E - Electronics (Avionics)Chapter 6 · 94 practice questions

Chapter 6: Navigation Systems - VOR, ILS, DME, ADF, GPS, GNSS, Transponder

Includes 8 animated diagrams — view them live in the interactive theory reader.

Diagram — Navigation Systems - VOR, ILS, DME, ADF, GPS, GNSS, TransponderBearing error varies withradialPhase detectornon-linearityNon-linearity causes errors thatchange with radial angle; correct at360° but erroneous at other radials180° bearing errorPhase reversal inreceiverReference and variable signalsinvertedBearing error increases withdistancePhase detector driftWeaker signals make receiver moresensitive to phase errorsIncorrect TO/FROM indicationPhase reversal or wiringerrorFaulty phase detector or wiringcauses flag reversal

Maintenance Considerations

  • Radial accuracy checks require verifying the phase detector alignment
  • Antenna cable mismatch causes signal loss, not radial-specific errors
  • Compass errors affect heading, not VOR bearing directly

2. ILS (Instrument Landing System)

ILS — Localizer and Glideslope ILS — Localizer and Glideslope LOC Antenna (108-112 MHz) 90 Hz 150 Hz Runway Centerline (DDM=0) Coverage Zone (±35° up to 10 NM) RUNWAY Threshold GS Antenna (UHF 329-335 MHz) Glideslope Path ~3° 90 Hz (Too High) 150 Hz (Too Low) OM (4-7 NM) MM (~3500 ft) TECHNICAL LEGEND: • DDM = Difference in Depth of Modulation • On course/slope: 90 Hz = 150 Hz (Zero DDM)

System Components

ILS consists of three independent subsystems:

Localizer (LOC)

  • Frequency range: 108.10–111.95 MHz
  • Provides lateral guidance along the runway centerline
  • Transmits 90 Hz and 150 Hz modulation signals
  • Course alignment determined by physical antenna array positioning

Glideslope (GS)

  • Frequency range: 329.15–335.00 MHz
  • Provides vertical guidance at a typical angle of 3°
  • Angle determined by mechanical alignment of antenna array
  • Transmits 90 Hz and 150 Hz modulation signals

Marker Beacons

  • Frequency: 75 MHz
  • Three types: Outer Marker (OM), Middle Marker (MM), Inner Marker (IM)
  • Provide distance-to-runway information along the approach path
  • Require both audio and visual indication per TC Standard 571

Common Failure Modes

Localizer Failures

Glideslope Failures

Marker Beacon Failures

Maintenance Considerations

  • Localizer alignment tolerance: ±0.5°
  • Glideslope angle tolerance: ±0.1° to ±0.2°
  • Modulation depth tolerance: ±5% to ±10%
  • All three marker beacon lights must be correctly wired to their respective circuits

3. DME (Distance Measuring Equipment)

DME — Round-Trip Pulse Timing and Slant Range DME — round-trip pulse timing measurement AIRCRAFT (interrogator) Transmission: pulse pairs Reception: station response Outbound (interrogation) Return (response) GROUND STATION (transponder) Fixed delay: 50 microseconds Receives and retransmits after 50 µs Slant range calculation D = c × (T_return − 50 µs) / 2 D = slant range · c = speed of light Tolerance (FAA AC 20-100B) ±0.25 NM or ±1.25% of distance (whichever is greater) Slant range correction near the station Near the station, slant range differs from horizontal distance: D_slant = √(D_horiz² + altitude²) — error decreases with distance Horizontal distance Altitude Slant range (measured)

Operating Principle

DME operates in the UHF band (962–1213 MHz) and measures slant range distance by calculating the round-trip time of interrogations and replies. The aircraft transmits interrogation pulses, and the ground station replies after a fixed delay.

