ALE

What it is

ALE is a desktop application implementing the Second Generation Automatic Link Establishment standard (2G ALE, MIL-STD-188-141B). ALE solves HF radio's most persistent operational problem: finding a frequency that actually works right now. Rather than requiring operators to manually probe frequencies and assess propagation, an ALE controller does this automatically and continuously, maintaining an up-to-date picture of which channels in the scan list are usable between which stations.

The application implements ALE in software using the PC's sound card as the radio interface, with Hamlib providing frequency and PTT control over a wide range of transceivers.

How ALE works

ALE operates by having each station in a network continuously scan a shared list of HF frequencies. On each channel, the station listens for ALE handshaking signals from other stations. Periodically, each station transmits a short sounding burst on each channel — a brief, structured transmission that identifies the sender and allows any receiving station to measure link quality. These quality measurements (called LQA, for Link Quality Analysis) accumulate in a table at each node, building a ranked picture of which frequencies currently support reliable communication between which pairs of stations.

When an operator initiates a call, the ALE controller selects the highest-ranked available frequency from the LQA table, transmits a calling sequence addressed to the destination station's address, and waits for an acknowledgement. If the link is established, both stations stop scanning and remain on that frequency for the voice or data session. If not, the controller tries the next best candidate.

The entire linking process — frequency selection, calling, acknowledgement — typically completes in under ten seconds on a well-maintained channel list. The operator simply selects a destination and initiates the call; ALE handles the rest.

The 2G ALE standard

MIL-STD-188-141B defines the waveform, protocol, and data formats for second-generation ALE. The standard specifies an 8-ary FSK modem operating at 125 baud within a 3 kHz SSB passband, with differential encoding for phase-ambiguity immunity and a Reed-Solomon-based coding scheme for the handshaking words. These design choices make the waveform robust to the multipath, Doppler spread, and selective fading conditions that characterize long-distance HF propagation.

2G ALE is widely deployed in military, government, and emergency communications networks worldwide and has been adopted by the amateur radio community as a practical tool for HF net management and interoperability with other ALE-equipped stations. The standard is public and interoperability with compliant equipment — military, commercial, and amateur — is a design goal.

Use cases

• —Amateur radio HF nets — coordinate across a channel list without manually surveying propagation; let ALE find the working frequency.
• —Emergency communications — ALE's automatic frequency management is particularly valuable when propagation is unpredictable and operators may be under stress.
• —Interoperability — establish links with other stations running MIL-STD-188-141B-compliant ALE equipment, including commercial and government systems.
• —Propagation monitoring — the continuous LQA sounding process produces a real-time picture of HF band conditions between network stations.

Platform support

Development status