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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.

How ALE works

Each ALE radio is an HF SSB radio transceiver with an internal microprocessor or computer attached to it. It has 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 (BER) 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 containing 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 it 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 unmutes the radio and answers the call then can talk in a regular conversation. Alternatively, in the case of an automated system, digital data can be exchanged either a built-in or external modem depending on the capability of the ALE node and situational needs. An additional built-in "short message" facility can be used to transfer short text messages or as an "orderwire" to enable the operators to coordinate external equipment such as phone patches or non-embedded digital links. [2][3]

Practical ALE use by skilled and unskilled operators

Due to the vagaries of ionospheric communications, HF radio as used by large governmental organizations in the mid-20th century was traditionally the domain of highly skilled and trained radio operators. One of the new characteristics that embedded microprocessors and computers brought to HF radio via ALE, was alleviation of the need for the radio operator to constantly monitor and change the radio frequency manually to compensate for ionospheric conditions or interference. For the average user of ALE, after learning how to work the basic functions of the HF transceiver, it became similar to operating a cellular mobile phone. For more advanced functions and programming of ALE controllers and networks, it became similar to the use of menu-enabled consumer equipment or the optional features typically encountered in computer software. In a professional or military organization, this does not eliminate the need for skilled and trained communicators to coordiate the per-unit authorized frequency lists and node addresses. It merely allows the deployment of relatively unskilled technicians as "field communicatiors" and end-users of the existing coordinated architecture.

Common applications provided by ALE

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 used

The essence of ALE techniques is the use of automatic channel selection, scanning receivers, selective calling, handshaking, and robust burst modems.[4] All received ALE signals, gathered by channel scanning, containing a station ALE address are tracked and stored in Link Quality Analysis (LQA) memory, with the signal's channel frequency, signal quality, and time stamp.[3] When a call is initiated, the LQA lookup table is searched and the best channel is used to rapidly establish communications between the calling and called stations. ALE techniques also include automatic signaling, automatic station identification ( sounding ), polling, , message store-and-forward, address linking protection, and anti-spoofing. 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]

ALE historic background

ALE evolved from HF radio selective calling technology. It put together scanning selective calling with microprocessors, burst transmissions, and transponding. Early ALE systems were developed in the late 1970s and early 1980s by several radio manufacturers.[6] The first ALE controller units were external rack mounted controllers connected to control military radios. 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 were 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 expense of equipment was high. By the late 1990s, most new government HF radios purchased were designed to meet the compatibility standard, so that they could interoperate. As the standards were adopted by other governments worldwide, more manufacturers produced competitively priced HF radios to meet this government standard, and civilian entities started using 2G ALE. By approximately 2000, so many civilian and government organizations worldwide were using ALE that it became the de facto HF interoperability standard. In the late 1990s, a third generation 3G ALE with higher performance was included in MIL-STD-188-141B,[5] retaining backward compatibility with 2G ALE, and was adopted in NATO STANAG 4538.

ALE as a force multiplier

ALE enables rapid unscheduled communication and message passing without requiring complex message centers, multiple radios and antennas, or extremely skilled 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 even in situations which keep the pilots too busy to operate a traditional HF radio. This ability to enable tactical communication in conditions where a dedicated, highly trained communications operator 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. Thus radio operator staffing and radio system installation requirements were considerably reduced, while enabling small mobile or portable stations to participate in multiple networks and subnetworks.

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, on a near-par with SATCOM systems.

World standards and protocols

The common basic world protocol standards for ALE are based on the original US MIL-STD 188-141A[5] and FED-1045[7], known as 2nd Generation or 2G ALE. 2G ALE uses non-synchronised scanning of channels, and it takes about 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.

Second generation (2G) ALE 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 one 7 bit long. The receive decoder is usually done by Digital Signal Processing techniques and can decode the 8FSK signal at a negative signal to noise ratio, meaning it can pull the signal out 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.[8]

Third generation (3G) ALE 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.[9] 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 efficency, 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 interoperable communications

Interoperation 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 interoperation on HF between governmental, non-governmental organizations, disaster relief agencies, emergency communications entities, and amateur radio. 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 [10]have utilized the AMD feature of ALE to expand the performance and connectivity.[11] In some cases, this has been successful, and in other cases, the use of proprietary preamble or embedded commands has led to interoperation problems.