Key Parameters

  • Lock-on time: Time required to acquire a valid reply
  • Reply efficiency: Percentage of interrogations receiving valid replies
  • Accuracy: Typically ±0.25 nm or ±1.25% of distance, whichever is greater (per FAA AC 20-100B)
  • Slant range correction: Required for accurate position when close to the station

Common Failure Modes

Critical Distinctions

  • Timing calibration errors cause consistent percentage errors (e.g., 1% or 2% of distance)
  • Delay calibration errors cause constant offset errors (same nm error at all ranges)
  • Antenna cable length adds negligible delay
  • Frequency offset or synthesizer drift causes complete loss of lock, not accuracy errors

Maintenance Considerations

  • DME requires line-of-sight to ground station
  • On ground, buildings and terrain may block signal
  • Calibration per manufacturer's data as specified in AC 43.13-1B

4. ADF (Automatic Direction Finder)

ADF — Loop Antenna Null Seeking and Sense Antenna ADF — loop antenna, null seeking and sense antenna 1. Motorized loop antenna — null seeking NULL The motor rotates the loop to find the minimum signal point (null), which indicates the direction of the NDB station. 2. Omnidirectional sense antenna — resolves ambiguity 180° ambiguity resolved by the sense antenna RESULT True direction The sense antenna receives the signal from all directions and determines whether the station is in front of or behind the aircraft. Functional diagram — normal operation Loop antenna (directional) Sense antenna (omnidirectional) Motor ADF receiver Compares signals RMI indicator Needle → station The motor rotates until the loop signal is minimal (null). The phase of the sense signal determines the direction (0° or 180°). Typical ADF system malfunctions Motor failure Needle fixed, does not follow the heading change Reversed wiring (180°) Needle points to the opposite (toward the aircraft tail) Sense antenna disconnected 180° ambiguity not resolved — random or no indication Illustration — 180° error and disconnected sense N Heading 000° NDB Correct Reversed wiring Sense disconnected Ambiguity not resolved Reversed loop antenna wiring causes a constant 180° error. A disconnected sense antenna makes the indication ambiguous (front/back impossible to distinguish).

Operating Principle

ADF operates in the LF/MF bands (190–1750 kHz) and uses a loop antenna to determine the direction of a ground station. The system employs:

  • Loop antenna: Rotates to find the null position (direction of station)
  • Sense antenna: Resolves 180° ambiguity

Common Failure Modes

Maintenance Considerations

  • Loop antenna must rotate freely and track the station
  • Sense antenna is essential for resolving ambiguity
  • Wiring errors cause fixed offset errors (180°, 90°, or other values)
  • AC 43.13-1B specifies loop antenna wiring requirements

5. GPS/GNSS (Global Positioning System/Global Navigation Satellite System)

GPS — Satellite Trilateration GPS — Satellite Trilateration & Geometry (GDOP) 3 SATELLITES = 2D POSITION Sat A Sat B Sat C Rx Aircraft Intersection of 3 circles = Latitude + Longitude 4 SATELLITES = 3D POSITION + TIME Sat 1 Sat 2 Sat 3 Sat 4 (t) Rx Aircraft 4 spheres resolve Lat, Lon, Alt + Clock Error GDOP (Geometric Dilution of Precision) GOOD GDOP Sats spread out POOR GDOP Sats clustered Accuracy depends on satellite geometry. Low GDOP (<2) indicates high accuracy. High GDOP (>6) degrades navigation solution. GPS CONSTELLATION 24+ Operational satellites (6 orbital planes, 55° incl.)

Operating Principle

GPS uses a constellation of satellites transmitting precise timing signals. The receiver calculates position by measuring time delays from multiple satellites. Key concepts include:

  • 3D fix: Requires at least 4 satellites for position, altitude, and time
  • RAIM (Receiver Autonomous Integrity Monitoring): Requires at least 5 satellites with good geometry
  • Dilution of Precision (DOP): Measure of satellite geometry quality

Factors Affecting Accuracy

Common Failure Modes

RAIM Requirements

RAIM — Satellite Geometry and Fault Detection RAIM — satellite geometry and fault detection GOOD GEOMETRY — LOW DOP AIRCRAFT SAT 1 SAT 2 SAT 3 SAT 4 SAT 5 LOW DOP High accuracy ≥ 5 well-distributed satellites → fault detection possible BAD GEOMETRY — HIGH DOP AIRCRAFT SAT 1 SAT 2 SAT 3 SAT 4 MISSING HIGH DOP Reduced accuracy < 5 satellites or grouped satellites → fault detection impossible AVAILABILITY VARIABLE ✓ RAIM AVAILABLE The receiver can detect a faulty satellite and exclude it from the navigation solution. ✗ RAIM UNAVAILABLE Insufficient NORMAL geometry condition — NOT a GPS receiver failure. AVAILABILITY VARIES BY LOCATION AND TIME 00:00 06:00 12:00 18:00 00:00 AVAILABLE UNAVAILABLE AVAILABLE DEGRADED AVAILABLE ■ RAIM available (≥ 5 satellites) ■ RAIM unavailable (< 5 satellites) ■ Marginal availability Variation due to satellite orbital positions
  • Minimum 5 satellites for fault detection
  • Availability varies with location and time
  • Remote areas may have fewer visible satellites
  • Does not indicate system failure, but insufficient geometry

