Communication System involves voice transmission and reception between aircraft to aircraft or aircraft to ground. For obvious reasons pilots cannot use sound as a medium during the flight to interact with other pilots, so we need a specialised system which can play an efficient role of transmitting important data (voice, signals) and simultaneously maintaining a good quality transmission. It can be said that safe aircraft operation is dependent to a large extent upon satisfactory performance of communication system. Radios are employed for fulfilling the purpose.
History – The first world war brought about an urgent need for communications. Voice communications from ground-to-air and from aircraft to aircraft were established. In the 1930s, the first all radio-controlled blind-landing was accomplished and by the end of the decade, use of high frequency radio waves was explored and included the advent of high-frequency radar. World War II injected urgency into the development of aircraft radio communication and navigation. Communication radios, despite their size, were essential on board aircraft. Very high frequencies were developed for communication and navigational purposes.
Frequency Band – Radio frequencies are being used in various field, like: FM radio broadcast, television broadcast, land mobile stations (emergency, business, and military), amateur radio, marine communications, aircraft communications, air traffic control, air navigation systems (e.g. VOR/DME, NDB)
- The use of the various radio frequencies all over the world is allocated by ITU (International Telecommunications Union).
- The radio frequency portion of the electromagnetic spectrum extends from 30 khz (kilohertz) to 30,000 mhz (megahertz).
- For the sake of convenience, this spectrum is divided into frequency bands.
Transmitters – Transmitters may be considered as a generator which changes electrical power into radio waves. It basically performs these functions:
- Generate RF (radio frequency) signal
- Amplify the RF signal
- Provide a means of placing intelligence on the signal
Receivers – Just opposite to transmitters, receivers:
- Select the desired radio frequency signals
- Amplifies the generated small AC voltage
- Convert the intelligence contained on these signals into a usable form with the help of demodulator. Demodulator which is used for AM signals is called detector, while the one which is used for FM waves is called discriminator.
- An antenna is a special type of electrical circuit designed to radiate and receive electromagnetic energy.
- As mentioned earlier, a transmitting antenna is a conductor which radiates EM waves when a radio frequency is passed through it, while receiving antenna intercepts the EM waves that are present in the air.
- They come in various size and shapes depending upon the frequency to be transmitted.
- Generally for aircraft application, same antenna is used for both transmission and reception of signals.
Antenna on aircraft
- Type and size of antenna varies with the different types of aircraft.
- The location of antenna depends on the design of aircraft.
VHF and HF
- VHF and HF communication systems use transceivers
- A transceiver is a self-contained transmitter and receiver
- The transmitter and receiver both operate at same frequency, and the microphone button determines when there is an output from the transmitter.
- In the absence of transmission the receiver is sensitive to incoming signals.
- This combination of transmitters and receivers saves a lot of space and weight and are hence largely used.
- VHF is a term used to describe the 30mhz to 300mhz portion of the radio spectrum.
- This range of frequencies will provide short-range LOS (line of site) communications.
- The range for VHF communications depends on equipment used, antenna height, and terrain (typically 2 to 20 miles).
- The frequency range is 3-30 mhz.
- This is used for long range communications because of its longer transmission range.
- This is the basic band for long-range communications mainly because its transmissions are reflected from the ionosphere.
- HF transmitters have higher power outputs than VHF transmitters.
- A tuner is used to match the impedance of the transceiver to the antenna.
The advantage of using HF communication system over VHF communication system is that it can transmit information over long distance as the output power of the HF transmitter is higher than that of the VHF transmitter.
The Communication System enables the crew to exchange voice communications and/or data with stations through VHF, HF radios, Satcom
Interphone – Function: it provides two way communication between the flight crew, cabin crew and ground mechanics. It is also used by the maintenance personnel to access the communication system and monitor the communication and the navigation receivers.
Flight crew interfaces are used by the flight crew for audio interface with the AMU.
- Headset (Boomset): It is used for both talking and listening. To talk you must press PTT (press to talk) on either the side stick or the ACP to turn the mic on.
- Hand mic: It is used for speaking only. It has a PTT switch.
- Oxygen mask: It is used in emergency cases like decompression or smoke detection case. When using it, PTT on ACP or the control column must be pressed.
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