avionics and aircraft systems chapter 2 aircraft communication systems
TRANSCRIPT
Avionics and Aircraft
Systems
Chapter 2 Aircraft Communication Systems
Radios used on present day aircraft are various and use different frequencies depending on the range they have to transmit to.
BAND Frequency
Wavelength
Propagation Via
Range
MF MediumFrequency
300-3,000kHz
1,000 -100m
Line of sightup to 100
miles
HF High Frequency
3-30MHz 100-10m Ionospheric Refraction
World wide depending on propagation
VHF Very High Frequency
30-300MHz
10-1m Rarely Ionospheric Refraction
Line of sight 200-250
miles
UHF Ultra High Frequency
300-3,000MH
z
100-10cm Line of sight 100-150
miles
Communication Systems
Allocation Of Frequencies
UHF – Mainly military use for voice communication.
VHF – Used by civilian and military for voice communicationand Data Link. Also used by navigation aids (VOR,TACAN).
HF – Used by civilian and military for voice communicationand Data Link.
MF – Used by civilian and military for navigation aids (ADF) and for civilian radio stations.
The range of these signals can vary due to the output power of the transmitters.
Communication Systems
Therefore by positioning a satellite in space over a specific areait can be used to extend the range of the VHF and UHF signals.
Communication Systems
VHF and UHF radio signals are line of sight and are not affected by the Ionosphere and pass directly through it.
VHFandUHF
There are two types of satellite relay:
Non-productive – Simple relay of original message.
Productive – Where the message is boosted.
HF range can be badly affected by weather and ionosphere effects.
On a good day you will be able to talk to the other side of the world, but on a bad day you may be lucky to reach 100 miles.
Communication Systems
HF uses the Ionosphere to bounce it’s signals off it and so extend the range.
HF
This means it can be badly affected by weather.
This is known as radio wave propagation.
A power supply circuit to transform the input electrical power to the higher voltages needed to produce the required power output.
Communication Systems Transmitters
Components.
An electronic oscillator circuit to generate the radio frequency signal. This usually generates a sine wave of constant amplitude often called the carrier wave, because it serves to "carry" the information through space.
A modulator circuit to add the information to be transmitted to the carrier wave produced by the oscillator. This is done by varying some aspect of the carrier wave.
An RF power amplifier to increase the power of the signal, to increase the range of the radio waves.
Communication Systems Transmitters
An impedance matching (antenna tuner) circuit to match the impedance of the transmitter to the impedance of the antenna (or the transmission line to the antenna), to transfer power efficiently to the antenna.
Components.
Power Supply
OscillatorCircuit
Modulator Amplifier Antenna
Communication Systems Transmitters - How They Work.
This modulation might be as simple as turning the energy on and off, or altering more subtle properties such as amplitude, frequency, phase, or combinations of these properties.
Information is provided to the transmitter in the form of an electronic signal (electrical energy), producing alternating current of a desired frequency of oscillation.The transmitter contains a system to modulate (change) some property of the energy produced to impress a signal on it.
Amplitude Modulation (AM):
This varies the strength of the transmitted carrier wave signal in proportion to the information being sent.
Frequency Modulation(FM):
This varies the frequency of the carrier wave signal.
Communication Systems
This signal may need amplification before the transmitter sends the modulated electrical energy to a tuned resonant antenna.
The transmitter combines the information signal to be carried with the radio frequency signal which generates the radio waves, which is often called the carrier.
Antenna (aerial)This structure converts the rapidly changing alternating current into an electromagnetic wave(energy) that can move through free space.These antenna can be of various shape and size to achieve the required range, direction and power.
There are two main types.
Omnidirectional
Directional
Transmitters - How They Work.
Communication Systems AntennaOmnidirectional or only weakly directional antennas which receive or radiate more or less in all directions.
AntennaCommunication Systems
Directional antennas which are intended to preferentially radiate or receive in a particular direction or directional pattern.
HF AntennaRadioAntenna
SATCOMAntenna
Communication Systems Antenna (aerial)
HF AerialsVHF Upper
Aircraft antenna are designed to give as little drag as possible.
Communication Systems Antenna (aerial)
No 2 UHF
IFF/SSR
No 2 TACAN
UHF-DF
No 2 ADF Loop No.1 HF
MATADOR(IF FITTED)
No.1 TACAN (BEACON)No.2 TACAN
No.1 and No.2
GLIDESCOPE
EGI
No 1 TACAN
No 1 ADF Loop
HF3(POST-SRIM 4085)
No.2 HF
ADF SENSE
No.2 VHF
MARKER
No.1 VHF
WEATHER RADAR
IFF/SSR
No.1 UHF
RADAR ALTIMETER
VHF NAV(VOR/LOC)
Depending on the equipment fitted to the aircraft, it may require many antenna.
