Fly High Aircraft Radio Receivers
Radio is a critical component of aviation, and all aircraft are equipped with at least one form of radio receiver and transmitter. Without wires to connect the on-board systems, or indeed the pilot, to anything on the ground or sharing the airspace, it is the only way of finding information about the outside world. Aircraft radio receivers are not only used for communications, though. They are also used for navigation.
Early aircraft radio receivers used low frequency bands of the radio spectrum. These have since mostly been replaced by higher-frequency systems that provide better quality, higher data bandwidth, better signal to noise ratio and less interference. Most low frequency beacons have been decommissioned, but some remain as a backup in case of a failure in the primary radio system. The low frequency aviation band lies between 200 kilohertz and 415 kilohertz. The range is not entirely used for aviation radio, though. There are some gaps left for other services. There is also a medium frequency radio system. This range is between 2850 kilohertz and to 3000 kilohertz. This is an amplitude modulation system, which refers to the method of encoding sound into the waves of electromagnetic radiation that a radio transmitter can produce. Sound is a wave and EM radiation is wave, so the first thought might be to simply use the sound source as the input to the transmitter. Unfortunately, the frequencies used by transmitters are in the MHz range (millions of cycles per second) while audible sound only reaches up to the KHz range (thousands of cycles per second). With this in mind, it's a simple step to decide that the sound can be encoded as changes in the amplitude of the EM gradation. Now we have a signal (the audio) and a carrier (the EM wave). So, we say that the carrier is modulated in amplitude by the signal - this is AM radio. AM is not a perfect method of modulating the carrier, however, especially at this end of the spectrum. Higher up the spectrum we have high frequency radio. In the past, this was used extensively for domestic aviation communications. With the advent of very high frequency radio, this has become almost obsolete, although it is still used in special instances. For example, international flights still use this band for voice communications because it works well over long distances. The most used and most modern system is very high frequency radio for communications and navigation. This band lies between 108 megahertz and 136 megahertz. Within this range are specially identified regions for things such as navigation, communications, distress signals, civil air patrol, aviation ground patrol, restricted channels, civil air patrol training beacons, flight schools, flight service stations, the flight advisory service, balloons, UNICOM controlled and uncontrolled airports, airplane to airplane for high altitude airliners, air shows, air to air helicopter communications, search and rescue, flight test, U.S. Army Golden Knights. There are also many specially reserved military ranges, including airlift command, military airports and federal aviation administration. |
