The ABC of ALE
In this month’s column, Nils Schiffhauer DK8OK throws some light on Automatic Link Establishment (ALE).
In this month’s column, Nils Schiffhauer DK8OK throws some light on Automatic Link Establishment (ALE). This technology has made HF communications nearly as comfortable as making a telephone call.
Even as late as the mid-1980s, it was still true to say that short wave communications were a combination of science, art and craft. But then ALE arrived and became a catalyst for massive changes.
Back in 1986, Siemens engineer Gerhard Braun published his 2nd and last edition of Planning and Engineering of Shortwave Links. All workflows to calculate the best HF channels for given circuits or areas throughout days, seasons and solar cycles were then virtually hand-made. You will not find the word ‘computer’ in any of the 338 pages of Braun’s book.
Only one year later, the scene had dramatically changed, as described by Nicholas Maslin in his HF Communications – A Systems Approach, also published in that year.
Three things, in particular, sparked the resurrection of HF communications at that time.
First, the creation of reliable algorithms, calculating HF propagation for stable communications. This included the arrival of more modern PCs to make up schedules in a very short time.
Second, microprocessor-controlled wide-band receivers and transmitters, which could change frequencies within milliseconds.
Third, error-correction and highly efficient codes and modulation schemes for digital communications appeared, as a by-product of space science.
These developments and concomitant technologies came together in 1985, when the USA founded an initiative to develop a Federal Standard (FED STD-1045A, in 1990) for automatic link establishment (ALE). This was quickly adopted by NATO (MIL-STD-188-141A) as well as commercial manufacturers and even by radio amateurs.
The general idea of ALE is to make HF communications as easy as a telephone call: just lift the receiver, dial a number and communicate. ALE is a tool to automatically find the best channel to establish communications, data or voice – regardless of day or night, season, sunspot cycle and any interference.
Operators just have to define and program a set of frequencies for all these situations (Fig. 1).
Following this, the radios regularly (at 30-minute intervals) transmit their identification on channel after channel for about ten seconds each.
These ‘sounding’ or ‘probing’ signals are received by their corresponding stations – sorted by quality and channel. They are then saved. If someone now lifts the receiver at such a station, communications are automatically established on the ‘best’ channel, according to the last sounding.
Some of the issues of planning and frequency management within a network of ten different stations and frequencies, can best be understood through graphs like the one in Fig. 2, which may remind some of Piet Mondrian’s painting New York Boogie Woogie.
In this form of ALE Boogie Woogie, each coloured block shows one station per colour. It transmits the sounding signal for 10 seconds – only to then switch to the next channel. One cycle lasts 120 seconds and is repeated every 30 minutes.
The small black caskets are 0,5s in length and show a receiver scanning all ten channels. You can easily see that, within 120s, each station transmitted its sounding signal on each channel and a scanning receiver captured each signal on each channel twice. This not just theory but has been derived from the worldwide HFGCS network of the US Air Force.
The image in Fig. 3 shows how, thanks to ALE, an aircraft flying from Nairobi to Bangkok, continuously maintains perfect contact with Camp Thunder Cove on Diego Garcia, by automatically choosing the best channel from a choice of ten that are checked every half hour.
According to a predictions of the (free online) tools used by the Australian Space Weather Services (SWS) nine frequency changes are required during this ten-hour flight by night and day.
A real-world example can be seen in Fig. 4, where I monitored some of the HF channels of the German Bundespolizei See – a part of the Maritime Safety and Security Centre (with the callsign BPLEZS).
In my example, they were calling the Coast Guard (Küstenwache) vessel Bad Bramstedt BP24 and I received five channels at my location. The operators sequenced their transmission from the highest channel down to the lowest one, to get the best signal-to-noise ratio, as fast as possible.
You can see the signal strength in the HF level of two consecutive scanning sessions. Having several sessions is a good idea, in order to avoid short-time interference. At my location, the best signal-to-noise-ratios were delivered on the three lower frequencies, with the highest one exhibiting by far the worst SNR.
