Airband

A typical aircraft VHF radio. The display shows an active frequency of 123.5MHz and a standby frequency of 121.5 MHz. The two are exchanged using the button marked with a double-headed arrow. The tuning control on the right only affects the standby frequency.

Airband or Aircraft band is the name for a group of frequencies in the VHF radio spectrum allocated to radio communication in civil aviation, sometimes also referred to as VHF, or phonetically as "Victor". Different sections of the band are used for radionavigational aids and air traffic control.^[1][2][3]

In most countries a license to operate airband equipment is required and the operator is tested on competency in procedures, language and the use of the phonetic alphabet.^[2][4]

Spectrum usage

Antenna array at Amsterdam Airport Schiphol

The VHF airband uses the frequencies between 108 and 137 MHz. The lowest 10 MHz of the band, from 108–117.95 MHz, is split into 200 narrow-band channels of 50 kHz. These are reserved for navigational aids such as VOR beacons, and precision approach systems such as ILS localizers.^[2][3]

As of 2012, most countries divide the upper 19 MHz into 760 channels for amplitude modulation voice transmissions, on frequencies from 118–136.975 MHz, in steps of 25 kHz. In Europe, it is becoming common to further divide those channels into three (8.33 kHz channel spacing), potentially permitting 2,280 channels. Some channels between 123.100 and 135.950 are available in the US to other users such as government agencies, commercial company advisory, search and rescue, military aircraft, glider and ballooning air-to-ground, flight test and national aviation authority use. A typical transmission range of an aircraft flying at cruise altitude (35,000 ft (10,668 m)), is about 200 mi (322 km) in good weather conditions.^[2][3][5][6]

Other bands

Aeronautical voice communication is also conducted in other frequency bands, including satellite voice on Inmarsat or Iridium,^[7] and high frequency voice. Usually these other frequency bands are only used in oceanic and remote areas, though they work over wider areas or even globally. Military aircraft also use a dedicated UHF-AM band from 225.0–399.95 MHz for air-to-air and air-to-ground, including air traffic control communication. This band has a designated emergency and guard channel of 243.0 MHz.^[2][8]

Some types of navaids, such as non-directional beacons and Distance Measuring Equipment, do not operate on these frequencies; in the case of NDBs, the low frequency and medium frequency bands are used between 190–415 kHz and 510–535 kHz. The ILS glide path operates in the UHF frequency range of 329.3–335.0 MHz, and DME also uses UHF from 962–1150 MHz.^[2]

Channel spacing

Channel spacing for voice communication on the airband was originally 200 kHz until 1947,^[9] providing 70 channels from 118 to 132 MHz. Some radios of that time provided receive-only coverage below 118 MHz for a total of 90 channels. From 1947–1958 the spacing became 100 kHz; from 1954 split once again to 50 kHz and the upper limit extended to 135.95 MHz (360 channels), and then to 25 kHz in 1972 to provide 720 usable channels. On 1 January 1990 the frequencies between 136.000 and 136.975 MHz were added, resulting in 760 channels.^[5]

Increasing air traffic congestion has led to further subdivision into narrow-band 8.33 kHz channels in the ICAO European region; all aircraft flying are required to have communication equipment for this channel spacing.^{[2][10][11][12]} Outside of Europe, 8.33 kHz channels are permitted in many countries but not widely used as of 2012.

The emergency communication channel 121.5 MHz is the only channel that retains 100 kHz channel spacing in the US; there are no channel allocations between 121.4 and 121.5 or between 121.5 and 121.6^[13]

Modulation

Aircraft communications radio operations worldwide use amplitude modulation, predominantly A3E double sideband with full carrier on VHF and UHF, and J3E single sideband with suppressed carrier on HF. Besides being simple, power-efficient and compatible with legacy equipment, AM and SSB permit stronger stations to override weaker or interfering stations. Additionally, this method does not suffer from the capture effect found in FM. Even if a pilot is transmitting, a control tower can "talk over" that transmission and other aircraft will hear a somewhat garbled mixture of both transmissions, rather than just one or the other. Even if both transmissions are received with identical signal strength, a heterodyne will be heard where no such indication of blockage would be evident in an FM system.^[14]

Alternative analog modulation schemes are under discussion, such as the "CLIMAX"^[15] multi-carrier system and offset carrier techniques to permit more efficient utilization of spectrum.

