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What is Sigint?

Sigint is a self-contained radio monitoring system that receives, demodulates, and decodes digital and analog radio signals in real time. It runs headless on a Raspberry Pi 5, serving a dark-themed web dashboard you can access from any browser on your network.

One RTL-SDR dongle. Seven protocol modes. Zero cloud dependencies.

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Key Features

7 Demodulation Modes — Hot-Swappable at Runtime

Switch between protocols without restarting the flowgraph. The GNU Radio signal chain locks, reconfigures, and unlocks in milliseconds.

Mode	Modulation	Notes
TETRA	π/4-DQPSK	Full L2/L3 decode via tetra-kit — voice, SDS, ISSI, cell info
DMR	4FSK	Tier II/III, 12.5 kHz TDMA, 2 timeslots
P25	C4FM / CQPSK	Phase I & II, IMBE / AMBE+2
NXDN	4FSK	6.25/12.5 kHz FDMA (Kenwood/Icom)
dPMR	4FSK	6.25 kHz FDMA, ETSI TS 102 658
Analog FM	NBFM	Direct audio demod, ±5 kHz
Analog AM	Envelope	Airband / HF / wideband

Frequency Scanner

An async scan engine cycles through a user-defined channel list, locking on any signal that breaks the squelch threshold. Scanner controls live directly in the dashboard topbar — start, stop, skip, and quick-save channels with one click.

Live Spectrum & Waterfall

A 256-point FFT with Hamming windowing streams at 15 fps over WebSocket. The dashboard renders both a spectrum trace and a scrolling waterfall using pure Canvas — no chart libraries.

In-Browser Audio

Decoded audio streams over WebSocket as raw PCM (16 kHz, 16-bit). The Web Audio API plays it back with adjustable volume and one-click WAV recording.

Event Database

Every decoded event — registrations, calls, SDS messages — is stored in SQLite via aiosqlite. The REST API serves them with filtering and pagination, and the dashboard displays them in a live-scrolling event stream.

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Architecture

Three systemd services coordinate via UDP on localhost:

tetra-sdr — GNU Radio flowgraph (tetra_rx_headless.py)

• Reads IQ samples from the RTL-SDR at 2 MS/s
• Frequency-translates and decimates to 36 kHz
• Routes through the active demodulator
• Emits symbols (UDP :42000), audio (UDP :42002), FFT (UDP :42003)
• Accepts tuning commands via XML-RPC (:42001)
• Exposes get_signal_level() for the scanner

tetra-decoder — Protocol decoder

• Consumes symbol stream on UDP :42000
• Runs tetra-kit for L2/L3 TETRA decoding
• Outputs structured JSON frames to UDP :42100

tetra-api — FastAPI application server (sigint-api-main.py)

• Ingests UDP streams (audio, FFT, decoded frames)
• Serves REST API + WebSocket streams on port 80
• Runs the async scan engine with XML-RPC control
• Hosts the single-file HTML dashboard
• Persists events to SQLite

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Quick Start

Hardware

• Raspberry Pi 5 (4 GB+ recommended)
• RTL-SDR v3/v4 (RTL2832U + R820T2)
• Appropriate antenna for your target band

Prerequisites

# On the Raspberry Pi (Debian 12+ / Ubuntu 24.04+)
sudo apt install gnuradio gr-osmosdr python3-numpy python3-aiosqlite python3-fastapi python3-uvicorn

Deploy

# Clone the repo
git clone https://github.com/petermartis/sigint.git
cd sigint

# Copy files to the Pi
scp tetra_rx_headless.py sigint-api-main.py sigint-ingestor.py sigint-ui.html admin@<PI_IP>:/opt/tetra-scanner/

# On the Pi — set up the config
sudo mkdir -p /opt/tetra-scanner/etc
cat <<'EOF' | sudo tee /opt/tetra-scanner/etc/tetra-scanner.conf
[radio]
frequency = 434000000
gain = 49.6
ppm = 0
mode = tetra

[scanner]
enabled = false
squelch = -50
dwell = 2.0
EOF

Systemd Services

Create three service units (see SIGINT-README.md for full service file contents):

sudo systemctl enable --now tetra-sdr tetra-decoder tetra-api

Open the Dashboard

Point your browser to:

http://<PI_IP>/

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How to Use

Tuning

Click the frequency display in the topbar or use the Settings panel to enter a frequency. The flowgraph retunes instantly via XML-RPC.

Switching Modes

Select a protocol from the mode dropdown. The GNU Radio chain hot-swaps demodulator blocks without restarting — the lock/unlock cycle takes milliseconds.

Using the Scanner

1. Add channels — Click + CH in the topbar to quick-save the current frequency, or open Settings → Scanner to manage the full channel list.
2. Start scanning — Click Scan. The engine cycles through channels, pausing on any signal above the squelch threshold.
3. Skip — Click Skip to move to the next channel immediately.
4. Configure — Adjust squelch level (dBFS) and dwell time (seconds) in Settings → Scanner.

Monitoring Audio

Click the speaker icon to unmute. Audio streams in real time via WebSocket → Web Audio API. Hit the record button to capture a WAV file directly in the browser.

Reviewing Events

The event stream panel shows decoded events as they arrive — registrations, group/individual calls, SDS messages, system info. Each event type is color-coded. Historical events are available via the REST API:

# Get the latest 50 events
curl http://<PI_IP>/api/events?limit=50

# Get SDS messages
curl http://<PI_IP>/api/sds

# Get active calls
curl http://<PI_IP>/api/calls

REST API

The full API is documented at http://<PI_IP>/docs (auto-generated Swagger UI).

Key endpoints:

• GET /api/radio — Current radio state (frequency, mode, gain, signal level)
• POST /api/tune — Set frequency
• POST /api/mode — Switch demodulation mode
• GET /api/events — Paginated event history
• GET /api/scan/status — Scanner state
• POST /api/scan/start — Start scanning
• POST /api/scan/stop — Stop scanning
• GET /api/scan/channels — List saved channels
• POST /api/scan/channels — Add a channel
• DELETE /api/scan/channels/{id} — Remove a channel

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Tech Stack

Layer	Technology
SDR Hardware	RTL-SDR v3/v4 (RTL2832U + R820T2)
Signal Processing	GNU Radio 3.10, gr-osmosdr
Protocol Decoder	tetra-kit (TETRA L2/L3)
API Server	Python 3.13, FastAPI, uvicorn, aiosqlite
Transport	UDP (symbols, audio, FFT, JSON), XML-RPC, WebSocket
Database	SQLite
Frontend	Single-file HTML/CSS/JS — Canvas, Web Audio API, zero dependencies
Platform	Raspberry Pi 5, Debian 13 (trixie), aarch64

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Project Structure

sigint/
├── tetra_rx_headless.py    # GNU Radio flowgraph (7 demod modes, FFT, XML-RPC, UDP output)
├── sigint-api-main.py      # FastAPI server (REST, WebSocket, scan engine, SQLite)
├── sigint-ingestor.py       # UDP ingestor module (audio, FFT, JSON frame listeners)
├── sigint-ui.html           # Single-file web dashboard (HTML + CSS + JS)
├── SIGINT-README.md         # Full technical reference & deployment guide
└── docs/
    ├── architecture.svg     # System architecture diagram
    ├── dashboard-mockup.svg # Dashboard UI mockup
    └── features.svg         # Protocol & feature overview

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Full Documentation

See SIGINT-README.md for the complete technical reference, including:

• Detailed service configuration
• systemd unit files
• UDP protocol formats
• XML-RPC API reference
• Troubleshooting guide
• Hardware setup notes

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