Growth of SELCAL
While SELCAL is an older technology by aviation standards, demand for SELCAL codes continues to increase by approximately 4% annually. This demand is being driven by the expanding civil aviation market and the requirement for almost all aircraft flying in remote regions to have HF communications. The resulting growth in the last decade has now oversubscribed the ICAO SELCAL register maintained by ASRI. The current database totals over 35,000 individual SELCAL code assignments while still increasing at rate of 3000 registrations a year. Given the 16 tone SELCAL system is limited to only 10,920 unique codes, this means multiple duplicate codes have been issued to different aircraft for what is supposed to be a unique identifier. While the ICAO Registrar (ASRI) attempts to mitigate the issue of duplicate SELCAL codes, the situation is expected to deteriorate with the growing number of airframes worldwide.
In one study by ASRI in 2012, a single ANSP ground station reported 266 occurrences of multiple aircraft responding to the same transmitted SELCAL code. Furthermore, several incidents have been reported with up to 6 different aircraft answering to a single transmission for what is supposed to be a unique assignment.
A second ANSP with a Flight Information Region (FIR) covering the NAT recorded duplicate SELCAL events listed below over the last five years.
| DATE | 2012 | 2013 | 2014 | 2015 | 2016 |
| TOTAL Flights | 400,480 | 404,787 | 420,423 | 442,645 | 470,000 |
| Recorded Duplicate SELCAL Events | 90 | 100 | 181 | 250 | 280 |
As the ICAO nominated registrar, ASRI is concerned that the duplication of codes is creating some potential human factors risk for aircrews and additional workload on service providers. Aviation HF and VHF voice continues to be used extensively world-wide and will be a backup communications media for the foreseeable future. The ICAO Global Air Navigation Plan (GANP) 5th edition has planned that HF systems will be operational beyond 2030. Expected HF usage has already been extended several times over the years due to existing equipage and demand for use from aircraft operators and ANSPs.
What is SELCAL 32?
To mitigate duplicate SELCAL code concerns, ASRI has been attempting to educate users about the potential duplication issue, while radio ground station operators have been proactively managing aircraft to minimize the issue. However, these measures can only mitigate the duplication issue so far, and therefore ASRI in cooperation with the affected Civil Aviation Authorities (CAAs), ANSPs, and avionics manufacturers, has been updating the SELCAL standards to allow for aviation growth. The new SELCAL proposal uses sixteen (16) new audio tones, in addition to the existing 16 audio tones, to create SELCAL codes from a total of 32 available audio frequencies (called SELCAL 32).
The updated standard labels the new audio frequencies with letters from T to Z, and the numbers 1 to 9. Creating SELCAL codes from these 32 different audio tones will result in up to 200,000 new unique SELCAL codes being available for assignment, mitigating the duplicate code issue for the foreseeable future.
The SELCAL 32 proposal will allow for new SELCAL avionics to operate without an increasing risk of SELCAL code duplication and the associated operational impacts. As this solution uses the existing 16 audio tones, it is backwards compatible with all existing aircraft avionics, while providing benefits to new aircraft with the updated SELCAL 32 standard. With growing support in the international aviation community, the new SELCAL 32 standard will eventually allow for continued operation of the SELCAL system to meet the increasing worldwide demand from aviation.