Navigation and Communication Systems Prepared by:

Posted on February 18, 2010


>VHF (very high frequency) is used by air traffic control and operates in the VHF band between 118 and 136.975 MHz

Range is 30 miles at 1000 feet and approximately 135 miles at 10,000 feet


HF (high frequency) used for extended range communication operates between 2.0 and 29.999 MHz

VHF & HF Systems

Both the VHF and HF system utilize transmitters, receivers and antennas.

Transceivers are units that include both the transmitter and receiver in one unit.

VHF and HF systems are completely independent of each other and utilize their own transmitters, receivers and antennas.

VHF systems are found in any aircraft capable of two way radio communication and are largely used for controlling traffic.

HF systems are found in large transport category aircraft that may need to communicate over large distances (overseas).

ACARS (ARINC Communication Addressing and Reporting System)

Transmits short messages from aircraft systems to central facility in Chicago

Two modes used

Demand mode – Flight crew transmits

Polled mode – Ground station transmits

Note: AIRCOM is the European and Australian equivalent

Secal Decoder

Used to “filter” messages on COMM radio receivers

Aircraft are assigned a tone combination for secal unit to monitor.

Secal unit alerts the crew to an incoming radio transmission


Utilizes satellites for transcontinental flight communications

More reliable the HF communication

Range is between latitudes 75º N and 75º S

Uses three sub-systems

Ground earth station

Aircraft earth station

Satellite system

Capable of of transmitting information from many different sources

AIRCOM, ACARS, flight-crew communications, passenger telephone, telex and fax

Navigation Systems






Marker beacons

Radio altimeters





VOR (VHF omni-directional range)

VOR’s operate between 108.0 to 117.9 MHz frequency band

System includes

VOR ground station or transmitter

VOR receiver in aircraft

In light aircraft this is often combined with the comm radio

Aircraft display

CDI course deviation indicator

TO/FROM indicator

OBS omni-bearing selector or course selector

ON/OFF flag to determine field strength


VOR Operation

VOR station continually transmits an infinite number of radials.

The VOR receiver in the aircraft receives the signal and operates the visual indicator.

The pilot determines the bearings of VOR station with respect to the aircraft.

(automatic direction finder)


The ADF receives NDB (non-directional beacon) signals in the 19 to 535 kHz AM broadcast low band.

The ADF display pointer (RMI or radio magnetic indicator) will indicate the relative bearing to the selected AM band in that range.

(instrument landing system)

Combination of several systems to provide pilot with the ability to land in conditions with poor visibility.


LOC (localizer)

Horizontal reference

GS (glide slope)

Vertical reference

Marker beacon

Distance from runway

Radio altimeter

Very accurate altitude measurement

DME (distance measuring equipment)

Very accurate distance measurement

LOC (localizer)

Combined with the VOR system

Utilizes 1 of 40 ILS channels between 108.10 to 111.95 MHz.


The ground transmitter is located at the far end of the runway and provides a valid signal up to 18 NM

The CDI (course deviation indicator) gives full fly left/right deviation of 700 feet at the runway threshold.

GS (glide slope)

Utilizes 1 of 40 channels between 329.15 to 335.00 MHz.

Operates on the same principles as the LOC.

The GS transmitter is located between 750 and 1250 ft. from the approach end of the runway and is offset 250 to 650 ft.

The indicator is either an ADI (attitude-director indicator) or HSI (horizontal-situation indicator).

Both indicators combine other indications for ease of use.

Marker Beacons

Marker beacon receivers operate at 75 MHz and sense the audio signature of 3 types of beacons.

Blue outer marker (5 miles from end of runway)

Modulated with 400 Hz

Amber middle marker (2/3 mile from end of runway)

Modulated with 1300 Hz

White inner marker (1500 feet from end of runway)

Modulated with 3000 Hz


As the aircraft flies over each maker the appropriate light will flash and an audible sound may be heard.

Radio Altimeters

The radio altimeter provides better accuracy then the pressure sensitive altimeters.


The transmitter sends out a VHF signal downward then receives the reflected signal.

The transmitter-receiver unit calculates the time needed for the signal to transmit and return to obtain AGL (above ground level) altitude.

DH (decision height) used for instrument landings may be incorporated in this system.

(distance measuring equipment)

Range is up to 199 NM at the high end of controlled airspace based on line of sight with accuracy of ½ mile or 3% of the distance.

