The PACSAT Project
A PACSAT is a packet-radio file server in orbit: stations request the directory, browse what’s on board, and download the files they want. AMSAT flew the first PACSAT in 1990 — and is now developing the next one in the open, with the flight software, board designs, and documentation developed openly on GitHub.
How a PACSAT works
A PACSAT can act as a digipeater, but it is also a file server you use over the radio. It can progressively and intelligently download data across multiple passes, making efficient use of bandwidth — and all of it is automated: you do not need to be present when the satellite passes over.
Request the directory
Your station asks the spacecraft what it is holding. The directory lists the messages and files on board.
Browse what’s on board
Ground software shows the directory: bulletins, messages, telemetry files. Pick the ones you want.
Download your files
Request the files you selected — or just the blocks you are missing — and the spacecraft sends them down. Upload your own messages for others to retrieve on later passes.
Four independent uplink receivers on the spacecraft service requests in parallel while a single downlink transmitter answers — the classic PACSAT architecture, rebuilt on modern silicon.
Operating a PACSAT
When a PACSAT flies, working it will look like this: a modest V/u satellite station, packet ground software, and a pass a few times a day. You upload and retrieve messages and files rather than talking through a transponder — store-and-forward lets two stations that never share a footprint exchange traffic. Operation is automated — your station can work a pass while you are away — and digipeater capability lets you make contacts through the satellite as well.
A typical digital-satellite station
- 2 m transmitter for the uplink (144–148 MHz)
- 70 cm receiver for the downlink (430–440 MHz)
- A computer running the PacSat Ground Station software; support for inexpensive SDR receivers is in progress
- Current Keplerian elements and a pass predictor
Exact modes and rates will be published as the design is finalized — the radios support FSK and PSK at speeds up to roughly 100 kb/s, with forward error correction in both directions.
Get ready now
- Work today’s digital satellites to build station skills — see the live digipeater list
- Follow the AMSAT News Service for project milestones
- Check the satellite status page for what’s active right now
- Join AMSAT to support the project
Put a PACSAT board on your spacecraft
The PACSAT board is designed to be more than a payload for one AMSAT mission: it is a complete, open, amateur-band communications and control board that a CubeSat team can integrate. Flying a store-and-forward amateur payload gives your mission a worldwide community of operators with a reason to receive it — and gives amateur radio a new satellite.
Reliable camera-image return
Downlink imagery as files with delay-tolerant, intelligent small-block transfers: a pass ending mid-image costs nothing, because your station asks for just the blocks it is missing on the next one.
Telemetry & whole-orbit data
Log housekeeping from the spacecraft bus to the fail-safe file store, timestamped by the on-board clock, and serve it as files — including whole-orbit data recorded far beyond your own ground station’s view.
Experiment data delivery
Send properly formatted CAN messages and they are stored automatically — in 8 MB of MRAM — for download over the radio. Higher data speeds help experiment partners retrieve more data, more quickly.
A worldwide download network
Files the team makes public can be retrieved by any equipped amateur station, so data return is not limited to the handful of passes a single campus ground station sees each day.
Command & spacecraft control
With an authenticated command uplink and CAN, serial, I2C, ADC, and GPIO interfaces, the board can serve as the spacecraft’s IHU (flight computer) or as a payload board alongside one — one board, two jobs.
A genuine amateur-radio service
A store-and-forward mailbox open to amateurs worldwide is a real contribution to the amateur-satellite service — the kind of mission element IARU frequency coordination looks for in an amateur-band request.
Communications + control in one design
- Single board compatible with the Pumpkin Space CubeSat Kit Bus (PC/104-style form factor)
- Space-proven TI Hercules TMS570 with dual lock-step cores, plus an auxiliary monitoring processor
- Five AX5043 FSK/PSK transceivers: four uplink receivers, one downlink transmitter — full duplex, 144–148 MHz up / 430–440 MHz down, up to ~100 kb/s with FEC
- Around 1 W to the antenna (final PA design pending), adjustable from the ground — throttle to 0.5 W or below to fit a tight power budget
- 8 MB MRAM for the message store and experiment data, plus a real-time clock
Made to fit into a bus
- CAN, serial, I2C, ADC, and GPIO to the rest of the spacecraft, plus dedicated connectors for experiment sensors and a power-switched external device
- Battery and voltage monitoring, with power limiting on every bus
- Optional active/standby pairing: two identical boards with RF switchover and state sync over CAN — few single points of failure
- Optional diplexer for a single dual-band antenna; auxiliary TX/RX connections for external converters to other bands
- Flight software is MIT-licensed; an open hardware license (from the CERN OHL v2 family) is being finalized
Interested in flying one?
The interface control document and design notes are in the
PacSatFSKBoard
repository (docs/ICD.md and docs/Design.md).
If a store-and-forward amateur payload could fit your mission, AMSAT would like
to hear from you — reach out through
amsat.org or open a discussion on GitHub.
- Open KiCad schematics and PCB layout
- Documented bus interfaces (CAN, serial, I2C, ADC, GPIO)
- MIT-licensed software; open hardware licensing in progress
- An operator community ready to use it
Help build it
The project is developed in the open by AMSAT volunteers, and there is work at every layer: embedded C on the flight software, RF and PCB design, assembly and test, ground-station software, and documentation. Development and testing happen on project hardware — the boards carry USB and JTAG interfaces for exactly that — so the best first step is to get in touch and the team will help you get set up.
The flight software: FreeRTOS tasks for the uplink, downlink, file store (Reliance Edge fail-safe file system), and command authentication, built as a TI Code Composer Studio project.
Complete KiCad design for the AX5043-based board: receivers, transmitter, power amplifier, diplexer, antenna control, and active/standby switching, plus the ICD and design documents.
Board-specific configuration folders that let the same flight software build for different targets, including the PACSAT boards, with a script to swap between them.
Project documentation: requirements, hardware references, SDR notes, telemetry definitions, and orientation material for new developers.
Ready to help?
Start by reaching out: contact AMSAT through the
volunteer page or join
the pacsat-dev mailing list, tell the team what you like working
on, and they will help you get set up. Browsing the repositories and the
documents in PacSatDocs is a good way to get oriented first.
- Embedded C flight software (FreeRTOS)
- RF and PCB design (KiCad)
- Assembly, test & integration
- Ground-station software
- Documentation & requirements
- Join the
pacsat-devmailing list
Thirty-five years of digital birds
The store-and-forward tradition runs from the first Microsats through the FalconSat era to today’s open-source rebuild.
The original PACSAT — a 9-inch Microsat cube launched as a dedicated file server in space.
USAFA spacecraft opened to amateurs, running the PACSAT protocol and serving as the reference for modern ground software.
Mauritius’ first satellite carried PACSAT service, extending protocol support to a new generation of operators.
A ground-up open-source rebuild — new board, new flight software — with heritage from AMSAT’s GOLF and ASCENT engineering programs.
For current digital-satellite status and frequencies, see the live status page.