Audio properties

The audio quality in the airband is limited by the RF bandwidth used. In the newer channel spacing scheme, the largest bandwidth of an airband channel might be limited to 8.33 kHz, so the highest possible audio frequency is 4.165 kHz.^[16] In the 25 kHz channel spacing scheme, an upper audio frequency of 12.5 kHz would be theoretically possible.^[16] However, most airband voice transmissions never actually reach these limits. Usually, the whole transmission is contained within a 6 kHz to 8 kHz bandwidth, corresponding to an upper audio frequency of 3 kHz to 4 kHz.^[16] This frequency, while low compared to the top of the human hearing range, is sufficient to convey speech. Different aircraft, control towers and other users transmit with different bandwidths and audio characteristics.

Digital radio

A switch to digital radios has been contemplated, as this would greatly increase capacity by reducing the bandwidth required to transmit speech. Other benefits from digital coding of voice transmissions include decreased susceptibility to electrical interference and jamming. The change-over to digital radio has yet to happen, partly because the mobility of aircraft necessitates complete international cooperation to move to a new system and also the time implementation for subsequent changeover.^[17][18] Another factor delaying the move to any digital mode is the need to retain the ability for one station to override another in an emergency.

Unauthorised use

It is illegal in most countries to transmit on the Airband frequencies without a suitable license, although an individual license may not be required, for instance in the US where aircraft stations are "licensed by rule.".^[19] Many countries' regulations also restrict communications in the airband. For instance, in Canada, airband communications are limited to those required for "the safety and navigation of an aircraft; the general operation of the aircraft; and the exchange of messages on behalf of the public. In addition, a person may operate radio apparatus only to transmit a non-superfluous signal or a signal containing non-profane or non-obscene radiocommunications."^[2]

Listening to airband frequencies without a license is also an offence in some countries, including the UK,^[20] though enforcement may vary. Such activity has been the subject of international situations between governments when tourists bring airband equipment into countries which ban the possession and use of such equipment.^[21][22]

See also

• Aircraft Communications Addressing and Reporting System
• Air traffic flow management
• Air traffic control
• Avionics
• Control tower
• Future Air Navigation System
• Radio horizon

References

1. H. P. Westman (ed), Reference Data for Radio Engineers Fifth Edition, Howard W. Sams and Co, 1968, page 1-6
2. Transport Canada (April 2014). "Com - 5.0 radio communications" (PDF). Retrieved 27 April 2013.
3. "Aviation Radio Bands and Frequencies". Smeter network 2011. Retrieved 16 February 2011.
4. "Radio Telephony Training Syllabus" (PDF). Cotswold Gliding Club - date undisclosed. Retrieved 16 February 2011.
5. "Requirements for 760 channel VHF radio for Aeronautical operations" (PDF). Federal aviation agency 1992. Retrieved 2011-02-14.
6. "VII. ELECTRONIC AIDS TO INSTRUMENT FLYING". FAA test company - date undisclosed. Retrieved 2011-02-17.
7. "Iridium Satellite Voice (SATVOICE) with Safety Services" (PDF). Retrieved 2016-09-18.
8. DAOT 5: C-12-118-000/MB-000 Operating Instructions CH118 Helicopter (unclassified), Change 2, 23 April 1987, Page 1-51. Department of National Defence
9. "8.33 kHz Channel spacing – what is this?". Roger-Wilco. 2010-04-03. Retrieved 2012-05-10.
10. Mise en oeuvre de l’espacement « à 8.33 kHz » au-dessous du FL 195
11. "Aircraft frequencies for UK and Europe". Garfnet organisation 2009. Retrieved 14 February 2011.
12. "8.33kHz Programme". Eurocontrol. Retrieved 2007-12-24.
13. 47 C.F.R. 87.173 as of 2012
14. EECE 252 Project Report, Amplitude Modulated Radio Applications in Aviation 17 April 2012
15. EuroControl, CLIMAX/8.33: To extend 8.33 kHz benefits, ICAO, October 2007
16. Poole, Ian. "Amplitude Modulation, AM Spectrum & Bandwidth". Radio-Electronics.com. Retrieved 26 September 2015.
17. "Affordable real-time digital voice transmission using Voip technology". Command NAVAIR 2 January 2010. Retrieved 16 February 2011.
18. "Aircraft centric digital CNS" (PDF). CITA 2010. Retrieved 14 February 2011.
19. 47 C.F.R. 87.18 as of 2012
20. Guidance on Receive-Only Radio Scanners Ofcom
21. "Greek drama in capital". Algarve resident 2011. Retrieved 16 February 2011.
22. "Plane-spotters 'ignored warnings'." BBC News, 25 April 2002. Retrieved: 14 March 2007. Quote: "Note-taking in conjunction with other activities may be detrimental (to Greek security)."