Also known as: ACARS
ACARS (Aircraft Communications Addressing and Reporting System) is a datalink for short, addressed text messages between aircraft and ground stations, originally over VHF and now also over HF and satellite.1 It carries operational traffic — position and status reports, weather requests, gate assignments, maintenance data, and the automatic OOOI (Out/Off/On/In) timestamps — that would otherwise clog voice frequencies. The classic VHF form (“Plain Old ACARS”, POA) transmits at 2400 bps using minimum-shift keying.
Overview
ACARS predates the modern IP aviation datalinks and remains ubiquitous. A message is a short block of text addressed by aircraft registration and a label indicating its type. The system is character-oriented and human-readable at heart, which is why hobbyists can decode plain ACARS with a simple VHF receiver and software. Higher-rate successors — VDL Mode 2 on VHF and satcom links — carry the same applications when more throughput is needed.
Technical characteristics
| Property | Value |
|---|---|
| Primary VHF channel | 131.550 MHz (plus 130.025, 131.725, others) |
| VHF modulation | MSK, 2400 bps, over an AM carrier |
| Message size | Short text blocks (up to ~220 characters) |
| Addressing | Aircraft registration + label + block ID |
| Integrity | Block check character / CRC |
| Bearers | VHF (POA), VDL Mode 2, HFDL, Inmarsat/Iridium satcom |
| Applications | OOOI, position, weather, free text, maintenance, ADS-C, CPDLC |
Plain Old ACARS keys minimum-shift keying onto an AM carrier at 2400 bps — a constant-envelope scheme that is spectrum-thrifty and easy to demodulate. Each transmission is a self-contained, acknowledged block; the ground network routes it to the relevant airline or service provider.
History
ARINC introduced ACARS in 1978 to automate the flight crew’s manual position and status reporting. It steadily absorbed new applications through the 1980s and 1990s and became the transport for FANS applications such as ADS-C and CPDLC over oceanic satcom and HF, extending its reach far beyond its VHF origins.
Deployment
ACARS is carried by virtually every airline aircraft worldwide and is one of the most popular aviation signals for SDR hobbyists precisely because the VHF form is unencrypted and easy to receive. Datalink service providers (ARINC/SITA) operate the ground networks. Modern aircraft use the higher-rate VDL Mode 2 and satcom bearers, but legacy VHF ACARS remains in daily use.
Decoding it with GopherTrunk
Not decoded. GopherTrunk is a land-mobile trunking scanner plus 1090 MHz ADS-B; it does not demodulate the aviation VHF ACARS channels or the HF/satcom bearers. VHF ACARS is very approachable with a cheap SDR and dedicated decoders, and this page is provided as honest context — the underlying modulation, MSK, and the successor links VDL Mode 2 and HFDL have their own pages.