The SWS tool Local Area Mobile Prediction is also of great help here. From a set of frequencies, it calculates the best one for each hour to cover a circle of 1,000km around a transmitter.
The images in Figs. 5a to c display maps relating to my monitoring of BPLEZS on 8th February and in mid-August 2018.
From the choice of channels, you can discern two overall strategies, which apply to ALE in general.
First, one channel each will routinely be assigned, to cover predictable, major, changes in propagation. Those major changes are, for instance, diurnal changes, varying sunspot numbers and fluctuations affecting remote partner stations across significant geographical distances.
Second, in many cases, one or more alternatives are assigned to a channel, in order to avoid interference.
Fig. 6 shows Germany’s customs headquarters (Zollleitstelle, ZLST) calling the customs vessel Kniepsand (ZKNI) in ALE. ZKNI responds by ‘handshaking’ and ZLST resumes data communications in another mode.
In general, this technique has remained unchanged for more than 30 years. It is also called 2G-ALE, to distinguish it from specific predecessors and non-standardized techniques.
ALE was a successful strategy to overcome some of the limits of HF. There are now thousands of stations and hundreds of networks in operation. This large investment renders a shift to the more advanced 3G-ALE very slow. Therefore, 2G-ALE still represents a good opportunity for monitors who wish to detect fascinating stations and communications networks around the world. This form of communication is backed by a strong combination of modulation and coding.
Eight Tones & Some Clever Math
The ALE signal consists of eight tones from 750 to 2,500Hz, at a 250Hz distance.
Each one is on the air for a mere 8 milliseconds (Fig. 7) and each tone carries three bits of information (23).
The content is coded in 7-bit ASCII and is transmitted in ALE standard ‘words’ of three characters each. In the example of USAF Diego Garcia, these are ‘JDG’.
There are also three trailing bits for the frame type (Fig. 8) This preamble may read ‘TO’, ‘THIS WAS’ or ‘FROM’, among other alternatives.
One such ‘word’ consists of 24 bits. To fight possible quality losses through weak and degraded HF propagation, redundancy (or forward error correction, FEC) is added.
The mathematics behind this clever coding was developed by Swiss engineer Marcel J.E. Golay and published in 1949. His text – just half a page in length – became one of the most influential publications in coding theory:
If you are interested in more detail on this, you can visit Stephen Wolfram’s interactive Golay Code Demonstrations Project, at this URL:
The outcome of this is a complete frame of 2 x 24 bits and one additional bit or 49 bits transmitted for 392ms.
Golay-coded transmissions can experience up to three bit errors per 24-bit standard word, caused by fading and interference and will duly repair the signal automatically during the decoding process on the receiver side.
This modulation, where each symbol is transmitted with full power and with a ‘smart’ code, leads to an extremely robust mode, even under adverse propagation conditions and often with a signal-to-noise ratio of down to 0dB. ‘Sounding’, ‘handshaking’ and information exchange; that is usually all that listeners will read.
Because it has been going on for a long time, ALE monitoring benefits from many lists, which have been published to detail what station or organization uses which ALE call signs.
For instance, ‘Hugh’ is maintaining a huge list of ALE callsigns at this website:
Callsigns often come in groups and patterns. Table 1 shows a few examples.
In a first group, there are those callsigns with a clear link to the organisation or entity using them, the location or the carrier (‘Group 1’ in the Table).
In a second batch, callsigns can refer to the position of a station in the hierarchy of a network and they can sometimes be ambiguous (‘Group 2’ in the Table).
A third, general, group has diverse members and locations (‘Group 3’ in the Table).
Modes of Transmission
Most ALE stations use USB, with the suppressed carrier frequency often ending in integer kHz or 500Hz.
A few nets show a more unique pattern: Those of the US Department of State and its Embassies usually end in 600Hz.