Tuned Radio Frequency (TRF) ReceiverCommunication Systems
The electromagnetic wave is intercepted by a tuned receiving antenna. This structure captures some of the energy of the wave and returns it to the form of oscillating electrical currents.
At the receiver, these currents are demodulated, which is conversion to a usable signal form by a detector sub-system.
The receiver is "tuned" to respond preferentially to the desired signals, and reject undesired signals.
Antenna
RFAmplifie
r
RF = Radio Frequency.
DetectorAudioAmplifier
AudioOutput
The principle of operation of the superheterodyne receiver depends on the use of heterodyning or frequency mixing.
Superheterodyne ReceiverCommunication Systems
The signal from the antenna is filtered sufficiently at least to reject the image frequency and amplified.
A local oscillator in the receiver produces a sine wave which mixes with that signal, shifting it to a specific intermediate frequency (IF), usually a lower frequency.
The IF signal is itself filtered and amplified and possibly processed in additional ways.
The demodulator uses the IF signal rather than the original radio frequency to recreate a copy of the original information (such as audio).
Antenna
RFFilter
RFAMP
Mixer
LocalOscillato
r
IFFilte
r
AMDetector
AudioOutput
IFAMP
RF = Radio Frequency. IF = Intermediate Frequency.
Superheterodyne Receiver - AMCommunication Systems
Superheterodyne Receiver - FMCommunication Systems
Antenna
RFFilter
RFAMP
Mixer
LocalOscillato
r
IFFilte
r
IFAMP
PhaseDetect
or
Low PassFilter
Voltage ControlledOscillator Audio
Output
Data LinkCommunication Systems
Is a system where information is collated and put in a digital form and then transmitted in a short burst transmission.
The information can come from various sources.
The aircraft Flight Management Computer.
Weather broadcasts.
Aircraft systems monitoring devices.
Airborne radar pictures.
Messaging service.
There are three types of Data Link System:
Simplex
Half-Duplex
Duplex
Communication in one direction Only.
Communication in both directions but not simultaneously.
Communication in both directions simultaneously.
In civil aviation, a data-link system (known as Controller Pilot Data Link Communications) is used to send information between aircraft and air traffic controllers.
Data LinkCommunication Systems
This is to overcome the problems with short range V/UHF radios and poor HF which can make voice communication impossible.
Such systems are used for aircraft crossing the Atlantic and Pacific oceans.
This system uses the aircraft's flight management computer to send location, speed and altitude information about the aircraft to the ATC.
ATC can then send messages to the aircraft regarding any necessary changes.
Communication Systems
The system can also be used as a company management tool.
Data about the operation of the aircraft can be transmitted automatically to the company ground station to be analysed.
A British Airways aircraft crossing the Atlantic received a message from it’s HQ, telling the crew to shut down No 2 engine.
The ground engineers had analysed the data from the aircraft and worked out that the engine would fail imminently.
Data Link
Data LinkCommunication Systems
In military aviation, a data-link may also carry weapons targeting information.
The Data Link messages can be encrypted to achievesecurity.
The E3D uses it to pass it’s radar picture to command HQand to other aircraft, allowing planning and allocation of targets to be more efficient.
Communication Systems
Emergency Locator Transmitter (ELT)
This is a small radio which is carried in most crew lifejacketsand aircraft dinghies.
It transmits either on 243.0 MHz(Military) and 121.5 MHz(Civil).
It must be positioned upright to allow the antenna to transmitthe correct cone signal.
When an aircraft is searching for survivors it flies a specific pattern to give best chance of finding them.
When it receives the signal from the ELT it continues the pattern until it enters the Cone Of Silence.
This indicates that the aircraft is directly over the survivors.
Communication Systems
Emergency Locator Transmitter (ELT)
If the transmitter is pointed at the search aircraft, the aircraft will be permanently in the cone of silence and never find the survivors.
Once started the transmitter must be left on continuously to ensure that the signal is available to the search aircraft.
The transmitter signal can be received at a distance outside the visual range of the survivors and therefore they may not be able to see the aircraft.
The transmitter can be automatically initiated by:
Inflation of the lifejacket.
An inertial switch on an aircraft.
Water pressure. i.e. sinking.
Communication Systems
Emergency Locator Transmitter (ELT)
Cone OfSilence
Modern ELTs also have a speech facility, but this drains the battery quickly.