Emergency / disaster relief or extraordinary situation response communications

ALE radiocommunication systems for both HF regional area networks and HF interoperative communications are in service among emergency and disaster relief agencies. 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 respond to situations or scenarios to anticipate the possibility of loss of instantaneous conventional communications. Natural disasters such as earthquakes, typhoons, hurricanes, tsunamis, tornadoes, and volcanic eruptions are typical examples of natural disaster situations in which organizations may deem ALE use to be part of their critical system backup plan. ALE networks are common among organizations engaged in extraordinary situation response, such as: man-made disasters, transportation, internet or telecommunication network failures, war, investigations, international hostility mitigation, peacekeeping, or anti-terrorism. Well-known examples of organizations using ALE for Emergency management, disaster relief, or extraordinary situation response include: Red Cross, FEMA, Disaster Medical Assistance Teams, NATO, Federal Bureau of Investigation, United Nations, AT&T, SHARES, State of California OES (Office of Emergency Services), other US States' OES, and Amateur Radio Emcomm.[10]

International HF telecommunications for disaster relief

The International Telecommunications 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 and IARU Global Simulated Emergency Tests have included ALE. [12]

ALE use in international 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.[10] After the event, hams developed more permanent ALE emergency/disaster relief networks, including internet connectivity, with a focus on interoperation between organizations. The amateur radio Automatic Link Establishment system uses an open net protocol to enable all amateur radio operators and amateur radio nets worldwide to participate in ALE and share the same ALE channels interoperably. Amateur radio operators may use it to call each other for voice or data communications.[2]

ALE adaptations to amateur radio for interoperability

Amateur radio operators commonly provide local, regional, national, and international emergency / disaster relief communications.[12] The need for interoperability on HF led to adaption of Automatic Link Establishment ALE open networks by hams. Amateur radio adapted 2G ALE techniques, by utilizing a common denominator the 2G ALE protocol, with a limited subset of features found in the majority of all ALE radios and controllers. Each amateur radio ALE station uses the operator's callsign as the address, also known as the ALE Address, in the ALE radio controller.[2] The lowest common denominator technique enables any manufacturer's radios or software to be utilized for interoperative communication and networking. Known as Ham-Friendly ALE, the amateur radio ALE standard is used to establish radio-communications, through a combination of active ALE on internationally recognized automatic data frequencies, and passive ALE scanning on voice channels and all other channels. In this technique, active ALE frequencies include pseudo-random periodic polite station identification, while passive ALE frequencies are silently scanned for selective calling. Ham-Friendly ALE technique is also known as 2.5G ALE, because it maintains 2G ALE compatibility while employing some of the adaptive channel management features of 3G ALE, but without the accurate GPS time synchronization of 3G ALE.

Global amateur radio high frequency network for interoperable emergency disaster relief communications

Hot standby nets are in constant operation 24/7/365 for International Emergency and Disaster Relief communications. The Ham Radio Global ALE High Frequency Network, which began service in June 2007, is the world's largest intentionally open ALE network for internet connectivity. It is a free open network staffed by volunteers, and utilized by amateur radio operators supporting disaster relief organizations.[12]

International amateur radio ALE coordination details

International amateur radio ALE High Frequency channels are frequency coordinated with all Regions of the International Amateur Radio Union(IARU entity of ITU),[10] for international, regional, national, and local use in the Amateur Radio Service. All Amateur Radio ALE channels use "USB" Upper Sideband standard. Different rules, regulations, and bandplans of the region and local country of operation apply to use of various channels. Some channels may not be available in every country. Primary or global channels are in common with most countries and regions.[13]

