Aircraft communications

Aircraft Communications By Jim Sparks October 1998 It has been about forty years since the Federal Aviation Administration required all aircraft operating in high traffic areas to be equipped with two-way communication radios. In this time...


Aircraft Communications

By Jim Sparks

October 1998

It has been about forty years since the Federal Aviation Administration required all aircraft operating in high traffic areas to be equipped with two-way communication radios. In this time period, technology has advanced radically in the area of solid state electronics. In fact, what is "new technology" today may be "obsolete" tomorrow. The type of aircraft communications that most frequently comes to mind is that which takes place between Pilot and Controller, and uses a "Very High Frequency" (VHF). Several other means are available and they include "High Frequency" (HF), "Satellite Communications" (SAT COM) — even some telephone systems can be used in flight.

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Audio Panel

VHF communication systems are the most widely used for maintaining contact between ground and aircraft. This employs "Line Of Sight" transmission, which translates to a range of about thirty miles for an aircraft operating at 1,000 feet above the ground, or about 135 miles with an aircraft operating at 10,000 feet.

Adjustments to the frequency range have been made over the years as the capacity of ground-based radio stations tends to double every sixteen years. Initially, the "Radio Frequency" (RF) range was from 118 to 132 MHz. Channel spacing was set at 200 kHz intervals. In 1958, the first expansion occurred and was accomplished by reducing the channel spacing to 100 kHz. This change doubled the number of available channels. The following year, the upper end of the usable frequency range was extended from 132 MHz to 136 MHz. In 1964, the next change in channel spacing was made and decreased the 100 kHz to 50 kHz, and was again repeated in 1974 when spacing went to 25 kHz. Extension of the frequency range occurred again in 1979 when it went up to 137 MHz. Military communications can use the frequencies from 137 to 151.975 with 25 kHz spacing. Several areas in Europe have already established a need for additional channels.

By reducing the channel spacing to 8.33 kHz, the ability to triple the number of usable frequencies is realized. This change will most probably not be implemented in the United States; however, "N" registered aircraft wishing to operate in countries where 8.33 is in effect will need to comply or obtain a waiver. The equipment required for this voice link includes a transceiver or transmitter/receiver, antenna, microphone, audio panel, and speaker. In many cases, a headset can be used in place of a hand microphone and loud speaker.

The transceiver is where most of the action is. This device has eight separate functions to perform.

First of all, as a transmitter, a "carrier wave" has to be generated at a specific frequency and wavelength. Secondly, this carrier wave requires amplification so it has the ability to travel through the rest of the transmitter. Once the carrier is established, the next step is to couple the information to be sent on the wave. This step is called modulation. After the intelligence is added, the signal goes to the final power amplifier where the signal strength is determined and the modulated wave then leaves the transmitter.

The "receiver" section of the transceiver works almost in reverse of the transmitter. Once the signal has been received, it is directed into the tuner. It is here that only the selected frequency is recognized and passed further on. Signal strength at this point is quite weak and requires some amplification before being directed to the demodulator. In this area, the intelligence that was added to the carrier wave in the transmitter is removed. This intelligence is in a rather weak state and requires amplification before it can be used by any of the aircraft audio equipment such as headphones or speakers.

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