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newSchoolWorks.com › VoTechWorks › Transportation › Aviation Maintenance › Avionics: Aircraft Electrical, Instrument, and Communication Systems › Lesson 9
TRN-AVIA22-04 · Lesson 9 · 51 clock hours · theory

Navigation Systems: VOR, ILS, GPS, and ADS-B

Objective. Students will describe the operating principles of VOR, ILS (localizer and glideslope), GPS, and ADS-B Out systems, identify the regulatory requirements for ADS-B Out equipage, and interpret a navigation avionics wiring diagram to locate antenna, receiver, and indicator connections.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Navigation Systems: VOR, ILS, GPS, and ADS-B

Why Navigation Avionics Matter to the Mechanic

Navigation avionics guide flight crews from departure to touchdown. As an aviation maintenance technician, you will inspect, troubleshoot, and return these systems to service. Understanding how each system works — and how they interconnect — is essential before you ever pick up a wiring diagram or a ramp tester.

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VOR — VHF Omnidirectional Range

VOR is a ground-based navigation aid that transmits signals in the VHF frequency band (108.0–117.95 MHz). The ground station radiates two signals simultaneously: a reference phase signal broadcast omnidirectionally, and a variable phase signal that rotates 30 times per second. The aircraft receiver measures the phase difference between the two signals. That phase difference corresponds directly to the aircraft's magnetic bearing from the station, called a radial.

The cockpit instrument is the Course Deviation Indicator (CDI). The pilot selects a desired radial (course) using the Omni-Bearing Selector (OBS). The CDI needle deflects left or right to show deviation from that course. Full-scale deflection is typically ±10° from the selected course, with each dot representing roughly 2°.

VOR antennas on the aircraft are usually V-shaped or blade-type antennas mounted on the fuselage. The coaxial cable from the antenna connects to the VOR/NAV receiver, which then drives the CDI indicator.

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ILS — Instrument Landing System

The ILS allows precision approaches in low-visibility conditions. It has two radio guidance components:

Localizer: Operates in the VHF band (108.10–111.95 MHz, odd tenths). The ground transmitter sends two overlapping lobes — one modulated at 90 Hz, one at 150 Hz — centered on the runway centerline. The aircraft receiver compares the depth of modulation (DDM) of the two signals. Equal modulation means the aircraft is on centerline. More 150 Hz indicates the aircraft is right of course; more 90 Hz indicates left. The localizer drives the lateral (left/right) needle of the CDI. Full-scale deflection equals approximately ±2.5° from centerline.

Glideslope: Operates in the UHF band (329.15–335.00 MHz). The same 90 Hz/150 Hz DDM technique is used, but in the vertical plane. The glideslope transmitter is located beside the runway threshold and projects a glide path, typically at 3°. The glideslope receiver drives the vertical needle of the CDI (or a separate indicator). Full-scale deflection equals approximately ±0.7° from the glideslope.

The localizer and glideslope frequencies are paired: selecting the localizer frequency on the NAV radio automatically tunes the glideslope receiver. Aircraft typically carry separate blade antennas for localizer (VHF, shared with VOR antenna) and for glideslope (UHF antenna, usually mounted on the nose or lower fuselage).

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GPS — Global Positioning System

GPS determines position by measuring the travel time of signals from multiple satellites. The receiver must track at least four satellites to calculate a three-dimensional position fix (latitude, longitude, and altitude). Each satellite broadcasts its precise time and orbital data; the receiver computes the pseudorange to each satellite and solves for position.

WAAS (Wide Area Augmentation System) is an FAA-operated network of ground reference stations that send correction data to geostationary satellites, which then rebroadcast corrections to WAAS-enabled GPS receivers. WAAS improves accuracy to within approximately 3 meters horizontally and provides integrity monitoring — essential for approach operations down to LPV minimums.

GPS antennas are patch-type, mounted on top of the fuselage with an unobstructed view of the sky. Active GPS antennas contain a built-in low-noise amplifier (LNA) and require a small DC voltage supplied through the coaxial cable from the receiver — confirm this voltage with a multimeter before condemning an antenna.

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ADS-B Out — Automatic Dependent Surveillance–Broadcast

ADS-B Out transmits the aircraft's GPS-derived position, altitude, velocity, and identification to ground stations and other ADS-B In-equipped aircraft. It operates on 1090 MHz (using an Extended Squitter transponder, Mode S ES) or 978 MHz (UAT — Universal Access Transceiver, used below 18,000 feet MSL in the US).

