Automatic link establishment

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Automatic Link Establishment, commonly known as ALE, is the worldwide de facto standard for digitally initiating and sustaining HF radio communications.[1] ALE is a feature in an HF communications radio transceiver system that enables the radio station to make contact, or initiate a circuit, between itself and another HF radio station or network of stations. The purpose is to provide a reliable rapid method of calling and connecting during constantly changing HF ionospheric propagation, reception interference, and shared spectrum use of busy or congested HF channels.

Mechanism

ATF Dingo of German Bundeswehr equipped with ALE capable HF-transceiver HRM-7000 in Afghanistan 2011

A standalone ALE radio combines an HF SSB radio transceiver with an internal microprocessor and MFSK modem. It is programmed with a unique ALE address, similar to a phone number (or on newer generations, a username). When not actively in contact with another station, the HF SSB transceiver constantly scans through a list of HF frequencies called channels, listening for any ALE signals transmitted by other radio stations. It decodes calls and soundings sent by other stations and uses the bit error rate to store a quality score for that frequency and sender-address.

To reach a specific station, the caller enters the ALE Address. On many ALE radios this is similar to dialing a phone number. The ALE controller selects the best available idle channel for that destination address. After confirming the channel is indeed idle, it then sends a brief selective calling signal identifying the intended recipient. When the distant scanning station detects ALE activity, it stops scanning and stays on that channel until it can confirm whether or not the call is for it. The two stations' ALE controllers automatically handshake to confirm that a link of sufficient quality has been established, then notify the operators that the link is up. If the callee fails to respond or the handshaking fails, the originating ALE node usually selects another frequency either at random or by making a guess of varying sophistication.

Upon successful linking, the receiving station generally emits an audible alarm and shows a visual alert to the operator, thus indicating the incoming call. It also indicates the callsign or other identifying information of the linked station, similar to Caller ID. The operator then un-mutes the radio and answers the call then can talk in a regular conversation or negotiates a data link using voice or the ALE built-in short text message format. Alternatively, digital data can be exchanged via a built-in or external modem (such as a STANAG 5066 or MIL-STD-188-110B serial tone modem) depending on needs and availability. The ALE built-in text messaging facility can be used to transfer short text messages as an "orderwire" to allow operators to coordinate external equipment such as phone patches or non-embedded digital links, or for short tactical messages.[2][3]

Common applications

An ALE radio system enables connection for voice conversation, alerting, data exchange, texting, instant messaging, email, file transfer, image, geo-position tracking, or telemetry. With a radio operator initiating a call, the process normally takes a few minutes for the ALE to pick an HF frequency that is optimum for both sides of the communication link. It signals the operators audibly and visually on both ends, so they can begin communicating with each other immediately. In this respect, the longstanding need in HF radio for repetitive calling on pre-determined time schedules or tedious monitoring static is eliminated. It is useful as a tool for finding optimum channels to communicate between stations in real-time. In modern HF communications, ALE has largely replaced HF prediction charts, propagation beacons, chirp sounders, propagation prediction software, and traditional radio operator educated guesswork. ALE is most commonly used for hooking up operators for voice contacts on SSB (single-sideband modulation), HF internet connectivity for email, SMS phone texting or text messaging, real-time chat via HF text, Geo Position Reporting, and file transfer. High Frequency Internet Protocol or HFIP may be used with ALE for internet access via HF.

Techniques

The essence of ALE techniques is the use of automatic channel selection, scanning receivers, selective calling, handshaking, and robust burst modems.[4] An ALE node decodes all received ALE signals heard on the channel(s) it monitors. It uses the fact that all ALE messages use forward error correction (FEC) redundancy. By noting how much error-correction occurred in each received and decoded message, an ALE node can detect the "quality" of the path between the sending station and itself. This information is coupled with the ALE address of the sending node and the channel the message was received on, and stored in the node's Link Quality Analysis (LQA) memory.[3] When a call is initiated, the LQA lookup table is searched for matches involving the target ALE address and the best historic channel is used to call the target station. This reduces the likelihood that the call has to be repeated on alternate frequencies. Once the target station has heard the call and responded, a bell or other signalling device will notify both operators that a link has been established. At this point, the operators may coordinate further communication via orderwire text messages, voice, or other means. If further digital communication is desired, it may take place via external data modems or via optional modems built into the ALE terminal.