There are also some stations transmitting on LSB. Both sidebands are easily distinguished if a pilot tone is sent on the carrier frequency. The image in Fig. 9 shows two stations of the SONATRACH net in Algeria, one transmitting on USB, the other on LSB.
Stations working double sideband exist too. One example is the Royal Saudi Air Force. However, this type of transmission is rare.
Some stations transmit their pilot tone not on the carrier frequency but on the lowest tone (MARS) on any other tone, or none at all.
Not all stations in a network might work on exactly the same frequency and some might deviate some 10Hz from their nominal channel.
If you face no decode at all or a part-decode of a weak station in a recording, try it again and shift the frequency by ±50Hz in steps of 10Hz. Switch your receiver to 3kHz bandwidth; this ought to be further reduced in conditions of severe interference only.
Adjust your AGC to the ‘short’ setting to cope with potentially different signal strengths within a net.
If none of this works, change the sideband.
Last but not least, there are some signals, which sound and look like ALE but are encrypted or following another (proprietary) protocol.
The Decoder End
Decoders offering ALE will provide a good output and will include a tuning aid. During my tests on the air as well as using a channel simulator, I found Sorcerer and MultiPSK to yield the largest number of accurate decoding results, even under adverse conditions. These two pieces of software will also read and show signals, which you may barely hear.
I prefer MultiPSK because it is very stable and works reliably, decoding, for example, 24 channels in parallel over a period of days.
This is also the case with PC-ALE and MARS-ALE for emergency communications among radio amateurs. However, they are not that sensitive in case of weak signals.
The reception of ALE signals is a fascinating and never-ending challenge for seasoned and novice utility signals monitors alike. More often than not, unknown stations pop up and new networks suddenly emerge.
I will come back to these eight-tone signals in the future.
Meanwhile, the US Air Force’s HF Global Communications System is a good starting point on 3137, 4721, 5708, 6721, 9025, 11226, 13215, 15043, 18003 and 23337kHz.
See you next month.
Table 1: Key to Main Groups of ALE Callsigns
GROUP 1 (Organisation, Location or Carrier)
CAPPELLETTI is the ALE call sign of ‘G. 94 Cappelletti’, a ship of Italian’s Guardia di Finanza, with BARI, VACCARO and others in their wake.
FEC0FEM – is a station (actually Bothell, WA) of the Federal Emergency Agency (FEMA) of the US. Most of their call signals end in ‘FEM’. Likewise, callsigns ending in CAP point to Civil Air Patrol.
FERB points to Ferghana in Uzbekistan. If you solved this one, ANDB, JIZB and GULB, in the same net, can be decoded as the Uzbek cities of Andijon, Jizzax and Guliston.
GERONIMO stands for USAF Altus, Oklahoma (because Apache leader Geronimo died at Ft. Sill, Oklahoma).
JFERNANDEZ is the outpost of a Chilean net on Juan Fernández Archipelago.
GROUP 2 (Station Position, Structure or Hierarchy inside a Network)
HQ1 is used by a Libyan net as well as by the FBI HQ.
NAVY1 marks the Iraqi Navy Operational Headquarters.
AA1, BB1, BB2 and BB3 are all call signs of the Israeli Air Force, with AAA their headquarters.
T1Z140 stands for the US Army in Kuwait (1st Battalion, 140th Aviation Regiment).
N.B.: A combination of location and structure is sometimes found, as in the case of the vast network of Turkish Emergency stations, made of only numbers. Here, the second and third cyphers represent the province. This number is identical to the first cypher(s) of their ZIP/ Postal code and regional car plates. For instance, 364013 stands for a station in Usak province and 8411 for one in Kocaeli province.
GROUP 3 (Various, Sometimes Ambiguous)
C3 for Royal Moroccan Army, DB5 for Iraqi Border Control Najaf and XS43 for the National Guard in Algeria.
This article was featured in the August 2018 issue of Radio User