1. Beacon Activation
Emergency beacons (EPIRBs, ELTs, PLBs) transmit distress signals at 406.025 MHz when activated manually or automatically.
gr-sarsat-modern
Decode the signals that save lives. GNU Radio blocks for receiving Cospas-Sarsat 406 MHz emergency distress beacons.
60,000+ lives saved since 1982
How the System Works
The Cospas-Sarsat system detects distress signals from emergency beacons and relays location data to rescue coordination centers worldwide.
2. Satellite Detection
LEOSAR, MEOSAR, and GEOSAR satellites receive the 406 MHz signal and relay it to ground stations.
3. Position Calculation
Local User Terminals process the signal, calculate position via Doppler shift or GPS data embedded in the message.
4. Rescue Dispatch
Mission Control Centers alert the appropriate Rescue Coordination Center, which dispatches SAR assets.
What This Project Decodes
| Beacon Type | Use Case | Activation |
|---|---|---|
| EPIRB | Maritime vessels | Float-free or manual |
| ELT | Aircraft | Crash-activated or manual |
| PLB | Personal/hikers | Manual only |
| SSAS | Ship security alerts | Covert manual |
The Signal Chain
gr-sarsat-modern provides three GNU Radio blocks that form a complete decoder:
Biphase-L Decoder
Decodes Manchester/Biphase-L encoded symbols with 2:1 decimation. Handles the 400 bps baseband signal.
PDS Frame Sync
Detects the 24-bit frame synchronization pattern and extracts 72-byte Processed Digital Signals frames.
SARP Extractor
Splits PDS frames into 24-byte SARP messages, validates sync words, and outputs decoded beacon data.
Signal Specifications
| Parameter | Value |
|---|---|
| Frequency | 406.025 MHz (±2 kHz) |
| Modulation | Phase modulation |
| Data Rate | 400 bps ±1% |
| Power | 5W (+37 dBm) typical |
| Burst Duration | 440ms (short) / 520ms (long) |
- Line Code: Biphase-L (Manchester)
- Error Correction: BCH(82,61) + BCH(38,26)
- Message Types: Short (112 bits) / Long (144 bits)
- Protocol Types: User, Location, RLS, ELT(DT)
Message Structure
Cospas-Sarsat beacons transmit in two formats: Short (112 bits) for basic identification and Long (144 bits) with encoded location data.
Short Message (112 bits)
Long Message (144 bits)
PDF-1 Field Breakdown
The first Protected Data Field contains the beacon’s identity — country code, protocol type, and unique identifier.
Quick Links
Quick Start Install and receive your first beacon signal in 5 minutes
Signal Chain Tutorial Build a complete decoder flowgraph step-by-step
Block Reference Complete API documentation for all blocks
Official Cospas-Sarsat Docs Official specifications from the international programme
First Cospas-Sarsat rescue — September 1982, Canada