This unusual usage of FEC redundancy is the primary innovation that differentiates ALE from previous selective calling systems which either decoded a call or failed to decode due to noise or interference. A binary outcome of "Good enough" or not gave no way of automatically choosing between two channels, both of which are currently good enough for minimum communications. The redundancy-based scoring inherent in ALE thus allows for selecting the "best" available channel and (in more advanced ALE nodes) using all decoded traffic over some time window to sort channels into a list of decreasing probability-to-contact, significantly reducing co-channel interference to other users as well as dramatically decreasing the time needed to successfully link with the target node.

Techniques used in the ALE standard include automatic signaling, automatic station identification (sounding), polling, message store-and-forward, linking protection and anti-spoofing to prevent hostile denial of service by ending the channel scanning process. Optional ALE functions include polling and the exchange of orderwire commands and messages. The orderwire message, known as AMD (Automatic Message Display), is the most commonly used text transfer method of ALE, and the only universal method that all ALE controllers have in common for displaying text.[5] It is common for vendors to offer extensions to AMD for various non-standard features, although dependency on these extensions undermines interoperability. As in all interoperability scenarios, care should be taken to determine if this is acceptable before using such extensions.

History and precedents

ALE evolved from older HF radio selective calling technology. It combined existing channel-scanning selective calling concepts with microprocessors (enabling FEC decoding and quality scoring decisions), burst transmissions (minimizing co-channel interference), and transponding (allowing unattended operation and incoming-call signalling). Early ALE systems were developed in the late 1970s and early 1980s by several radio manufacturers.[6] The first ALE-family controller units were external rack mounted controllers connected to control military radios, and were rarely interoperable across vendors.

Various methods and proprietary digital signaling protocols were used by different manufacturers in first generation ALE, leading to incompatibility.[3] Later, a cooperative effort among manufacturers and the US government resulted in a second generation of ALE that included the features of first generation systems, while improving performance. The second generation 2G ALE system standard in 1986, MIL-STD-188-141A,[5] was adopted in FED-STD-1045[7] for US federal entities. In the 1980s, military and other entities of the US government began installing early ALE units, using ALE controller products built primarily by US companies. The primary application during the first 10 years of ALE use was government and military radio systems, and the limited customer base combined with the necessity to adhere to MILSPEC standards kept prices extremely high. Over time, demand for ALE capabilities spread and by the late 1990s, most new government HF radios purchased were designed to meet at least the minimum ALE interoperability standard, making them eligible for use with standard ALE node gear. Radios implementing at least minimum ALE node functionality as an option internal to the radio became more common and significantly more affordable. As the standards were adopted by other governments worldwide, more manufacturers produced competitively priced HF radios to meet this demand. The need to interoperate with government organizations prompted many non-government organizations (NGOs) to at least partially adopt ALE standards for communication. As non-military experience spread and prices came down, other civilian entities started using 2G ALE. By the year 2000, there were enough civilian and government organizations worldwide using ALE that it became a de facto HF interoperability standard for situations where a priori channel and address coordination is possible.

In the late 1990s, a third generation 3G ALE with significantly improved capability and performance was included in MIL-STD-188-141B,[5] retaining backward compatibility with 2G ALE, and was adopted in NATO STANAG 4538. Civilian and non-government adoption rates are much lower than 2G ALE due to the extreme cost as compared to surplus or entry-level 2G gear as well as the significantly increased system and planning complexity necessary to realize the benefits inherent in the 3G specification. For many militaries, whose needs for maximized intra-organizational capability and capacity always strain existing systems, the additional cost and complexity of 3G are less problematic.

Reliability

ALE enables rapid unscheduled communication and message passing without requiring complex message centers, multiple radios and antennas, or highly trained operators. With the removal of these potential sources of failure, the tactical communication process becomes much more robust and reliable. The effects extend beyond mere force multiplication of existing communications methods; units such as helicopters, when outfitted with ALE radios, can now reliably communicate in situations where the crew are too busy to operate a traditional non-line of sight radio.[8] This ability to enable tactical communication in conditions where dedicated trained operators and hardware are inappropriate is often considered to be the true improvement offered by ALE.

ALE is a critical path toward increased interoperability between organizations. By enabling a station to participate nearly simultaneously in many different HF networks, ALE allows for convenient cross-organization message passing and monitoring without requiring dedicated separate equipment and operators for each partner organization. This dramatically reduces staffing and equipment considerations, while enabling small mobile or portable stations to participate in multiple networks and subnetworks. The result is increased resilience, decreased fragility, increased ability to communicate information effectively, and the ability to rapidly add to or replace communication points as the situation demands.