Maintenance Considerations

  • Antenna placement critical to minimize multipath
  • Oscillator stability checks required per TC guidance
  • Database currency affects approach procedures, not position accuracy
  • Software updates may require re-initialization

6. Transponder (Mode A/C/S)

Operating Principle

The transponder responds to ground-based interrogations with coded replies for identification (Mode A), altitude (Mode C), and data link (Mode S). Mode S provides additional capabilities including:

  • Diversity: Uses two antennas for omnidirectional coverage
  • Data link: Encodes/decodes messages
  • Surveillance: Responds to specific interrogations
  • Acquisition: Initial contact with ground radar
  • Identification: Transmits unique aircraft address

Common Failure Modes

Antenna/Cable Failures

Software/Configuration Failures

Altitude Reporting Failures

Frequency/Performance Failures

Critical Distinctions

  • Self-test passing indicates internal circuitry functional but does not verify antenna system
  • Antenna cable shield issues affect signal levels and diversity
  • Software version mismatches affect data functions, not basic reply capability
  • Valid codes (0000, 1200, 7500, 7777) do not cause failures

Maintenance Considerations

  • AC 43.13-1B specifies antenna installation requirements
  • Proper impedance matching essential for pulse shape
  • Diversity systems require both antennas functional
  • Altitude encoder must be aligned with altimeter

7. Integrated Systems: RNAV

RNAV — Multi-Sensor Integration RNAV — Multi-sensor VOR/DME/GPS integration VOR FM reference signal + Variable 30 Hz signal Phase → bearing 108.00 – 117.95 MHz DME Pulse interrogation Reply after 50 µs Round-trip time 962 – 1213 MHz GPS Satellite trilateration 3D position (lat, lon, alt) Atomic clock GNSS constellation IRS Gyroscopes + accelerometers Slant range correction Ground distance Alt Slant DME Dground = √(Dslant² − Alt²) MULTI-SENSOR FUSION RNAV computer Kalman filter Sensor weighting Computed position Lat / Lon / Alt + velocity vector Cross-check VOR/DME comparison vs GPS vs IRS Error detection Deviation > tolerance → Alert / downgrade Navigation display DISPLAY / USE Navigation display (CDI/HSI) Lateral deviation DISTANCE 12.4 NM GROUND SPEED 245 kt HEADING 087° ALTITUDE 5 200 ft RNAV MODE ACTIVE SOURCE RECALL Sensor data Correction Fusion Near the station, the slant DME distance differs from ground distance. The correction uses altitude.

Operating Principle

RNAV (Area Navigation) systems combine inputs from multiple navigation sensors to compute position. Common configurations include:

  • VOR/DME RNAV: Uses VOR bearing and DME distance
  • GPS RNAV: Uses satellite positioning
  • Multi-sensor: Combines GPS, VOR, DME, IRS

Critical Considerations

  • Slant range correction: Essential for VOR/DME RNAV when close to station
  • Without correction, significant position errors occur at low altitudes near stations
  • Expired databases affect waypoint data, not position computation

Important Formulas, Regulations, and Procedures

VOR — Reference vs Variable Signal Phase Comparison VOR — phase comparison of reference signal and variable signal GROUND STATION VOR Reference signal Omnidirectional — FM 9960 Hz Variable signal Directional — 30 Hz rotation Transmission AIRCRAFT VOR RECEIVER Antenna PHASE DETECTOR Compares the phases of both signals Linearity error possible Ref. Var. Δφ RADIAL Δφ ∝ radial angle (0° to 360°) Ex: 090° = 90° TO/FROM indicator Logic based on phase 180° phase shift = reversed VARIABLE BEARING ERRORS • Phase detector linearity error • Error varies with radial (e.g., correct at 360°, 10° error at 30° at 090°, 180°, 270°) • Drift: error increases with distance Reference signal FM 9960 Hz Variable signal 30 Hz Phase difference = radial Bearing output

Accuracy Standards

DME Accuracy (FAA AC 20-100B)

Allowable error = max(±0.25 nm, ±1.25% of distance)

