1. Navigation and Communication Systems
Prepared by:
Lindsay V. Oczak
Fall 2000

2. Communication Systems
VHF HF
ACARS / AIRCOM
Secal decoders
SATCOM
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

3. COMM Radios Two types commonly used for communication: VHF HF
VHF (very high frequency) is used by air traffic control and operates in the VHF band between 118 and MHz
Range is 30 miles at 1000 feet and approximately 135 miles at 10,000 feet HF
HF (high frequency) used for extended range communication operates between 2.0 and MHz
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

4. 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).
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

5. 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
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

6. 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
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

7. SATCOM 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
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

8. Navigation Systems VOR ADF ILS LOC GS Marker beacons Radio altimeters
DME
GPS
Transponders
ELT
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

9. VOR (VHF omni-directional range)
VOR’s operate between to 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
Antenna
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

10. 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.
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

11. ADF (automatic direction finder)
Operation
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.
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

12. ILS (instrument landing system)
Combination of several systems to provide pilot with the ability to land in conditions with poor visibility.
Components
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
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

13. LOC (localizer) Combined with the VOR system
Utilizes 1 of 40 ILS channels between to MHz.
Operation
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.
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

14. GS (glide slope)
Utilizes 1 of 40 channels between to 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.
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

15. 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
Operation
As the aircraft flies over each maker the appropriate light will flash and an audible sound may be heard.
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

16. Radio Altimeters
The radio altimeter provides better accuracy then the pressure sensitive altimeters.
Operation
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.
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

17. DME (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.
Operation
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.
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

18. GPS (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.
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

19. Transponders
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
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

20. 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)
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

21. ELT (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 (civil) and (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
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

22. ELT Testing Three switch positions: AUTO, OFF and ON
Testing may be done under the following conditions:
Tune VHF COMM receiver to 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
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

23. 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
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

24. Inspections for NAV/COMM Equipment
System inspections
Antenna inspections
Static discharge inspections
Operational checks or any additional inspections required by the manufacturer
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

25. 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.
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

26. 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.
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

27. 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.
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

28. 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
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

29. 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
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

30. 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.
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

31. 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.
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2

32. Additional Inspections
Transponder
Per FAR 14 Part and
ELT
Per FAR Part 91.52
Functional checks of all other COMM and NAV systems per the manufacturer’s instructions
References: Aircraft Electricity and Electronics pg: , AC B Chapter 12 Section 2