When combined with Near Vertical Incidence Skywave (NVIS) techniques and sufficient channels spread across the spectrum, an ALE node can provide greater than 95% success linking on the first call, nearly on par with SATCOM systems. This is significantly more reliable than cellphone infrastructure during disasters or wars yet is mostly immune to such considerations itself.

Standards and protocols

Global standards for ALE are based on the original US MIL-STD 188-141A[5] and FED-1045,[7] known as 2nd Generation (2G) ALE. 2G ALE uses non-synchronised scanning of channels, and it takes several seconds to half a minute to repeatedly scan through an entire list of channels looking for calls. Thus it requires sufficient duration of transmission time for calls to connect or link with another station that is unsynchronised with its calling signal. The vast majority of ALE systems in use in the world at the present time are 2G ALE.

2G technical characteristics

2G ALE Signal

The more common 2G ALE signal waveform is designed to be compatible with standard 3 kHz SSB narrowband voice channel transceivers. The modulation method is 8ary Frequency Shift Keying or 8FSK, also sometimes called Multi Frequency Shift Keying MFSK, with eight orthogonal tones between 750 and 2500 Hz.[5] Each tone is 8 ms long, resulting in a transmitted over-the-air symbol rate of 125 baud or 125 symbols per second, with a raw data rate of 375 bits per second. The ALE data is formatted in 24-bit frames, which consist of a 3-bit preamble followed by three ASCII characters, each seven bits long. The received signal is usually decoded using digital signal processing techniques that are capable of recovering the 8FSK signal at a negative decibel signal-to-noise ratio (i.e., the signal may be recovered even when it is below the noise level). The over-the-air layers of the protocol involve the use of forward error correction, redundancy, and handshaking transponding similar to those used in ARQ techniques.[9]

3G technical characteristics

Newer standards of ALE, called 3rd Generation or 3G ALE, use accurate time synchronization (via a defined time-synch protocol as well as the option of GPS-locked clocks) to achieve faster and more dependable linking. Through synchronization, the calling time to achieve a link may be reduced to less than 10 seconds. The 3G ALE modem signal also provides better robustness and can work in channel conditions that are less favorable than 2G ALE.[10] Dwell groups, limited callsigns, and shorter burst transmissions enable more rapid intervals of scanning. All stations in the same group scan and receive each channel at precisely the same time window. Although 3G ALE is more reliable and has significantly enhanced channel-time efficiency, the existence of a large installed base of 2G ALE radio systems and the wide availability of moderately priced (often military surplus) equipment, has made 2G the baseline standard for global interoperability.

Basis for HF interoperability communications

Interoperability is a critical issue for the disparate entities which use radiocommunications to fulfill the needs of organizations. Largely due to the ubiquity of 2G ALE, it became the primary method for providing interoperability on HF between governmental and non-governmental disaster relief and emergency communications entities, and amateur radio volunteers. With digital techniques increasingly employed in communications equipment, a universal digital calling standard was needed, and ALE filled the gap. Nearly every major HF radio manufacturer in the world builds ALE radios to the 2G standard to meet the high demand that new installations of HF radio systems conform to this standard protocol. Disparate entities that historically used incompatible radio methods were then able to call and converse with each other using the common 2G ALE platform. Some manufacturers and organizations[11] have used the AMD feature of ALE to expand the performance and connectivity.[12] In some cases, this has been successful, and in other cases, the use of proprietary preamble or embedded commands has led to interoperability problems.

Tactical communication and resource management

ALE serves as a convenient method of beyond line of sight communication. Originally developed to support military requirements, ALE is useful to many organizations who find themselves managing widely located units. United States Immigration and Customs Enforcement and United States Coast Guard are two members of the Customs Over the Horizon Enforcement Network (COTHEN), a MIL-STD 188-141A ALE network.[13] All U.S. armed forces operate multiple similar networks. Similarly, shortwave utility listeners have documented frequency and callsign lists for many nations' military and guard units, as well as networks operated by oil exploration and production companies and public utilities in many countries.

