# gr-sarsat-modern

GNU Radio 3.10+ blocks for decoding **Cospas-Sarsat** 406 MHz emergency distress beacons.

Ported from [zleffke/gr-sarsat](https://github.com/zleffke/gr-sarsat) (2018, GNU Radio 3.7).

## What is Cospas-Sarsat?

**Cospas-Sarsat** is the international satellite-based search and rescue (SAR) system that has helped save over **60,000 lives** since 1982. It detects and locates emergency beacons carried by:

- **EPIRBs** (Emergency Position Indicating Radio Beacons) - Maritime
- **ELTs** (Emergency Locator Transmitters) - Aviation
- **PLBs** (Personal Locator Beacons) - Hikers, adventurers

When activated, these beacons transmit a 406 MHz signal that is picked up by a constellation of satellites and relayed to ground stations (LUTs) for position determination.

## The Satellite Constellation

Cospas-Sarsat uses **three types of satellites** to provide global coverage:

### LEOSAR (Low Earth Orbit)
| Satellite | Operator | Orbit |
|-----------|----------|-------|
| NOAA-15, 18, 19 | USA | ~850 km polar |
| MetOp-A, B, C | EUMETSAT | ~817 km polar |
| COSMOS (Sarsat-10,11,12) | Russia | ~1000 km polar |

LEO satellites can **Doppler-locate** beacons (no GPS needed) but only see a portion of Earth at any time.

### MEOSAR (Medium Earth Orbit)
| Constellation | Satellites | Coverage |
|---------------|------------|----------|
| GPS | Block IIF/III (18 SAR payloads) | Global |
| Galileo | 24 satellites | Global |
| GLONASS | 24 satellites | Global |

MEOSAR provides **near-instantaneous global coverage** and can locate GPS-equipped beacons within minutes.

### GEOSAR (Geostationary)
| Satellite | Position | Coverage |
|-----------|----------|----------|
| GOES-16/17 | 75°W / 137°W | Americas |
| MSG (Meteosat) | 0° / 41.5°E | Europe/Africa |
| INSAT-3D/3DR | 82°E / 74°E | Indian Ocean |
| MTSAT/Himawari | 140°E | Asia-Pacific |

GEO satellites provide **immediate alerting** but cannot determine position (beacon must have GPS).

## Signal Specifications

| Parameter | Value |
|-----------|-------|
| **Frequency** | 406.025 MHz (±1 kHz) |
| **Modulation** | Phase modulation, Biphase-L encoded |
| **Bit Rate** | 400 bps |
| **Power** | 5W (37 dBm) minimum |
| **Message Length** | 112 or 144 bits |
| **Burst Duration** | ~440 ms every 50 seconds |

### Frame Structure

```
┌─────────────┬──────────────────────────────────────────────────────┐
│ Frame Sync  │                    PDS Frame (576 bits)              │
│  15 bits    │                                                      │
│   0x7FF     ├──────────────────┬──────────────────┬────────────────┤
│             │  SARP Msg 1      │  SARP Msg 2      │  SARP Msg 3    │
│             │  (24 bytes)      │  (24 bytes)      │  (24 bytes)    │
│             │  Sync: 0xD60     │  Sync: 0xD60     │  Sync: 0xD60   │
└─────────────┴──────────────────┴──────────────────┴────────────────┘
```

## Blocks

| Block | Type | Description |
|-------|------|-------------|
| **Biphase-L Decoder** | `decim_block` | Decodes Manchester/Biphase-L symbols (2:1 decimation) |
| **PDS Frame Sync** | `sync_block` | Detects frame sync, extracts 72-byte PDS frames |
| **SARP Message Extractor** | `basic_block` | Splits frames into 24-byte SARP messages, validates sync |

## Installation

```bash
# Clone
git clone https://git.supported.systems/rf/gr-sarsat-modern.git
cd gr-sarsat-modern

# Install Python package
pip install -e .

# Install GRC blocks
cp grc/*.yml ~/.local/share/gnuradio/grc/blocks/
```

## Signal Chain

```
┌──────────────┐     ┌──────────────┐     ┌─────────────────┐
│  RTL-SDR     │────▶│  FM Demod    │────▶│ Clock Recovery  │
│  406.025 MHz │     │              │     │ MM, 400 bps     │
└──────────────┘     └──────────────┘     └────────┬────────┘
                                                   │
                                                   ▼
┌──────────────────────────────────────────────────────────────────┐
│  Correlate Access Code - Tag                                      │
│  Pattern: 0x7FF (15 ones), Tag: "pds_sync"                        │
└──────────────────────────────────────────────────┬───────────────┘
                                                   │
                                                   ▼
┌──────────────────┐     ┌──────────────────┐     ┌────────────────┐
│ Biphase-L Decode │────▶│ PDS Frame Sync   │────▶│ SARP Extract   │
│ (2:1 decim)      │     │ (PDU out)        │     │ valid/invalid  │
└──────────────────┘     └──────────────────┘     └────────────────┘
```

## Example Flowgraph

A minimal flowgraph for receiving SARSAT beacons:

1. **Source**: RTL-SDR @ 406.025 MHz, 250 kHz sample rate
2. **Filter**: Low-pass, 15 kHz bandwidth
3. **FM Demod**: Quadrature demod
4. **Clock Recovery**: M&M, omega=625 (250k/400), mu=0.5
5. **Binary Slicer**: Threshold at 0
6. **Correlate Access Code - Tag**: Pattern `0x7FF`, Threshold 0
7. **Biphase-L Decoder**: Our block
8. **PDS Frame Sync**: Our block, tag_name="pds_sync"
9. **SARP Message Extractor**: Our block

## Beacon Message Contents

A decoded SARP message contains:

- **Protocol Flag**: Standard or National Location
- **Country Code**: 10-bit MID (Maritime Identification Digit)
- **Beacon ID**: Unique identifier (MMSI for EPIRBs)
- **Position**: Encoded lat/lon or GPS coordinates
- **BCH Codes**: Error correction (can recover from bit errors)

## References

- [Cospas-Sarsat System Overview](https://www.cospas-sarsat.int/en/system-overview)
- [406 MHz Beacon Specification (T.001)](https://www.cospas-sarsat.int/en/documents-pro/t-001-specification-for-cospas-sarsat-406-mhz-distress-beacons)
- [Original gr-sarsat by zleffke](https://github.com/zleffke/gr-sarsat)

## License

MIT License

- Original work (c) 2018 zleffke
- GNU Radio 3.10 port (c) 2025 supported.systems