ALE (automatic link establishment)
Short automatic 'calls' that shortwave radios send to find a working frequency and link up without an operator.
ALE sounds like a short, bubbly, chirping warble, from a second or two to several seconds long, like a fast electronic bird call, often heard again moments later on a different frequency.
What does it sound like?
A brief, chirpy, bubbling warble, from about a second to several seconds long, repeating on different channels.
Who uses it?
Military and government networks, emergency-management agencies, and some hams.
Where and when can I hear it?
Anywhere from 2 to 30 MHz; stations 'sound' every so often, around the clock.
The science
Second-generation ALE, defined in MIL-STD-188-141 and Federal Standard 1045, sends one of eight audio tones between 750 and 2500 Hz, spaced 250 Hz apart, at 125 symbols per second, so each 8-millisecond tone carries three bits. The basic unit is a 24-bit word, a 3-bit preamble plus three 7-bit ASCII characters, which is Golay-encoded and interleaved into 48 bits plus one stuff bit and then sent three times in a row, taking 392 milliseconds. Radios scan their assigned channels, use soundings to rank each channel by signal strength and recent performance, and then connect with a three-way handshake: call, response, acknowledgment.
Why use it?
Picking a working shortwave frequency used to take an experienced operator who understood how propagation changes. ALE automates channel selection, sounding and scanning, so anyone can place an HF call and the radios choose among the best-ranked channels on their own.
Lore
Government test engineers at the Institute for Telecommunication Sciences produced an audio compact disc of standard ALE test tones so radios could be tested against the federal standards the same way. A prototype went free of charge in November 1993 to members of the HF Industry Association and interested U.S. government agencies, and the final version followed in December 1994.
What do the decoded parts mean?
- Addresses: Short station names such as 'TO ABC' and 'TIS XYZ' (this is XYZ).
Easily confused with
- PACTOR, from the same family
- STANAG 4285 (NATO modem), from the same family
How to recognize it
The common "second generation" ALE uses eight tones between 750 and 2500 Hz, changing tone 125 times per second. That rapid hopping among eight pitches gives a busy, burbling sound, more melodic than RTTY and much faster than FT8. Transmissions are short: an answer or acknowledgment lasts a second or two, and a call to a station that is scanning its channels can run longer, up to about 15 seconds.
Stations also send "sounding" calls on a timer, simply announcing their name so other stations can measure which frequencies are working. If you park on one frequency, you may hear these soundings every few minutes to hours.
Easily confused with:
- Olivia and other multi-tone ham modes: continuous for minutes rather than brief calls.
- RTTY: two tones only, with a steady back-and-forth.
- Newer 3G and 4G ALE: 3G sends fast phase-shift bursts in the same 3 kHz channel that sound more like noise; 4G (wideband) ALE can use much wider channels.
Where and when to hear it
ALE is spread across the whole shortwave range, from about 2 to 30 MHz, since the point is to test many frequencies. Military, government and emergency-management networks use it around the clock, so scanning the HF bands with upper sideband will find it sooner or later.
Radio amateurs run the HFN (High Frequency Network) on coordinated channels including 3.596, 7.102, 10.1455, 14.109, 18.106, 21.096 and 28.146 MHz (upper sideband), mainly for emergency and disaster communications practice.
The best way to catch ALE is to let a receiver sit on one of these channels, or step slowly across a band, and wait. Activity comes in short, unpredictable flurries: a round of soundings, a call and reply, then long quiet stretches. Evenings and times of disaster exercises tend to be busier, and the same station often shows up minutes later on another channel as its radio tests the next frequency on its list.
A little history
Before ALE, making a long-distance HF contact meant an operator guessing which frequency would work at that hour and calling by hand. ALE automated that: radios scan a list of channels, listen for each other's calls, rate the link quality, and connect on the best one without a skilled operator.
The U.S. military standardized ALE in MIL-STD-188-141A in the late 1980s, followed in 1990 by the matching federal standard FED-STD-1045. Later revisions added faster, more capable generations of the technique. Hams adopted the second-generation system from the early 2000s for emergency preparedness.
Common questions
Is it legal to listen to ALE?
Receiving ALE is ordinary shortwave listening in the United States and most other places. The calls contain station addresses rather than conversations, and amateur ALE is fully open.
What does an ALE call contain?
Mostly addresses: who is calling ("TIS" means "this is"), whom it is calling ("TO"), and sometimes a short text or link quality data. The actual conversation that follows a link is usually in voice or another data mode.
Why does ALE jump between frequencies?
The whole point of ALE is to find the frequency that works best at that moment. Stations scan and call on several channels, so the same station can pop up on different frequencies within minutes.
How do I decode ALE?
PCALE, Windows software that is free for ham use, can decode second-generation ALE from a shortwave receiver's audio in upper sideband, and many public KiwiSDR web receivers include an ALE decoder.