Emergency / disaster relief or extraordinary situation response communications

ALE radio communication systems for both HF regional area networks and HF interoperability communications are in service among emergency and disaster relief agencies as well as military and guard forces. Extraordinary response agencies and organizations use ALE to respond to situations in the world where conventional communications may have been temporarily overloaded or damaged. In many cases, it is in place as alternative back-channel for organizations that may have to respond to situations or scenarios involving the loss of conventional communications. Earthquakes, storms, volcanic eruptions, and power or communication infrastructure failures are typical situations in which organizations may deem ALE necessary to operations. ALE networks are common among organizations engaged in extraordinary situation response such as: natural and man-made disasters, transportation, power, or telecommunication network failures, war, peacekeeping, or stability operations. Organizations known to use ALE for Emergency management, disaster relief, ordinary communication or extraordinary situation response include: Red Cross, FEMA, Disaster Medical Assistance Teams, NATO, Federal Bureau of Investigation, United Nations , AT&T, Civil Air Patrol, SHARES, State of California Emergency Management Agency (CalEMA), other US States' Offices of Emergency Services or Emergency Management Agencies, and Amateur Radio Emergency Service (ARES).[11]

International HF telecommunications for disaster relief

The International Telecommunication Union (ITU), in response to the need for interoperation in international disaster response spurred largely by humanitarian relief, included ALE in its Telecommunications for Disaster Relief recommendations.[4] The increasing need for instant connectivity for logistical and tactical disaster relief response communications, such as the 2004 Indian Ocean earthquake tsunami led to ITU actions of encouragement to countries around the world toward loosening restrictions on such communications and equipment border transit during catastrophic disasters. The IARU Global Amateur Radio Emergency Communications Conferences (GAREC) and IARU Global Simulated Emergency Tests have included ALE.[14]

Use in amateur radio

Amateur radio operators began sporadic ALE operation on a limited basis in the early to mid-1990s,[3] with commercial ALE radios and ALE controllers. In 2000, the first widely available software ALE controller for the Personal Computer, PCALE, became available, and hams started to set up stations based on it. In 2001, the first organized and coordinated global ALE nets for International Amateur Radio began. In August 2005, ham radio operators supporting communications for emergency Red Cross shelters used ALE for Disaster Relief operations during the Hurricane Katrina disaster.Cite error: Closing </ref> missing for <ref> tag

International channels

This listing is current as of February 2020. See HFLINK for more information about Amateur Radio Service ALE Automatic Link Establishment.[14]

