What is gr-apollo?

The Apollo Unified S-Band (USB) system was the backbone of all communications between ground stations and Apollo spacecraft from 1967 through 1975. A single 2287.5 MHz carrier handled everything: PCM telemetry, astronaut voice, ranging, and command uplinks. gr-apollo decodes these signals using modern software-defined radio.

What gr-apollo does

gr-apollo provides a set of GNU Radio blocks and pure-Python engines that implement the full Apollo USB downlink demodulation chain. Feed it complex baseband samples and it produces decoded telemetry frames, voice audio, and AGC computer data.

PCM Telemetry

128-word frames at 51.2 kbps with 32-bit sync correlation, A/D voltage scaling, and automatic complement-on-odd frame detection.

Voice Audio

1.25 MHz FM subcarrier demodulated to 300—3000 Hz audio at 8 kHz output rate. Listen to astronaut comms.

Virtual AGC Bridge

TCP bridge to the Virtual AGC emulator. Send uplink commands, receive decoded AGC downlink lists (navigation state, autopilot data, DSKY display).

Test Signal Generator

Synthetic USB baseband generator with configurable SNR, known payloads, and voice. No RF hardware needed to develop and test.

Signal chain overview

The downlink receiver processes a phase-modulated carrier through five stages. Each stage has a standalone GNU Radio block, or you can use usb_downlink_receiver which chains them together.

flowchart TB
    A["Complex<br/>Baseband<br/>5.12 MHz"] --> B["PM Demod<br/><em>Carrier PLL +<br/>phase extraction</em>"]
    B --> C["Subcarrier<br/>Extract<br/><em>BPF + translate<br/>1.024 MHz → DC</em>"]
    C --> D["BPSK Demod<br/><em>Costas loop +<br/>symbol sync</em>"]
    D --> E["Frame Sync<br/><em>32-bit correlator<br/>SEARCH → LOCKED</em>"]
    E --> F["PCM Demux<br/><em>128-word frames<br/>+ AGC channels</em>"]

    B --> V["Voice<br/>Extract<br/><em>1.25 MHz FM</em>"]
    V --> VA["Audio<br/>8 kHz"]
    F --> T["Telemetry<br/>PDUs"]
    F --> AG["AGC Data<br/>Ch 34/35/57"]

Key parameters

Parameter	Value
Downlink frequency	2287.5 MHz
PM peak deviation	0.133 rad (7.6 deg)
PCM subcarrier	1.024 MHz, BPSK
PCM bit rate	51.2 kbps (high) / 1.6 kbps (low)
Voice subcarrier	1.25 MHz, FM +/-29 kHz
Frame size	128 words x 8 bits = 1024 bits
Frame rate	50 fps (high rate)
Master clock	512 kHz
Baseband sample rate	5.12 MHz

Two ways to use it

gr-apollo splits cleanly into two layers:

Pure-Python engines (no GNU Radio required) — the signal generator, frame sync engine, demux engine, downlink decoder, and AGC bridge client all work standalone. Use these for scripting, testing, or integration with other tools.

GNU Radio blocks (require gnuradio runtime) — each engine is wrapped in a GR block with proper streaming I/O and message ports. Use these in GRC flowgraphs or programmatic top_block scripts.

# Pure Python — always available
from apollo import generate_usb_baseband, FrameSyncEngine, DemuxEngine

# GNU Radio blocks — require gnuradio installed
from apollo import pm_demod, bpsk_demod, pcm_frame_sync, usb_downlink_receiver

What you will need

• Python 3.10+
• uv (for dependency management and running)
• NumPy (installed automatically)
• GNU Radio 3.10+ (only for the GR blocks — the pure-Python engines work without it)

Tip

You can use the signal generator and frame processing engines without GNU Radio installed. This is useful for offline analysis, unit testing, or environments where GR is hard to install.

References

gr-apollo is based on these primary sources:

• NAA Telecommunication Systems Study Guide (Course A-624, 1965) — the original North American Aviation training manual for the Apollo USB system. Defines all frequencies, modulation parameters, frame formats, and SCO specifications used in gr-apollo.
• Virtual AGC Project — Ron Burkey’s Apollo Guidance Computer emulator and document archive. The AGC bridge protocol and downlink list decoding are based on the yaAGC source code.
• NASA Technical Reports Server — primary source for Apollo spacecraft telecommunications system descriptions and MSFN ground station specifications.

Community

• CuriousMarc (Marc Verdiell) — Marc and his team (Mike Stewart, Ken Shirriff, and others) have been restoring and operating original Apollo S-Band flight hardware. Their YouTube channel documents the restoration process in detail — see Building an Apollo transmit station with Keysight instruments for a look at how modern test equipment talks to 1960s flight hardware.
• Ken Shirriff’s Blog — detailed reverse-engineering posts on Apollo-era hardware, from AGC core rope memory to the USB signal processing chains.

Next step

Installation Install gr-apollo and verify everything works.