompression & Oxygen Requirements 14.3 Master Minimum Equipment List (MMEL), MEL & CDL Procedures 14.4 Flight Controls, Autoflight, Fuel, Oil, Brakes, and Fire Protection
15 Chapter 15: Hazardous Materials, Duty Regulations & Emergency Procedures
15.1 Dangerous Goods Carriage by Air (49 CFR Part 175) & NOTOC 15.2 Dispatcher Duty Limitations, Rest Requirements & Fatigue 15.3 Emergency Authority, Declaration & Diversion Decision-Making 15.4 NTSB Part 830 Accident Reporting & Wreckage Preservation 15.5 Security, Overdue Aircraft, and FAA Emergency Services
16 Chapter 16: Communications, ATC, and Flight Plan Procedures
16.1 Voice, Data Link, Position Reports, and Lost Communications 16.2 ATC Facilities, Separation, ICAO Flight Plans, and Traffic Management
17 Chapter 17: Human Factors and Practical Dispatch Applications
17.1 Dispatch Resource Management, Human Error, and Risk Decisions 17.2 Manual Flight Planning, PIC Briefing, Flight Watch, and Postflight Closeout
6.2 Ground-Based Navigation Aids & Standard Service Volumes
Key Takeaways Explain This Topic AI
VOR radial and heading are distinct; radials describe directions from the station and must be interpreted with the correct reference.
DME measures slant range; directly above a sea-level station at 36,000 ft, the ideal distance is approximately 5.9 NM.
SSV altitude boundaries use height above the transmitter; the legacy H volume includes 130 NM from 18,000 through 45,000 ft ATH, subject to restrictions and blockage.
MON supports conventional reversion during GNSS disruption; actual terrain, weather, equipment, facility status, and ATC conditions still matter.
ILS localizer and glide slope provide different guidance and have different frequency bands and service volumes.
Last updated: October 2026
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.
- 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:
Reference Phase Signal: Radiated omnidirectionally in all azimuths as a frequency-modulated (FM) 30 Hz signal imposed on a 9960 Hz subcarrier.
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.
- 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:
The aircraft interrogator transmits a stream of paired pulse interrogations to the ground transponder at a randomized pulse repetition frequency (PRF).
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.
The airborne receiver locks onto its own unique randomized pulse signature, measures total round-trip elapsed time ( t total t_{\text{total}} t total ), subtracts the 50 µs ground station delay, and calculates line-of-sight distance using the speed of light ( c ≈ 161,875 NM/s c \approx 161{,}875\text{ NM/s} c ≈ 161 , 875 NM/s or 300,000 km/s 300{,}000\text{ km/s} 300 , 000 km/s ): Slant Distance (NM) = c × ( t total − 50 μ s ) 2 \text{Slant Distance (NM)} = \frac{c \times (t_{\text{total}} - 50\;\mu\text{s})}{2} Slant Distance (NM) = 2 c × ( t total − 50 μ s )
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: Slant Distance = ( Ground Distance ) 2 + ( Altitude Above Station ) 2 \text{Slant Distance} = \sqrt{(\text{Ground Distance})^2 + (\text{Altitude Above Station})^2} Slant Distance = ( Ground Distance ) 2 + ( 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 6 2 + 5.92 2 ≈ 8.43 NM \sqrt{6^2 + 5.92^2} \approx 8.43\text{ NM} 6 2 + 5.9 2 2 ≈ 8.43 NM —an error exceeding 40%.
Tip
Operational Rule of Thumb for DME Slant-Range Error: Slant-range error becomes negligible (less than 1%) when the aircraft's horizontal distance from the station is at least 1 nautical mile for every 1,000 feet of altitude above the station elevation (e.g., at least 30 NM away when cruising at 30,000 feet AGL). Inside this distance, flight crews and dispatchers cannot use raw DME as an exact measure of ground position.
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.
- Designator Published principal altitude/range boundaries
- T 1,000–12,000 ft ATH inclusive: 25 NM
- L 1,000–18,000 ft ATH inclusive: 40 NM
- H 1,000–14,500 ft: 40 NM; 14,500–60,000 ft: 100 NM; 18,000–45,000 ft: 130 NM
- VL 1,000 to below 5,000 ft: 40 NM; 5,000 to below 18,000 ft: 70 NM
VH 1,000 to below 5,000 ft: 40 NM; 5,000 to below 14,500 ft: 70 NM; 14,500–60,000 ft: 100 NM; 18,000–45,000 ft: 130 NM
- DL Below 12,900 ft: published line-of-sight boundary; 12,900 to below 18,000 ft: 130 NM
- DH Below 12,900 ft: published line-of-sight boundary; 12,900–60,000 ft: 100 NM; 12,900–45,000 ft: 130 NM
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.
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.
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.
Warning
The Critical Dispatch Trap: 2-Nav Facility Dependency To utilize the 2-Nav rule, the two approaches must rely on independent ground navigational facilities . If Runway 09 has an ILS and Runway 27 has an ILS, but both approaches require a single on-field VOR/DME for the initial approach fix or missed approach procedure, an outage of that single VOR disqualifies the 2-Nav rule! The dispatcher must then revert to the 1-Nav rule or select another alternate airport entirely.
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.