International amateur radio ALE channels

Channel Freq (kHz)[12] SSB Common Use NET Description[13]
01 1806.0 USB VOICE/DATA QRZ
02 1840.5 USB VOICE/DATA QRZ
03 1845.0 USB VOICE/DATA HFL International Emergency/Disaster Relief
04 1996.0 USB VOICE/DATA QRZ
05 3584.5 USB DATA/VOICE QRZ
06 3596.0 USB PRIMARY DATA HFN Internet connectivity[12], sounding
07 3617.0 USB DATA/VOICE HFN IARU Region 1 Internet, sounding
08 3626.0 USB DATA/VOICE QRZ
09 3791.0 USB VOICE HFL International Emergency/Disaster Relief[12]
10 3845.0 USB VOICE HFL North America Emergency/Disaster Relief
11 3996.0 USB VOICE HFL North America Emergency/Disaster Relief
12 5371.5 USB VOICE QRZ Emergency Only
13 5403.5 USB VOICE QRZ Emergency Only
14 7040.5 USB DATA HFN IARU Region 1 Internet, sounding
15 7065.0 USB VOICE QRZ
16 7099.5 USB DATA QRZ
17 7102.0 USB PRIMARY DATA HFN Internet connectivity[12], sounding
18 7110.5 USB DATA QRZ
19 7185.5 USB VOICE HFL Intnat'l. & North Am. Emergency/Disaster Relief[12]
20 7296.0 USB VOICE HFL North America Emergency/Disaster Relief
21 10136.5 USB DATA/VOICE QRZ
22 10142.5 USB DATA QRZ
23 10145.5 USB PRIMARY DATA HFN International Emergency/Relief[12], Internet
24 14100.5 USB DATA QRZ
25 14109.0 USB PRIMARY DATA HFN Internet connectivity[12], sounding
26 14112.0 USB DATA QRZ
27 14342.5 USB VOICE QRZ
28 14346.0 USB VOICE HFL International Emergency/Disaster Relief[12]
29 18104.5 USB DATA QRZ
30 18106.0 USB PRIMARY DATA HFN Internet connectivity[12], sounding
31 18117.5 USB VOICE/DATA HFL International Emergency/Disaster Relief[12]
32 18157.5 USB VOICE QRZ
33 21096.0 USB PRIMARY DATA HFN Internet connectivity, sounding
34 21116.0 USB DATA QRZ
35 21437.5 USB VOICE HFL International Emergency/Disaster Relief[12]
36 24926.0 USB PRIMARY DATA HFN Internet connectivity, sounding
37 24932.0 USB VOICE HFL International Emergency/Disaster Relief
38 28146.0 USB PRIMARY DATA HFN Internet connectivity, sounding
39 28312.5 USB VOICE/DATA HFL International Emergency/Disaster Relief[12]
40 28327.5 USB VOICE QRZ
41 50162.5 USB VOICE/DATA QRZ
42 144162.5 USB VOICE/DATA QRZ

Channel list note: This listing is current as of August 2008. For more information about Amateur Radio ALE Automatic Link Establishment and channel updates please see HFLINK.COM

International amateur radio ALE standard configurations

Note Configuration Standard
1 ALE System MIL-STD 188-141A ; FED-1045 (8FSK, 2kHzBW)[5]
2 Transmission duration Calling optimum 22 seconds; Maximum 30 seconds.
3 Scan rate 1 or 2 channels per second.
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 text[5]
7 Sounding Type TWS Sounding (This Was Sound)[5]

International amateur radio open ALE nets

Net Member Slots Entity or Purpose
HFL 10 All ALE voice SSB stations[12], open selective calling
HFN 10 Ham Radio Global ALE High Frequency Network[10]
QRZ 3 Open calling on all channels
GPR 3 Geo Position Reporting
RPT 3 Station Status Reporting

ALE in international amateur radio popular culture

Amateur radio has a long tradition of free communications with global reach, predating the internet. A popular message-passing or "pure communication" subculture within international amateur radio has been active since the early days of radio itself.[14] Indeed, the US ARRL was originally formed for such purposes. Amateur radio is famous for pioneering technology for radio communications in the 20th century, and modernization through the use of ALE has recently become part of ham radio's digital technological advancement in the 21st century.[14] ALE is recognized among the international ham community.[13] Mention of the word Ale among hams often leads to a joke or pun, with reference to the term beer. The community of hams using HF is also quite sensitive to comparisons between amateur radio and the internet, and some see the interconnection of internet with ham radio as heresy or contradictory to what they perceive as the pure essence of ham radio, RF (Radio Frequency) only. Others see any interconnection with the internet as leading to dependence on the internet which is often unavailable in disasters and emergencies.

ALE in US amateur radio popular subculture

Amateur radio operators in the US also have adopted ALE for hobby purposes, including DXing, NVIS, award seeking, keeping in touch with friends, and personal education.[13] Although ham radio often uses evolving technology, the evolution of HF technology within the US ham popular culture is often fraught with resistance to change. Like the luddites of 19th century Britain, [15] a subculture of hams in US includes individuals[who?] who do not embrace the changes that new technology, such as ALE development, brings.[16] Some[who?] see ham radio as a hobby that is best pursued only for personal gratification and sporting competition, [17] and as such, should not focus on providing free emergency or disaster relief services such as embodied in popular ALE use.[10] Tradition runs deep; for example, ham radio is one of the few remaining radio services that continues to actively use and vehemently promote Morse Code for nostalgic, traditional, competitive, social, and practical reasons.[14] Perception of the need for preservation of 20th century social values and manual operating traditions and skills of ham radio operation[14] has led to advocacy for luddism,[15] or the seeking of active suppression of some types of HF technological advancement in ham radio.[16] It is a generally-recognized pejorative in amateur radio circles to call someone an "appliance operator" [18].