DME operates on frequencies from 962 to 1213 MHz.


The aircraft transmitter sends out paired pulses at specific spacing.

The ground station receives the pulses and then responds with paired pulses at the same spacing but a different frequency.

The aircraft receiver measures the time it takes to transmit and receive the signal which is transmitted into distance.

(global positioning system)

Utilizes a 24 hour satellite system that is accurate within 100 meters and is unaffected by weather.

Has 3 independent segments

Space segment – satellites

Control segment – ground based monitoring

User segment – aircraft

Database updating and antenna maintenance are the primary concerns to the GPS user.

Will be the most widely used system in the near future.


An automatic receiver and transmitter that can receive a signal (be interrogated) from a ground station and send a reply back to the station.

Used to identify aircraft on radar

Identification or squawk is 1200 for VFR flight

Squawk assigned by ATC for IFR flight

Used for emergency transmissions

Transponder operation

Three modes of operation

Mode A

Location only, non-altitude reporting

Mode C

Location and altitude reporting

Mode S

Can do Mode A and C and also responds to TCAS (traffic collision avoidance systems)

(emergency locator transmitter)

Required on all aircraft to provide a signal on crash landings that will enable search aircraft or ground stations to locate the aircraft.

Consists of a dual frequency radio transmitter and battery power supply with a whip antenna.

Transmits on international distress signals of 121.5 (civil) and 243.0 (military) MHz.

Activated by impacts of 5g or more or manually.

Transmits up to 100 miles at receiver altitude of 10,000 ft for 50 continuous hours.

Located in an area of the aircraft where impact damage will be minimal.

Tail cone area

Aft top of cabin

ELT Testing

Three switch positions: AUTO, OFF and ON

Testing may be done under the following conditions:

Tune VHF COMM receiver to 121.5 MHz

Only within the first 5 minutes of an hour

Only three pulses should be activated

Listen for an audible signal when switched to ON position

ELT Servicing

The battery pack must be changed in accordance with the date stamped on the unit.

The battery pack must also be replaced or recharged when it has been in use for more than one cumulative hour, or when 50% of the useful life or charge has expired.

Testing should be performed regularly.

Inspections must be made every 12 calendar months.

Regulations FAR Part 91.52

Inspections for NAV/COMM Equipment

System inspections

Antenna inspections

Static discharge inspections

Operational checks or any additional inspections required by the manufacturer

Inspection of NAV/COMM Systems

Inspect the condition and security of equipment including wiring bundles.

Check for any indications of overheating in the equipment or wiring.

Check for poor electrical bonding

Requirements are specified by the manufacturer.

Cables should be kept as short as possible, except antenna cable which have a specific length determined in installation.

Proper bonding on the order of .003 ohms is important to the performance of avionics equipment.

Inspection of NAV/COMM Systems

Check instruments and radios for secure attachment to the instrument panel.

Check that all avionics are free of dust or contaminates.

Equipment ventilation openings must not be obstructed.

Check all plugs, connectors, switches, controls for operation and condition.

Inspection of NAV/COMM Systems

Check all instruments for placards as needed.

Check all instrument lighting and annunciator lights for operation.

Check circuit breaker panel for placards labeling each circuit breaker installed.

Antenna Inspection

Check for:

broken or missing antenna insulation

lead through insulators

Safety wires

Cracked antenna housing

Missing or poor sealant at base of antenna

Antenna Inspection

Check for:

Correct installation

Signs of corrosion

Condition of paint/bonding and grounding

Bonding of each antenna from mounting base to the aircraft skin.

Tolerance 1 ohm, maximum

Inspection of Static Dischargers/Wicks

Check for:

Physical security of mounting attachments, wear or abrasion of wicks, missing wicks, etc.

Assurance that one inch of the inner braid of flexible vinyl cover wicks extends beyond the vinyl covering.

Assurance that all dischargers are present and securely mounted to their base.

Inspection of Static Dischargers/Wicks

Check for:

Assurance that all bases are securely bonded to the skin of the aircraft.

Any sign of excessive corrosion or deterioration of the discharger tip.

Any lighting damage shown by pitting of the metal base.

The ohm value of the static wick itself per manufacturer’s instructions.

Additional Inspections


Per FAR 14 Part 91.411 and 91.413


Per FAR Part 91.52

Functional checks of all other COMM and NAV systems per the manufacturer’s instructions

Posted in: AVIATION