PACSAT
A PACSAT is a packet-radio file server in orbit. You upload a message on one pass and it is broadcast to every station under the footprint on the next — no internet, no infrastructure, just a radio and a view of the sky. AMSAT has flown this idea since 1990, and the protocol is being carried forward onto the next generation of spacecraft.
One downlink, many uplinks — everyone listens
Rather than serving each station a private copy, a PACSAT sends a file once and every station in view captures the frames it is missing. Requests go up; the broadcast comes down for all.
Ground station
You transmit a directory or file request on one of several shared uplink channels using an FM radio and a TNC.
Spacecraft
A multitasking flight computer runs the mailbox and an AX.25 driver in parallel, holding files in memory and broadcasting them to the whole footprint.
Thirty-five years of digital birds
The store-and-forward tradition runs from the first Microsats through the FalconSat era to today’s experimental spacecraft prototyping a file-based next generation.
The original PACSAT — a 9-inch Microsat cube with 10 MB of static RAM, launched as a dedicated file server in space.
USAFA spacecraft opened to amateurs, running the PACSAT broadcast protocol and serving as the reference target for modern ground software.
Mauritius’ first satellite carried PACSAT service, extending protocol support to a new generation of operators.
An experimental spacecraft prototypes file-based store-and-forward for the coming generation of AMSAT digital payloads.
What comes down the pipe
Personal mail
Point-to-point messages using the satellite as an endpoint, entered directly or pre-formatted for fast unattended upload.
Bulletins
Items of general interest — AMSAT News, orbit predictions, gateway traffic — sent once for reception by many.
Realtime telemetry
Current spacecraft values: solar-array power, internal temperatures and bus status, framed to stand alone.
Whole-orbit data
Telemetry logged over time — an entire orbit of readings, downloaded as a file and forwarded on the ground.
Ground station software
Open, cross-platform tools decode the PACSAT protocol: request the directory, ask for files to be broadcast, and forward telemetry — on Windows, Linux, or macOS.
PacSat Ground Station
The reference decoder for the PACSAT protocol, originally built for FalconSat-3 and extended toward future store-and-forward satellites.
1200 bps BPSK
Coherent phase detection with Doppler-tracking AFC recovers the downlink well into the noise — a soft-modem or classic hardware TNC.
Native clients
Community efforts are moving the ground station toward native builds that stay responsive on Raspberry Pi–class hardware.
You can work a PACSAT with simple equipment
The barrier is low by design: a dual-band radio for the uplink, a 70 cm SSB receiver for the downlink, a TNC or sound-card modem, and free ground software. Point an antenna at a pass and start capturing the directory.
New to digital satellites? Start by just receiving — decode the beacon and watch the file directory scroll before you ever key up.
- 2 m FM transmitter for the uplink
- 70 cm SSB receiver for the downlink
- TNC or sound-card BPSK modem
- PacSat ground-station software
- Current Keplerian elements & a pass predictor
- A clear view of the sky
PACSAT store-and-forward has flown on AMSAT spacecraft since 1990. For current digital-satellite status and frequencies, see the live status page.