Frequency kHz Mode ALE or Selcall Channel Number Channel Label North America Net Europe Net UK Net Japan Net Australia- NZ Net ITU Region 1 Net ITU Region 2 Net ITU Region 3 Net Preamble Time (seconds)
00473.0 USB SEL 00A 00ASEL HFS HFS HFS HFS HFS HFS HFS HFS 15.0
00475.5 USB ALE 00B 00BALE HFL HFL HFL HFL HFL HFL HFL HFL 15.0
01838.0 USB SEL 01A 01ASEL HFR HFR HFS HFS HFR HFR HFR 15.0
01843.0 USB ALE 01B 01BALE HFN HFL HFL HFL HFL HFL HFL 15.0
01908.0 USB SEL 01C 01CSEL HFS HFS 15.0
01909.0 USB ALE 01D 01DALE HFL HFL 15.0
01990.0 USB SEL 01E 01ESEL HFS HFS HFS HFS 15.0
01996.0 USB ALE 01F 01FALE HFL HFN HFL 15.0
03527.0 USB ALE 03A 03AALE HFN 15.0
03529.0 USB SEL 03B 03BSEL HFR 15.0
03590.0 USB SEL 03C 03CSEL HFR HFR HFR HFR HFR HFR HFR 15.0
03596.0 USB ALE 03D 03DALE HFN HFN HFN HFN HFN HFN HFN 15.0
03600.5 USB ALE 03E 03EALE HFL HFL HFL 15.0
03605.0 USB SEL 03F 03FSEL HFS HFS HFS 15.0
03710.0 USB SEL 03G 03GSEL HFX HFX HFX 15.0
03791.0 USB ALE 03H 03HALE HFL HFL HFL HFL 15.0
03795.0 USB SEL 03I 03ISEL HFS HFS HFS HFS HFS 15.0
03845.0 USB SEL 03J 03JSEL HFS 15.0
03995.0 USB SEL 03K 03KSEL HFS 15.0
03996.0 USB ALE 03L 03LALE HFL 15.0
05102.0 USB SEL 05A 05ASEL HFX 15.0
05346.5 USB SEL 05B 05BSEL HFR 15.0
05354.5 USB ALE 05C 05CALE HFL HFL HFL HFL HFL 15.0
05355.0 USB SEL 05D 05DSEL HFR HFR HFR HFR HFR 15.0
05357.0 USB ALE 05E 05EALE HFL HFL HFL HFL HFL 15.0
05363.0 USB SEL 05F 05FSEL HFS HFS HFS HFS 15.0
05371.5 USB ALE 05G 05GALE HFL HFL 15.0
05403.5 USB SEL 05H 05HSEL HFS HFS 15.0
07044.0 USB SEL 07A 07ASEL HFR HFR HFR HFR HFR HFR HFR 15.0
07049.5 USB ALE 07B 07BALE HFL HFL HFL HFL HFL 15.0
07100.0 USB SEL 07C 07CSEL HFR 15.0
07102.0 USB ALE 07D 07DALE HFN HFN HFN HFN HFN HFN HFN HFN 15.0
07185.0 USB ALE 07E 07EALE HFL HFL HFL HFL HFL HFL HFL 15.0
07195.0 USB SEL 07F 07FSEL HFS HFS HFS HFS HFS HFS HFS HFS 15.0
07291.0 USB SEL 07G 07GSEL HFS 15.0
07296.0 USB ALE 07H 07HALE HFL 15.0
10126.0 USB SEL 10A 10ASEL HFS HFS HFS 15.0
10131.0 USB ALE 10B 10BALE HFL HFL HFL 15.0
10144.0 USB SEL 10C 10CSEL HFR HFR HFR HFR HFR HFR HFR HFR 15.0
10145.5 USB ALE 10D 10DALE HFN HFN HFN HFN HFN HFN HFN HFN 15.0
14094.0 USB SEL 14A 14ASEL HFR HFR HFR HFR HFR HFR HFR HFR 15.0
14109.0 USB ALE 14B 14BALE HFN HFN HFN HFN HFN HFN HFN HFN 15.0
14122.0 USB SEL 14C 14CSEL HFX HFX HFX 15.0
14343.0 USB SEL 14D 14DSEL HFS HFS HFS HFS HFS HFS HFS HFS 15.0
14346.0 USB ALE 14E 14EALE HFL HFL HFL HFL HFL HFL HFL HFL 15.0
18106.0 USB ALE 18A 18AALE HFN HFN HFN HFN HFN HFN HFN HFN 15.0
18107.0 USB SEL 18B 18BSEL HFR HFR HFR HFR HFR HFR HFR HFR 15.0
18113.0 USB SEL 18C 18CSEL HFX HFX HFX 15.0
18117.5 USB ALE 18D 18DALE HFL HFL HFL HFL HFL HFL HFL HFL 15.0
18163.0 USB SEL 18E 18ESEL HFS HFS HFS HFS HFS HFS HFS HFS 15.0
21094.0 USB SEL 21A 21ASEL HFR HFR HFR HFR HFR HFR HFR HFR 15.0
21096.0 USB ALE 21B 21BALE HFN HFN HFN HFN HFN HFN HFN HFN 15.0
21228.0 USB SEL 21C 21CSEL HFX HFX HFX 15.0
21427.0 USB SEL 21D 21DSEL HFS HFS HFS HFS HFS HFS HFS HFS 15.0
21432.5 USB ALE 21E 21EALE HFL HFL HFL HFL HFL HFL HFL HFL 15.0
24924.0 USB SEL 24A 24ASEL HFR HFR HFR HFR HFR HFR HFR HFR 15.0
24926.0 USB ALE 24B 24BALE HFN HFN HFN HFN HFN HFN HFN HFN 15.0
24932.0 USB ALE 24C 24CALE HFL HFL HFL HFL HFL HFL HFL HFL 15.0
24977.0 USB SEL 24D 24DSEL HFS HFS HFS HFS HFS HFS HFS HFS 15.0
28143.0 USB SEL 28A 28ASEL HFR HFR HFR HFR HFR HFR HFR HFR 15.0
28146.0 USB ALE 28B 28BALE HFN HFN HFN HFN HFN HFN HFN HFN 15.0
28305.0 USB SEL 28C 28CSEL HFS HFS HFS HFS HFS HFS HFS HFS 15.0
28312.5 USB ALE 28D 28DALE HFL HFL HFL HFL HFL HFL HFL HFL 15.0
29520.0 FM SEL 29A 29ASEL HFM HFM HFM HFM HFM HFM HFM HFM 6.0

Frequency table notes: Automatic Link Establishment ALE channel frequencies in the Amateur Radio Service are internationally coordinated with selective calling Selcall channels for interoperability purposes. Net is the ALE net address or Selcall net name.