Example: At 20 nm, 1.25% = 0.25 nm, so tolerance is ±0.25 nm

Altimeter System (CAR 625 Appendix B)

  • At 5,000 ft: ±35 ft allowable error
  • Tolerances vary with altitude

ILS Alignment Tolerances

  • Localizer course: ±0.5°
  • Glideslope angle: ±0.1° to ±0.2°
  • Modulation depth: ±5% to ±10%

Regulatory References

Maintenance Procedures

DME Calibration

  • Verify internal timing per manufacturer's data
  • Check antenna cable for attenuation
  • Confirm lock-on time within specifications
  • Verify reply efficiency

VOR Calibration

  • Test radial accuracy at multiple radials
  • Check phase detector linearity
  • Verify TO/FROM indication
  • Confirm station identification

Transponder Testing

  • Verify reply frequency stability
  • Check receiver sensitivity
  • Test pulse shape and level
  • Confirm Mode S functions (if equipped)
  • Verify altitude reporting accuracy

Relationships Between Concepts

System Interdependencies

VOR/DME Integration

  • VOR provides bearing, DME provides distance
  • Combined for position fixing
  • RNAV systems use both for navigation updates
  • DME slant range error affects RNAV accuracy near stations

ILS Component Relationships

  • Localizer, glideslope, and marker beacons are independent systems
  • All three required for precision approach
  • Failure of one does not affect others
  • DME may be paired with ILS for distance information

GPS/RAIM Relationship

  • RAIM requires minimum satellite geometry
  • Availability varies with location and time
  • Not related to database currency
  • "RAIM not available" is a normal condition, not a failure

Transponder Mode Relationships

  • Mode A: Identification (squawk code)
  • Mode C: Altitude reporting (requires encoder)
  • Mode S: Data link, surveillance, identification (requires software)
  • Diversity: Antenna configuration (requires two antennas)

Common Failure Patterns

Antenna System Failures

  • Affect multiple systems: VOR, DME, transponder, marker beacon
  • Symptoms vary: signal loss, reduced sensitivity, erratic operation
  • Cable shield issues cause different symptoms than open circuits

Receiver Internal Failures

  • Phase detector issues affect VOR bearing accuracy
  • Audio output stage failures affect marker beacon audio
  • Oscillator drift affects transponder frequency and GPS timing
  • Timing calibration errors affect DME accuracy

Wiring/Configuration Errors

  • Crossed wires cause wrong indications (marker beacon lights, ADF bearing)
  • Software mismatches affect Mode S functions
  • Encoder misalignment affects altitude reporting
  • Phase reversals cause 180° errors in VOR and ADF

Troubleshooting Approach

When diagnosing navigation system failures:

  1. Determine if the problem is consistent or variable
  • Consistent errors suggest calibration or wiring issues
  • Variable errors suggest intermittent connections or environmental factors
  1. Check related systems
  • If VOR works but DME fails, focus on DME-specific components
  • If multiple systems fail, check common elements (power, antenna)
  1. Verify test equipment
  • Ensure test set is functioning correctly before diagnosing aircraft system
  • Compare aircraft indication with test set reference
  1. Consider environmental factors
  • Ground operations may block line-of-sight signals
  • Solar activity affects GPS accuracy
  • Building proximity causes multipath interference

Summary

This chapter has covered the six primary navigation and surveillance systems found on modern aircraft. Key takeaways for the AME include:

  • VOR: Phase detector linearity is critical for accurate bearing; non-linearity causes radial-specific errors
  • ILS: Physical alignment of antenna arrays determines course and glideslope accuracy
  • DME: Timing calibration errors cause consistent percentage offsets; antenna cable issues affect lock-on and efficiency
  • ADF: Loop antenna motor and wiring integrity essential for accurate bearing
  • GPS/GNSS: Satellite geometry, ionospheric conditions, and oscillator stability affect accuracy
  • Transponder: Antenna system integrity and software version matching are critical for Mode S functions

Understanding the relationships between these systems and their common failure modes enables the AME to efficiently diagnose and rectify navigation system problems, ensuring aircraft remain airworthy and capable of safe operation in all phases of flight.

Diagram

Practice this chapter

Reinforce Navigation Systems - VOR, ILS, DME, ADF, GPS, GNSS, Transponder with 94 Transport Canada–style practice questions, matched to your weak areas.

Try Free Practice QuestionsView Pricing