The first version of Selcal in 1985 used only 12 tones (A through M except I). In 1994, the standard was extended to have 16 tones, adding tones P, Q, R, and S. With the 16 tone format, the number of possible allowable codes was extended to 10,920. Selcal codes assigned to aircraft previously use a subset of only twelve letters/tones. Therefore, more than one aircraft may be designated by the same code
Aviation Spectrum Resources, Inc. (ASRI) is the worldwide International Civil Aviation Organization (ICAO) registrar for all Selective Calling (SELCAL) system codes (This was formerly done by Aeronautical Radio, Incorporated (ARINC).) To avoid confusion from two or more aircraft using the same SELCAL code, ASRI tries to assign code duplicates to aircraft that do not usually operate in the same region of the world or on the same HFHigh Frequency (3-30 MHz) radio frequencies. However, aircraft commonly move between different geographical regions and it is now routine for two aircraft with the same SELCAL code to be found flying in the same region. Therefore, air crew always verify both SELCAL and call sign (i.e., aircraft tail registration, or telephony designator and flight identification) to be sure their aircraft is the intended recipient.
The ICAO Selcal signal itself is USBUpper Side Band Modulation (Radio, referring to reception and modulation mode)Universal Serial Bus (Computer, referring to USB Ports and cables) modulated with a full carrier. The radio transmissions that follow a Selcal are typically USB modulated voice without a carrier.
The Selcal follows a 2 x 2 format. The first group comprises of two simultanious tones, made of 2 of the 16 availiable tones, and lasts for 1 second. Then there is a 200ms pause before the second group, which follows the same format as the first group. Essentially, every Selcal will have a XX-XX format, where the X's are the letter codes of the tones transmitted.
The following is a sample group of 16 Selcalls, 4 Selcalls repeated 4 times each. The decoded tones for each selcall is: AB-CD, EF-GH, JK-LM, PQ-RS. This can be used to tune your decoder.
- Hobby Level Software
- Sorcerer in Selcall decoder Tone Selcalls, dropdown menu ICAO
- MultiPSK Listed in Professional Modes as "SELCAL"
- Professional Equipment/Software
- 8867 kHz SP-6 International aeronautical HF radio
- SWL Aeronautical SELCAL
- Aircraft HF radio SELCAL - 6.628 Mhz USB
- SELCAL: See-and-Hear Radio v2.0 HD
- WAVECOM Database: ICAO Selcal
- Tone Signaling Charts
- Wikipedia Selcal
- Connect Systems Incorporated Programming Manual AVCALL / SELCALL
- Skybrary SELCAL info