Standard configurations

Note Configuration Standard
1 ALE System MIL-STD 188-141B; FED-1045 (8FSK, 2 kHz bandwidth)[5]
2 Transmission duration Calling optimum 15 seconds; or preamble 15 seconds.
3 Scan rate 1, 2, or 5 channels per second. Minimum dwell time 120 milliseconds per channel for ALE and 300 milliseconds for selcall.
4 Sounding Interval 60 Minutes or more (for same channel)
5 Audio Centre Frequency 1625 Hz for digital mode text and data
6 Messaging standard AMD (Automatic Message Display) Universal short texting[5]
7 Sounding Type TWS Sounding (This Was Sound)[5]
8 Tune Time 3000 milliseconds or approximately 3 seconds[5]

International nets

NET Protocol Content Status Sounding Net Slots Purpose
HFL ALE Voice Open Manual 3 Normal communications and emergency
HFN ALE Texting Open Auto 1 hour 3 Normal communications
HFR Selcall Texting Open Auto 1 hour 1 Normal communications
HFS Selcall Voice Open Manual 1 Normal communications and emergency
HFM Selcall Texting or Voice Open Manual 1 Normal communications
HFX ALE or Selcall Texting or Voice Open Manual 1 Inactive or auxiliary frequencies

See also

References

  1. ↑ Telecom Bureau, ITU-D/SG (2000-12-14). "Frequency Agile Systems in the MF/HF Bands" (doc). International Telecommunication Union. http://www.itu.int/ITU-D/study_groups/SGP_1998-2002/SG2/Documents/2001/080R1e.doc. 
  2. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named ARRL1
  3. ↑ 3.0 3.1 3.2 3.3 Menold, Ronald E., AD4TB (February 1995). "ALE--The Coming of Automatic Link Establishment". ARRL, QST Volume 79, Number 2. http://www.arrl.org/tis/info/pdf/9502068.pdf. 
  4. ↑ 4.0 4.1 "ITU ALE Handbook". International Telecommunication Union ITU. http://hflink.com/itu/ITU_ALE_Handbook.pdf. 
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 "MIL-STD 188-141B". US Government. http://hflink.com/standards/ALE_standard_188_141B.pdf. 
  6. ↑ "ALE--The Coming of Automatic Link Establishment". ARRL, QEX, 1990-JAN, Reprint of NTIA ITS. January 1990. http://www.arrl.org/tis/info/pdf/9001qex003.pdf. 
  7. ↑ 7.0 7.1 "Federal Standard 1045A". US Government. http://www.its.bldrdoc.gov/fs-1045a/. 
  8. ↑ Fiedler, D (1994). "Automated HF communications for nap-of-the-earth flying". United States Army. http://www.gordon.army.mil/ocos/ac/articles/fiedler/dfale.pdf. [yes|permanent dead link|dead link}}]
  9. ↑ Klingenfuss, J. (2003). Radio Data Code Manual (17th ed.). Klingenfuss Publications. pp. 72–78. ISBN 3-924509-56-5. 
  10. ↑ Johnson, E. (2008-08-17). "Simulation Results for Third-Generation HF Automatic Link Establishment". New Mexico State University. http://wireless.nmsu.edu/hf/papers/milcom99.pdf. 
  11. ↑ 11.0 11.1 Cite error: Invalid <ref> tag; no text was provided for refs named GAREC
  12. ↑ "Codan Automatic Link Management CALM" (PDF). Codan. http://hf-ssb-transceiver.at-communication.com/en/codan/hf_ssb_transceiver_ngt.html. 
  13. ↑ "COTHEN - The RadioReference Wiki". http://wiki.radioreference.com/index.php/COTHEN. 
  14. ↑ 14.0 14.1 IARU (2007-08-17). "ALE for Emergency Disaster Relief Communications". International Amateur Radio Union IARU. http://hflink.com/garec/ALE_IARU_GlobalEmCommConference2007.pdf. 



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