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Abstract
VHF systems are the most reliable alternative for avionic communication systems. This is generally explained by the high reliability and selectivity of VHF radio transmission. The principles and rules of VHF transmission are helpful for creating safe navigation, operation, control and other systems.
VHF Communication
The purposes of VHF communication in aviation are various. This band is used in commercial and general aviation, and the main aim is the radio-navigational aid, traffic control and similar purposes. The frequency range of VHF is 30 to 300 MHz, however, the airband is 108 137 MHz. Military aircrafts operate on 225 400 MHz frequencies. (Andersen, Rappaport, et.al., 1995) The wave propagation may be impeded by various physical obstacles and objects that create reflections, as the signal may come to the receiving point by multiple ways and with numerous interferences. The main factor of the successful radio transmission is the observation of sight-line transmission rule. Consequently, antennas of the land service network can not be surrounded by buildings, mountains, or located in low places. (Dexter-Smith, 1998)
VHF station spacing in Australia is defined as 25 kHz. The Spacing that is accepted in European air traffic control network is 8.33 kHz. This is explained by the fact that highly loaded air traffic control system of Europe requires more channels for communication in comparison with Australian one. (Freeman, 2002) The limitations, that may be place on the operator if the communication system does not comply with the required spacing is obvious the operator will not be able to operate within the air space of the European continent until the communication system of the required standards will be used on the planes. (CCIR Report, 2001)
The VHF communication systems on numerous aircraft types are powered by either 28Vdc Battery Bus or the 28Vdc Essential Bus and never by the main power bus because of several reasons. Navigation and communication are the key factors required for flight, hence, communication systems should be independent from the main power source for the pilot and avionic systems could communicate with the air traffic control systems before the engine startup and in the case when main electric supply system is down. (Herrick, 2006)
One of the reasons why antenna is put on the top of the fuselage is that it is an effective lightning rod. If a lightning strikes a plane, it hits the antenna but not a random point on the fuselage. From the perspective of radio transmission, no 1 antenna (on the top) is used when the plane is on the ground, and no 2 antenna (on the bottom) is used when the plane is in the air. (Erwin, 2007)
3 VHF COM is used for using the Inmarsat SatCom interface, Iridium Interface datalink communication, as well as additional Amplitude Modulated short range voice communication of air-to-air and air-to-ground systems. Some aircrafts use this system for communicating the flight and technical data to maintenance and engineering teams, and do not require pilot interference, as they work independently on the aero-navigation communication.
VHF Problem Case Analysis
In spite of the fact MEL points out that either VHF system may be inoperative, this does not mean that the problem does not need to be fixed. The maintenance system is capable to remove any problem if it is observed independently on the MEL requirements. Surely, it is not a reason to delay a flight, however, the maintenance team will inevitably perform the required actions for fixing the appeared problem.
In accordance with the requirements of CAR 35/36 certification, the offered way of problem fixing may be authorized, as this certificate offers the required modification of the construction areas of avionics, propulsion, performance, structures, propellers and rotors, systems and equipment. Additionally, the certified specialists are free to make all the required replacements if the plane is operating on the territory of Australia. These changes may be performed if this is the only solution of the problem, however, the changes of cable location will not influence the performance of other systems within the plane, consequently, this action may be performed freely in accordance with the CAR 35 and CAR 36 certification.
Reference List
Andersen, J.B., Rappaport, T.S. and Yoshida, S. 1995. Propagation Measurements and Models for Wireless Communications Channels, IEEE Communications Magazine, pp. 42-49.
CCIR Report 567-4. 2001 Propagation data and prediction methods for the terrestrial land mobile service using the frequency range 30 MHz to 3 GHz. International Telecommunication Union, Geneva.
Dexter-Smith, J. 1998. High Frequency Communication. African Business 37.
Erwin, S. I. 2007. High Frequency. National Defense, 92, 43.
Freeman, R.L. 2002. Radio System Design for Telecommunications. Wiley & Sons publishing, New York.
Herrick, H. 2006. Radio Communications in the Digital Age: VHF/UHF Technology. Harris Corporation, RF Communications Division. Vol. 2.
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