Communication

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ommunication—the conveying of information-is essential to avia­tion and spaceflight. It would be almost impossible to communicate in these fields without radio. Pilots and astronauts use radio to communicate with each other and with air traffic, or mission, controllers on the ground. Aircraft and spacecraft also send and receive text and data by radio.

Commercial aircraft carry a variety of communications equipment. Aircraft crews use some equipment to talk to air traffic controllers and other pilots. Other equipment is used for sending and receiving text messages.

How Radio Works

Information sent by radio travels as a stream of invisible energy waves mov­ing at the speed of light, which is

186,0 miles per second (300,000 kilo­meters per second). A radio wave is actually two waves, one electrical and one magnetic, traveling together. A wave of this kind is called an electro­magnetic wave. Other electromagnetic waves include light and X-rays. The only difference between the different waves is their lengths—radio waves are longer than the other types of waves.

The length of a radio wave is referred to as its wavelength. The number of waves passing by every second is called the frequency. Radio frequency is meas­ured in waves per second, also called cycles per second, or hertz.

Radio signals are divided into fre­quency bands. The high frequency (HF) and very high frequency (VHF) bands are used for aircraft communications. A radio signal is transmitted at a particu­lar frequency, or waves per second. To receive the signal, a radio has to be tuned in to the same frequency. Airports and air traffic control centers have their own radio frequencies. During a flight, a pilot has to keep retuning a plane’s radio to match local frequencies.

VHF signals travel in a straight line from transmitter to receiver. When a transmitter and receiver are no longer in line because an airplane has traveled below the horizon, VHF radio contact is lost. Pilots can use VHF radio to talk to air traffic controllers up to only 230 miles (370 kilometers) or so away.

HF radio is used for communicating over longer distances. HF signals can travel beyond the horizon, because they bounce off a layer of Earth’s atmosphere called the ionosphere.

TECH lbTALK

FREQUENCY BANDS IN AVIATION

Band: High frequency (HF).

Frequency: 3-30 megahertz.

Band: Very high frequency (VHF). Frequency: 30-300 megahertz.

(1 megahertz = 1 million hertz)

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О This diagram shows the wavelength and fre­quency measurements for electromagnetic waves. Radio waves are long compared to X-rays, and so their frequency is comparatively low.

A radio link for sending signals up to an aircraft or spacecraft is called the uplink. A radio link for sending signals from an aircraft or spacecraft down to the ground is the downlink. The infor­mation to be sent by radio-whether a pilot’s voice or data from instruments-is added to a radio signal called a carrier wave. The information changes, or mod­ulates, the carrier wave. When the radio signal is received, the carrier wave is filtered out, leaving the voice or data.