INMARSAT-C Codec

This skill converts between text and INMARSAT-C satellite messaging audio files. INMARSAT-C is a mandatory Global Maritime Distress and Safety System (GMDSS) component since 1999, providing store-and-forward text messaging, position reporting, distress alerting, and SafetyNET maritime safety broadcasts via geostationary Inmarsat satellites in the L-band (1.5–1.6 GHz).

Each message is encoded with convolutional forward error correction (rate 1/2, constraint length 7), block interleaving for burst protection, and modulated as FSK audio representing the BPSK baseband.

The generated WAV files are protocol-correct INMARSAT-C transmissions with authentic message framing, FEC, and interleaving. They represent what the baseband signal would sound like when converted to audio tones.

Quick reference: the INMARSAT-C signal

An INMARSAT-C transmission consists of:

1. Unique Word (UW) — Known sync pattern (32+ bits) for frame synchronization. Allows the receiver to detect and lock onto the transmission start.

2. Message Header — Contains:

   • Message type (distress, urgency, safety, routine, EGC SafetyNET, etc.)
   • Priority level
   • Source and destination Inmarsat Mobile Numbers (IMN, 9 digits each)
   • Message reference number
   • Packet sequence number

3. Payload — Text data:

   • ITA2 (5-bit Baudot) encoding for telex-compatible messages
   • Or IA5 (7-bit ASCII) for data messages
   • Up to ~32 KB per message

4. CRC-16 — Error detection checksum over header + payload.

5. FEC (Forward Error Correction) — Rate 1/2 convolutional code:

   • Constraint length K=7
   • Generator polynomials: G1=0o171 (0b1111001), G2=0o133 (0b1011011)
   • Viterbi decoder used for hard-decision decoding

6. Block Interleaving — Protects against burst errors:

   • Write by rows, read by columns
   • Typical: 16 rows × variable columns

7. FSK Modulation — Two-tone audio representation:

   • 600 bps mode: Mark=1200 Hz, Space=1800 Hz
   • 1200 bps mode: Mark=1200 Hz, Space=2400 Hz
   • Continuous-phase FSK

Message types and priority levels

Type	Code	Use case
Distress	0x01	Life-threatening emergency; highest priority
Urgency	0x02	Urgent problem; mediate priority
Safety	0x03	Safety information; low priority
Routine	0x04	Normal messaging; lowest priority
EGC SafetyNET	0x10	Maritime safety broadcasts (MSI, weather, NAVAREA)

How to use this skill

There are three Python scripts in the scripts/ directory. Use them rather than writing INMARSAT-C logic from scratch.

Encoding (text to INMARSAT-C WAV)

python3 <skill-path>/scripts/inmarsat_encode.py <output.wav> [options]

The encoder:

1. Reads text from a file (with --text-file) or command line (with --text)
2. Encodes as IA5 (7-bit ASCII) or ITA2 (5-bit Baudot)
3. Builds message header with type, priority, addresses, sequence numbers
4. Appends CRC-16 checksum
5. Applies rate 1/2 convolutional FEC encoding (Viterbi-ready)
6. Applies block interleaving
7. Prepends unique word for frame sync
8. FSK-modulates with continuous-phase onto audio
9. Writes a 16-bit mono WAV at 44100 Hz

Options:

• --mode 600|1200 — Data rate: 600 bps (return) or 1200 bps (forward), default 1200
• --type TYPE — Message type: distress, urgency, safety, routine, egc (default routine)
• --source IMN — Source Inmarsat Mobile Number (9 digits, default 412345678)
• --dest IMN — Destination IMN (default 000000000 for broadcast)
• --text TEXT — Message text (literal string)
• --text-file FILE — Read message from text file
• --encoding ascii|ita2 — Text encoding (default ascii)
• --no-fec — Skip FEC encoding (for testing only)

Decoding (INMARSAT-C WAV to text)

python3 <skill-path>/scripts/inmarsat_decode.py <input.wav> [output.txt] [options]

The decoder:

1. Reads the WAV (any sample rate — resamples to 44100 if needed)
2. FSK-demodulates by measuring mark/space energy per bit period
3. Detects unique word for frame sync
4. De-interleaves the bit stream
5. Viterbi-decodes the FEC to recover original bits
6. Extracts header, verifies CRC
7. Decodes payload (auto-detects IA5 vs ITA2)
8. Displays message type, priority, addresses, text

Options:

• --mode 600|1200 — Data rate (default: auto-detect by trying both)
• --no-fec — Skip FEC decoding

Testing

python3 <skill-path>/scripts/inmarsat_test.py [--verbose]

Runs the full validation suite: convolutional encode/Viterbi roundtrips, interleaving roundtrips, ITA2/IA5 encoding roundtrips, CRC verification, full message encode/decode at 600 and 1200 bps, different message types, and distress message formats.

Typical workflow

User wants to encode a maritime distress message as audio:

1. Run the encoder with --type distress and distress text
2. Optionally verify by decoding the WAV back and comparing
3. Deliver the WAV file to the user

User wants to decode an INMARSAT-C recording:

1. Run the decoder on their WAV
2. Show them the decoded message, type, addresses
3. Note: real-world recordings may have noise or fading. The decoder works best on clean signals.

User wants a roundtrip demonstration:

1. Encode a test message to WAV
2. Decode WAV back to text
3. Compare the original and recovered text
4. Report the match quality

User asks about INMARSAT-C format details: The quick reference above covers the key parameters. Main things people care about: rate 1/2 convolutional FEC with K=7, block interleaving for burst protection, 600/1200 bps modes, and message types (distress, safety, routine, etc.).

Dependencies

The scripts use only numpy and the standard library wave module. Install if needed:

pip install numpy --break-system-packages