Pactor
| Pactor | |
|---|---|
| Mode type | HF data mode with automatic error correction |
| Developed by | SCS (Special Communications Systems), Germany |
| Versions | Pactor I, II, III and IV |
| First version | Pactor I, early 1990s |
| In use | |
| Equipment | Dedicated hardware modem (later versions) |
| Main uses | Winlink radio email, marine email, emergency messaging |
| Bands | HF |
| Related modes | Packet, AMTOR, VARA |
Pactor is a family of HF data modes designed to move text and files reliably over long-distance radio paths. It was developed in Germany, and its later versions are products of the German company SCS (Special Communications Systems), whose modems remain the only practical way to use them. The name joins packet and AMTOR, the two earlier modes whose ideas it combined. For decades Pactor has been the workhorse of HF email, carrying Winlink messages for hams and email for sailors at sea.
How it works
Pactor is a link between two stations, not a broadcast. Data is sent in blocks, and the receiving station checks each one and immediately tells the sender whether it arrived intact. A damaged block is sent again, a method called ARQ (automatic repeat request), so the text that comes out at the far end is free of errors. Pactor also combines repeated copies of a weak block, rather than throwing failed attempts away, which helps it get through deep fading.
Later versions adapt to conditions. When the path is good the modem shifts to faster modulation; when signals weaken it drops back to slower, tougher settings. Data is also compressed before sending, which makes ordinary text travel much faster than the raw speed suggests.
Versions
| Version | Introduced | Notes |
|---|---|---|
| Pactor I | Early 1990s | Two-tone FSK. Publicly documented, and built into many older multimode controllers |
| Pactor II | Mid-1990s | Two-carrier phase modulation in a narrow channel, about 500 Hz; much faster and more robust |
| Pactor III | Early 2000s | Many carriers across a full voice-width channel, about 2.4 kHz |
| Pactor IV | 2010s | Single-carrier high-speed modulation in a voice-width channel |
Pactor II, III and IV need an SCS modem, such as the PTC series or the Dragon models, which connect to the radio and to a computer. These are dedicated pieces of hardware and cost far more than the sound-card interfaces used by VARA or other software modems. Each new version stays compatible with the older ones, so a modern modem can still connect to a station running an earlier version.
In the US, the fastest version was long kept off the amateur bands by the old FCC limit on symbol rate, the number of signal changes per second, for HF data. In 2024 that limit was replaced by a maximum bandwidth of 2.8 kHz on most HF bands. Check the current Part 97 rules and band plan before operating any wide data mode.
Clover and G-TOR
Two other ARQ modes for HF appeared in the first half of the 1990s, each tied to one maker's hardware. Clover, developed by Ray Petit and sold in data controllers from HAL Communications, kept its signal within 500 Hz and chose automatically among six phase and amplitude modulation formats, reaching up to 750 bits per second on a good path. Its ARQ mode resent only the blocks that its Reed–Solomon error correction could not repair. G-TOR, a Kantronics trademark introduced for its KAM Plus controller in 1994, used FSK like Pactor and packet, at 100, 200 or 300 symbols per second depending on conditions, and combined ARQ with Golay forward error correction. The FCC's data-mode rule in §97.309(a)(4) still names CLOVER, G-TOR and PacTOR as examples of publicly documented techniques that amateurs may use, and the ARRL keeps technical descriptions of Clover and G-TOR on its website.
Uses
- Winlink. Pactor was the main HF transport for Winlink for many years, and many gateway stations still offer it alongside VARA.
- Marine email. Outside amateur radio, services such as SailMail use Pactor on marine HF frequencies to give boats email and weather information far from shore.
- Emergency and expedition use. Relief groups and remote expeditions have used Pactor where satellite links were unavailable or too expensive.
Monitoring controversy
On the amateur bands, Pactor has been the subject of a long argument. Amateur rules require that messages not be hidden from others, and the tradition is that any ham can listen in on any transmission. Critics have argued that the later Pactor versions, with their proprietary modulation and compression, cannot in practice be read by anyone without the right hardware and the right software, which makes enforcement and self-policing difficult. Others have complained about interference from automatically controlled data stations that transmit without hearing a signal already on the frequency.
Supporters respond that an SCS modem can monitor Pactor traffic and that the messages themselves are compressed, not encrypted. Winlink has also provided ways for licensed amateurs to view messages passed through its system. The debate has reached the FCC more than once, and the arguments apply in similar form to other proprietary modes.