Selective Calling System (SELCAL)

The Selective Calling System (SELCAL) is a specialized signaling method used in aviation communication. It allows a ground station to alert a specific aircraft that a message is intended for its crew. Instead of requiring pilots to continuously monitor noisy communication channels, SELCAL provides a selective audio-visual alert when ground personnel wish to establish contact.

This system has been particularly valuable in long-haul operations, where High Frequency (HF) communication remains necessary, and where background noise and frequency congestion make continuous monitoring impractical.

Selective Calling System (SELCAL)

Definition

Selective Calling (SELCAL):
A system that enables ground stations to transmit a coded signal to an aircraft, triggering an onboard alert (light, chime, or both), indicating that the ground station wishes to communicate with that aircraft.

• Source of Codes: SELCAL codes are assigned by Aviation Spectrum Resources, Inc. (ASRI).
• Registration Basis: Codes are assigned to operators, not individual aircraft.

Description and Function

How SELCAL Works

1. Ground Transmission:
   • A ground station uses a SELCAL encoder to transmit a signal composed of four distinct audio tones.
   • The transmission lasts approximately two seconds.
2. Aircraft Reception:
   • The aircraft’s SELCAL decoder monitors the audio output of the receiver.
   • If the transmitted four-tone sequence matches the aircraft’s assigned SELCAL code, the system triggers a cockpit alert.
3. Flight Deck Alert:
   • The alert can be:
     • A visual signal (light),
     • An audible chime,
     • Or a combination of both.
   • The crew is then aware that a message is incoming and can establish two-way communication on the appropriate frequency.

Operational Benefits

• Reduced Pilot Workload: Relieves the flight crew from maintaining a constant listening watch on noisy HF or congested VHF channels.
• Safety Enhancement: Minimizes the risk of missed calls in high-noise environments.
• Efficiency: Frees crews to focus on cockpit tasks without distraction, while still remaining accessible to Air Traffic Control (ATC).

Frequency and Modes

• Transmission Mediums:
  • VHF (Very High Frequency)
  • HF (High Frequency) – the most common, especially in oceanic and remote operations.
• HF Mode Considerations:
  • SELCAL signals are transmitted in full carrier mode, not in HF Single Sideband (SSB) suppressed carrier mode.
  • Many aircraft HF SSB transceivers are therefore designed to detect SELCAL signals in full carrier mode, even if the switch is set to suppressed carrier.
  • Transceivers without this feature require manual switching:
    • Full carrier mode for SELCAL reception.
    • Suppressed carrier mode for voice communication.

Code Assignment

• Assignment Process:
  • SELCAL codes are assigned by ASRI on behalf of ICAO.
  • Codes are registered to an aircraft operator rather than to an individual aircraft.
  • The assigned SELCAL code must be entered in Field 18 of the ICAO flight plan, using the SEL/ indicator.
• Purpose:
  • Provides ground stations a unique method to identify and contact a particular operator’s aircraft.
  • Reduces unnecessary transmission and avoids disturbing other flights.

SELCAL Code Structure

Early Systems (12-Tone Basis)

• Originally, SELCAL was based on 12 available tones.
• Four tones were combined in pairs to generate unique SELCAL codes.
• Due to limited combinations, codes were shared among multiple operators worldwide.
• To reduce duplication conflicts, ASRI attempted to allocate codes by geographical region of operation.

Modern Systems (16-Tone Basis)

• Newer SELCAL units use 16 available tones, providing a larger pool of unique code combinations.
• However, codes are still shared globally, as demand has outpaced the number of available codes.
• Despite this expansion, there are no current ICAO plans to increase tone numbers further.

Limitations and Challenges

1. Duplicate Codes:
   • With limited codes, it is not uncommon for two aircraft operating in the same geographical region to share the same SELCAL identification.
   • This can cause confusion if both crews respond to the same alert.
2. Mitigation:
   • Crews are trained to verify not just the SELCAL alert but also the Flight Identification (call sign) that follows.
   • This ensures they only act on messages intended for their flight.
3. HF Reliance:
   • While SELCAL reduces the need for constant HF monitoring, its usefulness is tied to regions where HF communication remains essential (e.g., transoceanic and polar routes).
   • With modern systems such as SATCOM and CPDLC (Controller–Pilot Data Link Communications), SELCAL’s role is gradually reducing in some areas.

Applications

• Long-Haul International Operations
  • Especially on transoceanic flights, where HF remains a critical means of communication.
• Polar Routes
  • Where satellite coverage may be limited, and HF communications continue to be relied upon.
• Maritime and Remote Area Aviation
  • SELCAL provides efficient communication management in regions with sparse ATC infrastructure.

Future Outlook

• Integration with Modern Systems:
  • As datalink systems such as CPDLC expand, reliance on SELCAL may decline.
  • However, it remains a low-cost, robust backup for global operators.
• Code Pool Limitations:
  • The current pool of SELCAL codes is nearly exhausted, even with 16 tones.
  • Future solutions may involve:
    • Expanding the tone set further.
    • Transitioning to digital addressing systems.
    • Greater reliance on satellite-based communication networks.

Key Takeaways

• SELCAL is a selective signaling system that alerts a specific aircraft when ground stations wish to communicate.
• It reduces pilot workload by eliminating the need for continuous frequency monitoring, especially on HF channels.
• Codes are globally managed by ASRI, assigned to operators, and filed in the flight plan.
• Older 12-tone systems and newer 16-tone systems coexist, but both face challenges due to limited code availability.
• Flight crews must always verify Flight ID alongside SELCAL alerts to prevent miscommunication.
• While modern datalink technologies may overshadow SELCAL in the future, it remains a vital, reliable, and cost-effective communication tool for long-haul and remote aviation operations.