Avionics and Aircraft Systems

1. Avionics and Aircraft • Systems Chapter 2 Aircraft Communication Systems

2. Communication Systems Radios used on present day aircraft are various and use different frequencies depending on the range they have to transmit to.

3. Communication Systems Allocation Of Frequencies UHF – Mainly military use for voice communication. VHF – Used by civilian and military for voice communication and Data Link. Also used by navigation aids (VOR, TACAN). HF – Used by civilian and military for voice communication and 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.

4. Communication Systems VHF and UHF radio signals are line of sight and are not affected by the Ionosphere and pass directly through it. VHFand UHF Therefore by positioning a satellite in space over a specific area it can be used to extend the range of the VHF and UHF signals. There are two types of satellite relay: Non-productive – Simple relay of original message. Productive – Where the message is boosted.

5. Communication Systems 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. HF uses the Ionosphere to bounce it’s signals off it and so extend the range. This is known as radio wave propagation. HF This means it can be badly affected by weather.

6. Communication Systems Transmitters Components. • A power supply circuit to transform the input electrical power to the higher voltages needed to produce the required power output. 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.

7. Communication Systems Transmitters Components. 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. Power Supply Oscillator Circuit Modulator Amplifier Antenna

8. Communication Systems Transmitters - How They Work. 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. 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. 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.

9. Communication Systems Transmitters - How They Work. 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. This signal may need amplification before the transmitter sends the modulated electrical energy to a tuned resonant  antenna. 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

10. Communication Systems Antenna Omnidirectional or only weakly directional antennas which receive or radiate more or less in all directions.

11. Communication Systems Antenna • Directional antennas which are intended to preferentially • radiate or receive in a particular direction or directional pattern. Radio Antenna HF Antenna SATCOMAntenna

12. Communication Systems Antenna (aerial) Aircraft antenna are designed to give as little drag as possible. VHF Upper HF Aerials

13. Communication Systems Antenna (aerial) No 2 ADF Loop UHF-DF No.1 HF IFF/SSR No 1 TACAN No 2 UHF No 1 ADF Loop No.2 HF No 2 TACAN HF3 (POST-SRIM 4085) EGI WEATHER RADAR No.1 VHF IFF/SSR No.2 VHF VHF NAV (VOR/LOC) MARKER MATADOR (IF FITTED) No.1 UHF ADF SENSE RADAR ALTIMETER No.1 and No.2 GLIDESCOPE No.1 TACAN (BEACON) No.2 TACAN Depending on the equipment fitted to the aircraft, it may require many antenna.

14. Communication Systems Tuned Radio Frequency (TRF) Receiver The electromagnetic wave is intercepted by a tuned receiving  antenna. 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. Detector RFAmplifier AudioAmplifier The receiver is "tuned" to respond preferentially to the desired signals, and reject undesired signals. Audio Output RF = Radio Frequency.

15. Communication Systems Superheterodyne Receiver The principle of operation of the superheterodyne receiver depends on the use of heterodyning or frequency mixing. The signal from the antenna is filtered sufficiently at least to reject the image frequencyand amplified. A local oscillator in the receiver produces a sine wave which  mixeswith 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).

16. Communication Systems Superheterodyne Receiver - AM Antenna AMDetector IFFilter Mixer RFFilter RFAMP IFAMP Local Oscillator RF = Radio Frequency. IF = Intermediate Frequency. Audio Output

17. Communication Systems Superheterodyne Receiver - FM Local Oscillator Antenna Low Pass Filter Phase Detector Mixer IFFilter RFFilter RFAMP IFAMP Voltage Controlled Oscillator Audio Output

18. Communication Systems Data Link 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: Communication in one direction Only. Simplex Half-Duplex Communication in both directions but not simultaneously. Duplex Communication in both directions simultaneously.

19. Communication Systems Data Link 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. 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.

20. Communication Systems Data Link 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.

21. Communication Systems Data Link In military aviation, a data-link may also carry weapons targeting information. The E3D uses it to pass it’s radar picture to command HQ and to other aircraft, allowing planning and allocation of targets to be more efficient. The Data Link messages can be encrypted to achieve security.

22. Communication Systems Emergency Locator Transmitter (ELT) This is a small radio which is carried in most crew lifejackets and 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 transmit the 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.

23. 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.

24. Communication Systems Emergency Locator Transmitter (ELT) Cone Of Silence Modern ELTs also have a speech facility, but this drains the battery quickly.