SELCAL (Selective Calling) is a system designed to give Flight Crews a possibility to guard a certain frequency without actually having to listen to the audio and the noises an HF radio received normally makes

The system comprises a Ground based SELCAL encoder and an on board decoder. Each SELCAL equipped aircraft is assigned a four-letter SELCAL code. Each letter represents a tone and the SELCAL code consists of the tones of the first two letters followed by the tones of the last two letters in the code. The on board decoder will detect the tone combination and will sound a chime and illuminate a light in the cockpit if the SELCAL decoded matches that of your aircraft.

The distribution of the SELCAL codes is administered by Aviation Spectrum Resources Inc. Maryland US.
>> www.asri.aero/selcal

SELCAL check of two radio installations can best be performed using the following procedure:

1. Select the same frequency on both radios.
2. Call STO radio on Radio #1 and request a SELCAL check.
3. The transmitted code should trigger both SELCAL decoders.
4. Acknowledge receipt of the SELCAL check using Radio #2.