Technology Overview
The ACARS Landscape
ACARS (Aircraft Communications Addressing and Reporting System) has been the backbone of digital air-ground communication since 1978. Originally developed by ARINC as a simple way to automate routine pilot-to-airline communications, ACARS has evolved into a diverse ecosystem of transmission mediums, protocols, and applications that touch nearly every aspect of modern aviation operations.
Transmission Mediums at a Glance
| Medium | Frequency | Range | Decoder | Difficulty |
|---|---|---|---|---|
| VHF ACARS (POA) | 129–131 MHz | Line-of-sight (~200 nm) | acarsdec | Beginner |
| VDL Mode 2 | 136–137 MHz | Line-of-sight (~200 nm) | dumpvdl2 | Beginner |
| HFDL | 2–22 MHz | Thousands of miles | dumphfdl | Intermediate |
| Inmarsat AERO (L-Band) | ~1.5 GHz | Geostationary coverage | JAERO | Advanced |
| Inmarsat AERO (C-Band) | ~3.4–4.2 GHz | Geostationary coverage | JAERO | Expert |
| Iridium (AoI) | ~1.6 GHz | Global (incl. polar) | iridium-toolkit | Advanced |
Transmission Mediums
ACARS data doesn't travel over a single radio link. It spans multiple transmission mediums, each with different characteristics, coverage areas, and use cases:
- VHF ACARS (Plain Old ACARS / POA) — The original and most common form. Operates on VHF frequencies (primarily 129.125 MHz and 131.550 MHz) with line-of-sight range. This is what most hobbyists start with.
- VDL (VHF Data Link) — A digital evolution of VHF ACARS with higher throughput and efficiency. VDL Mode 2 is the most widely deployed, operating in the 118–136.975 MHz range.
- HFDL (HF Data Link) — Uses High Frequency radio for long-range communication, particularly over oceans and polar regions where VHF coverage doesn't reach. A network of ground stations worldwide provides global coverage.
- SATCOM (Satellite Communications) — Encompasses multiple satellite-based systems:
- Inmarsat AERO (L-Band and C-Band) — Geostationary satellite-based ACARS, providing coverage over oceans and remote areas via the Inmarsat constellation.
- Iridium (ACARS over Iridium / AoI) — Uses the Iridium LEO satellite constellation for true global coverage, including polar regions.
Service Providers
Two major data link service providers manage the global ACARS network infrastructure:
- ARINC (now Collins Aerospace / RTX) — The original ACARS developer and primary service provider, particularly in the Americas.
- SITA — The other major global provider, with strong presence in Europe and Asia-Pacific.
Modern Evolution
The ACARS landscape continues to evolve:
- ACARS over IP (AoIP) — The newest transport option, leveraging broadband cellular connectivity on the ground and IP-capable SATCOM in the air. This addresses growing data volume demands as next-generation aircraft generate up to four times the ACARS data of their predecessors.
- FANS (Future Air Navigation System) — Enables direct controller-pilot datalink communications (CPDLC) and ADS-C (Automatic Dependent Surveillance - Contract) position reporting over ACARS, critical for oceanic and remote airspace management.
- Integration with NextGen/SESAR — ACARS is being integrated with next-generation air traffic management systems for increased automation and real-time analytics.
What Airframes Captures
Airframes aggregates data across all of these transmission mediums. Our feeder network uses software-defined radios (SDRs) to receive signals that are then decoded by clients like acarsdec, dumpvdl2, dumphfdl, JAERO, and others. This decoded data is sent to the Airframes aggregator for processing, storage, and distribution.
For a deeper look at each technology, see the individual sections on ACARS, VDL, HFDL, and SATCOM.
Start with VHF ACARS or VDL2 — they need only an inexpensive RTL-SDR and a simple antenna. See Getting Started or Receiver Systems for guidance on choosing your first setup.