Under 14 CFR Part 91, ADS-B Out equipage is required in Class A, B, and C airspace, above the ceiling of and within the lateral boundaries of Class B or Class C airspace up to 10,000 feet MSL, Class E airspace at and above 10,000 feet MSL excluding airspace below 2,500 feet AGL, and within 30 nautical miles of certain airports. The regulation specifies performance requirements including position accuracy and integrity standards.

The ADS-B transponder is connected to a dedicated ADS-B-approved GPS position source and a directional antenna (blade or whip, typically bottom-mounted).

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Reading a Navigation Avionics Wiring Diagram

A composite navigation wiring diagram will show:

When annotating, trace each coaxial run from antenna to receiver in red, mark every shield ground point with a circled G, and label each circuit breaker with its protected equipment and amperage rating.

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Safety Reminders

Key terms8

RadialA magnetic bearing measured outward from a VOR station; the CDI shows the aircraft's deviation from a selected radial.
Course Deviation Indicator (CDI)The cockpit instrument that displays lateral (and, during ILS, vertical) deviation from a selected navigation course.
Difference in Depth of Modulation (DDM)The ILS signal comparison technique; the receiver measures the difference in the 90 Hz and 150 Hz modulation depths to determine deviation from localizer centerline or glideslope.
WAAS (Wide Area Augmentation System)An FAA satellite-based augmentation system that broadcasts GPS correction data, improving position accuracy and providing integrity monitoring for precision approach use.
ADS-B OutA surveillance technology that automatically broadcasts GPS-derived aircraft position, altitude, velocity, and identification on 1090 MHz or 978 MHz to ATC ground stations and other aircraft.
PseudorangeThe approximate distance from a GPS receiver to a satellite, calculated from signal travel time before clock error corrections are applied.
Shield Ground PointA location on a wiring diagram where the outer braid of a coaxial cable is bonded to aircraft structure to prevent RF interference and ensure signal integrity.
GlideslopeThe vertical guidance component of an ILS system, operating in the UHF band and projecting a precision descent path — typically 3° — to the runway threshold.
In class: Complete a structured reading of FAA-H-8083-31 navigation systems chapters and the relevant sections of 14 CFR Part 91 for ADS-B Out requirements, then complete a 25-question fill-in and multiple-choice quiz on system principles, signal characteristics, and regulatory applicability. · Trace a composite navigation avionics wiring diagram (VOR/ILS receiver, CDI indicator, GPS/WAAS unit, ADS-B transponder, and associated antennas) identifying each component, wire run, shield-ground point, and circuit-breaker protection, annotating the diagram in red pen. · In pairs, use an avionics ramp tester or PC-based avionics simulator to perform a VOR receiver sensitivity check and an ILS CDI deflection test, recording all readings on a data sheet and comparing to manufacturer specifications.
Materials: FAA-H-8083-31, 14 CFR Part 91 ADS-B excerpt, composite navigation avionics wiring diagram set, avionics ramp tester or avionics PC simulator, multimeter, red pen, annotation worksheets, safety glasses
Check: 25-question quiz scored at 80% or higher; wiring diagram annotation checked by instructor confirming all components and ground points correctly identified — both required for pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Avionics Systems
1. A VOR ground station radiates signals that allow an aircraft receiver to determine the aircraft's position relative to the station. What physical quantity does the receiver measure to compute the radial?
Avionics Systems
2. During an ILS approach, the CDI lateral needle deflects fully to the right. According to ILS signal principles, what does this indicate?
Avionics Systems
3. An active GPS patch antenna is suspected of being faulty. Before replacing it, the technician should verify which of the following at the antenna connector using a multimeter?
Federal Aviation Regulations
4. Under 14 CFR Part 91, ADS-B Out equipment is required when operating in which of the following airspace?
Avionics Systems
5. On a composite navigation avionics wiring diagram, you identify a symbol indicating a bond between a coaxial cable braid and aircraft structure. This point is called a:
Avionics Systems
6. The glideslope receiver in an ILS-equipped aircraft is tuned automatically when the pilot selects the localizer frequency. The glideslope operates in which frequency band?
Safety and Ground Operations
7. A technician is using a ramp tester near an aircraft to perform a VOR receiver sensitivity check. Which safety precaution is most critical before activating the RF test signal?
Avionics Systems
8. WAAS improves standard GPS for aviation use primarily by:
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