# gr-sarsat-modern

GNU Radio 3.10+ compatible Cospas-Sarsat 406 MHz beacon decoder.

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

## Blocks

| Block | Description |
|-------|-------------|
| **Biphase-L Decoder** | Decodes Manchester/Biphase-L encoded bit stream |
| **PDS Frame Sync** | Synchronizes and extracts 72-byte PDS frames |
| **SARP Message Extractor** | Splits PDS frames into 24-byte SARP messages |

## Installation

```bash
# Install Python package
cd gr-sarsat-modern
pip install -e .

# Copy GRC blocks to GNU Radio
cp grc/*.yml ~/.local/share/gnuradio/grc/blocks/
```

## Signal Chain

```
RF Input (406 MHz)
    |
    v
[FM Demod] -> [Clock Recovery] -> [Slicer]
    |
    v
[Correlate Access Code - Tag]  (Frame sync: 0x7FF, tag: "pds_sync")
    |
    v
[Biphase-L Decoder]
    |
    v
[PDS Frame Sync]
    |
    v
[SARP Message Extractor]
    |
    +---> valid (sync word OK)
    +---> invalid (sync word mismatch)
```

## Protocol Background

Cospas-Sarsat is the international satellite-based search and rescue system:

- **Frequency**: 406.025 MHz
- **Modulation**: Phase-modulated carrier, Biphase-L encoded
- **Bit rate**: 400 bps
- **Frame structure**:
  - 15-bit frame sync (0x7FF)
  - 576-bit PDS frame (3 × 24-byte SARP messages)
  - Each SARP message starts with 0xD60 sync word

## License

MIT License - Original work (c) 2018 zleffke