Anti-ALE advocacy in US amateur radio popular culture

Some US radio hams have advocated abolishment or suppression of automation trends, and in 2007, a ham formally petitioned the FCC,[19] to abolish ALE and other popular automatic data systems and modes on HF.[16] [19] According to FCC documents,[16] over 600 comments were received,[16] with the overwhelming majority urging FCC[16] to rule against the petition that became widely known[verification needed] in US amateur radio popular culture as RM11392. In May 2008 the FCC did rule against the petition,[19] in an Official FCC Order.[16] In The Official Order page 6, the FCC said: "...we believe that amending the amateur service rules to limit the ability of amateur stations to experiment with various communications technologies or otherwise impeding their ability to advance the radio art would be inconsistent with the definition and purpose of the amateur service. Moreover, we do not believe that changing the rules to prohibit a communications technology currently in use is in the public interest."

After the Official FCC Order was published, there remained a small number of US hams who continued to argue in social forums against ALE and other modes, saying they still feel that ALE and automatic operation on HF should be against the rules;[citation needed] or that ALE is an HF propagation beacon.[citation needed] ALE is seen by some American ham operators[who?] as a direct affront to their personal skills and abilities to manually operate a radio set.[citation needed] Some have sought to rally support for their cause by calling HF automatic data stations evil robots.[citation needed] The amateur radio community is a broad cross-section of humanity, and there will probably continue to be heated social debate and interest in all things that deal with automation trends on High Frequency radio. (End of unverifiable paragraph)

See also

References

  1. ^ Telecom Bureau, ITU-D/SG (2000-12-14). "Frequency Agile Systems in the MF/HF Bands" (doc). International Telecommunications Union.
  2. ^ a b c Crystal, B. (2008-03-31). "ARRL We Do That: What Is ALE?" (html). ARRL, National Association for Amateur Radio.
  3. ^ a b c d Menold, Ronald E., AD4TB (1995-February). "ALE--The Coming of Automatic Link Establishment" (pdf). ARRL, QST Volume 79, Number 2. {{cite web}}: Check date values in: |date= (help)CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link)
  4. ^ a b "ITU ALE Handbook" (PDF). International Telecommunications Union ITU.
  5. ^ a b c d e f g h "MIL-STD 188-141B" (PDF). US Government.
  6. ^ Adair, R.; Peach, D. (1990-January). "ALE--The Coming of Automatic Link Establishment" (pdf). ARRL, QEX, 1990-JAN, Reprint of NTIA ITS. {{cite web}}: Check date values in: |date= (help)CS1 maint: multiple names: authors list (link)
  7. ^ a b "Federal Standard 1045A" (html). US Government.
  8. ^ Klingenfuss, J. (2003). Radio Data Code Manual (17th Ed.). Klingenfuss Publications. p. 72-78. ISBN 3-924509-56-5.
  9. ^ Johnson, E. (2008-08-17). "Simulation Results for Third-Generation HF Automatic Link Establishment" (pdf). New Mexico State University.
  10. ^ a b c d e f Crystal, B. ; Barrow, A. (2007-08-17). "ALE for Emergency / Disaster Relief Communications" (html). International Amateur Radio Union IARU.{{cite web}}: CS1 maint: multiple names: authors list (link)
  11. ^ "Codan Automatic Link Management CALM" (PDF). Codan.
  12. ^ a b c d e f g h i j k l m n o p IARU (2007-08-17). "ALE for Emergency Disaster Relief Communications" (pdf). International Amateur Radio Union IARU. {{cite web}}: Unknown parameter |size= ignored (help)
  13. ^ a b c d ARRL (2005-08-01). "ARRL Technical Information Service page:ARRLWeb: ALE (Automatic Link Establishment" (html). ARRL, National Association for Amateur Radio.
  14. ^ a b c d ARRL (2008-02-12). "ARRLWeb: Ham Radio History" (html). ARRL, National Association for Amateur Radio.
  15. ^ a b Pynchon, T. (1984-10-28). "Is it OK to be a Luddite?" (html). The Modern World.
  16. ^ a b c d e f g "Digital Stone Age FCC Petition RM-11392" (PDF). US Government Federal Communications Commission FCC.
  17. ^ ARRL. "DX The Ultimate Contact Sport" (ppt). ARRL.
  18. ^ Template:Cite web url="http://dzabcik.home.texas.net/ham.html"
  19. ^ a b c "FCC DA 08-1082" (PDF). US Government Federal Communications Commission FCC.
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