Also known as: Pactor, PACTOR
Pactor (styled PACTOR, from the Latin pactor, “one who makes a contract”) is a family of HF radio data modes designed to deliver error-free data over fading, noisy shortwave paths. It pairs phase-shift-keying (and, in later versions, QAM) with an ARQ (Automatic Repeat reQuest) handshake and adaptive speed, so the two stations continuously renegotiate rate and re-send any block that fails its checksum.1 It is the classic data mode behind Winlink radio email and marine/HF messaging.
Overview
Pactor is a synchronous, half-duplex master/slave protocol. The calling station becomes master and sends fixed-length data frames; the other end replies each cycle with a short control burst that acknowledges good frames and reports link quality. A defining trick is memory ARQ: rather than discarding a corrupt frame, the receiver stores the soft samples and combines them with later retransmissions, so several marginal copies add up to a correct decode. The modem also adapts its speed level in real time — dropping to slower, more robust signalling as conditions worsen and climbing back as they improve.
Technical characteristics
| Property | Value |
|---|---|
| Access | ARQ, half-duplex master/slave |
| Pactor-I | 100/200 baud FSK, ~200 bps, open spec |
| Pactor-II | DPSK in ~500 Hz, memory ARQ, up to ~800 bps |
| Pactor-III | Multi-carrier PSK in ~2.4 kHz, up to ~5.2 kbps |
| Pactor-IV | Adaptive PSK/QAM, up to ~10+ kbps |
| Error control | CRC per frame + memory ARQ + FEC |
| Carrier | SSB audio, HF |
History
Pactor-I appeared in 1991 as an amateur mode blending the best of AMTOR and packet radio, and its specification is open. From Pactor-II (1994) onward the modes were developed and sold by the German firm SCS as proprietary hardware modems, adding DPSK, memory ARQ, and strong FEC; Pactor-III (2002) widened the signal for higher throughput, and Pactor-IV (2013) added adaptive QAM.1 The proprietary nature of II–IV means legal transmission generally requires an SCS modem.
Deployment
Pactor is used for amateur HF email via Winlink, by sailors and expeditions for shore messaging, and historically by some commercial and government HF links needing reliable low-rate data. Because the later versions are proprietary and (in the US) limited by symbol-rate and bandwidth rules, VARA has taken over much amateur traffic, but Pactor remains valued for its robustness.
Decoding it with GopherTrunk
GopherTrunk does not decode Pactor. Pactor-II/III/IV are proprietary SCS modes with no open decoder, and even Pactor-I is an HF amateur mode outside GopherTrunk’s land-mobile trunking scope. GopherTrunk implements the general building blocks Pactor draws on — PSK demodulation and FEC — but not this protocol’s framing or its ARQ state machine.