cospas-sarsat-codec

COSPAS-SARSAT 406 MHz Beacon Codec

Comprehensive codec for encoding and decoding emergency distress beacons used in the international search-and-rescue system.

What is COSPAS-SARSAT?

COSPAS-SARSAT is the satellite-based search and rescue system operated by an international consortium. When an emergency beacon is activated:

1. The beacon transmits on 406.028 MHz (~0.5 second burst every 50 seconds)
2. LEOSAR (low-earth orbit) and MEOSAR (medium-earth orbit) satellites detect the signal
3. Satellites relay the signal to ground stations (LUTs — Local User Terminals)
4. Rescue coordination centers decode the beacon information and dispatch help

Three types of beacons transmit on this frequency:

• EPIRB — Emergency Position Indicating Radio Beacon (maritime vessels)
• ELT — Emergency Locator Transmitter (aircraft)
• PLB — Personal Locator Beacon (land, personal use)

Signal Format

Transmission characteristics:

• Carrier frequency: 406.028 MHz
• Modulation: Phase modulation (effectively BPSK — binary phase shift keying)
• Data rate: 400 bits per second (2.5 ms per bit)
• Message length: 112 bits (short, no position) or 144 bits (long, with position)
• Burst duration: ~0.52 seconds
• Repetition: Every ~50 seconds until rescue or manual deactivation

Audio baseband representation (for SDR processing):

• Use 1000 Hz carrier frequency (instead of 406 MHz) in audio domain
• BPSK modulation: phase reversal (180°) encodes bit=1; no phase shift encodes bit=0
• Sample rate: 44100 Hz recommended (or 48 kHz)
• Output: WAV format (mono, 16-bit signed)

Message Structure

All beacon messages follow this 15-bit to 144-bit structure:

Fixed Preamble

• Bits 0-14 (15 bits): Bit synchronization pattern 000101111101100 — optimized for autocorrelation detection
• Bits 15-23 (9 bits): Frame synchronization 011010000 — identifies normal message (not self-test)

Message Type and Format

• Bit 24 (1 bit): Format flag — 0 = short message (112 bits), 1 = long message (144 bits)
• Bit 25 (1 bit): Protocol flag — 0 = user protocol, 1 = location protocol (with position)

Registration and Type

• Bits 26-35 (10 bits): Country code (ITU Maritime Identification Digits)
  • Example: 338 = USA, 226 = France, 412 = Japan
• Bits 36-39 (4 bits): Protocol code — identifies beacon type:
  • 0010 = Maritime EPIRB (user protocol) or Standard Location EPIRB
  • 0011 = Standard Location ELT (aviation)
  • 0100 = Standard Location PLB (personal) or Serial user protocol
  • 0110 = Aviation ELT (user protocol) with ICAO address
  • 1110 / 1100 = Test beacons

Beacon Identification (46-50 bits, content varies by type)

Maritime EPIRB (user protocol):

• 30-bit MMSI (Maritime Mobile Service Identity) — unique ship identifier
• 16 spare bits

Aviation ELT (user protocol):

• 24-bit ICAO aircraft address — unique aircraft identifier
• 22 spare bits

Personal Locator Beacon (PLB):

• 20-bit serial number or similar identifier
• 26 spare bits

Position Data (long format only, bits 86-132)

Coarse Position (20 bits):

• Latitude: 1 sign bit (0=North, 1=South) + 7 bits for degrees (0-90) + 2 bits for quarter-degrees
• Longitude: 1 sign bit (0=East, 1=West) + 8 bits for degrees (0-180) + 1 bit for half-degree
• Precision: ~0.25° (roughly 25 km)

Fine Position Offset (25 bits, optional):

• Supplementary latitude refinement (10 bits)
• Supplementary longitude refinement (10 bits)
• Additional identification or auxiliary data (5 bits)

Error Correction

Primary BCH Code (bits 26-85, 60 data bits):

• 21 parity bits (bits 86-106) protect the registration and identification fields
• Uses BCH(82,61) code — can correct up to 6-bit errors
• Generator polynomial: x^21 + x^19 + x^18 + x^17 + x^16 + x^15 + x^14 + x^10 + x^9 + x^8 + x^6 + x^5 + x^2 + x + 1

Secondary BCH Code (optional, long format):

• 12 parity bits protect the supplementary position data

How to Use This Codec

Encoding (Beacon Data → WAV File)

Create an audio WAV file containing an encoded beacon signal:

python3 sarsat_encode.py output.wav \
  --country 338 \
  --type epirb \
  --mmsi 311000001 \
  --lat 40.7 \
  --lon -74.0 \
  --long-format

Options:

• --country CODE — 3-digit ITU country code (default: 338 = USA)
• --type {epirb,elt,plb,test} — Beacon type (default: epirb)
• --mmsi NNNNNNNNN — MMSI number for maritime beacons
• --icao NNNNNN — ICAO hex address for aviation (e.g., A0B1C2)
• --serial NNNN — Serial number for PLB/serial protocol
• --lat DD.DDDD — Latitude (positive=North, negative=South)
• --lon DD.DDDD — Longitude (positive=East, negative=West)
• --long-format — Use 144-bit format with position (default)
• --short-format — Use 112-bit format without position
• --bursts N — Number of beacon bursts (default: 1)
• --burst-interval N — Seconds between bursts (default: 50)

Decoding (WAV File → Beacon Information)

Extract beacon information from an audio WAV recording:

python3 sarsat_decode.py input.wav output.txt -v

Output includes:

• Beacon hex ID (15 hexadecimal digits used by SAR authorities)
• Country of registration
• Beacon type (EPIRB/ELT/PLB/test)
• Identification (MMSI, ICAO address, or serial number)
• Position if encoded
• BCH error correction status (valid/corrected/error)

Testing

Run the comprehensive test suite to verify correct implementation:

python3 sarsat_test.py

Tests cover:

• BPSK modulation/demodulation
• BCH error correction (encoding/decoding)
• Position encoding/decoding
• Message building (short/long formats)
• Message parsing
• Beacon type handling
• Country code lookup
• Sync pattern detection
• Full WAV encode/decode roundtrips
• Multi-burst recordings

Common Country Codes

Code	Country	Code	Country
201-299	Europe	301-399	Americas
226	France	303	Canada
230	Finland	338	USA
211	Germany	244	Netherlands
412	Japan	501	Australia
431	China	636	South Africa

File Structure

cospas-sarsat/
├── SKILL.md                    # This documentation
├── README.md                   # Extended usage guide
├── scripts/
│   ├── sarsat_common.py        # Core codec library
│   ├── sarsat_encode.py        # Beacon encoder CLI
│   ├── sarsat_decode.py        # Beacon decoder CLI
│   └── sarsat_test.py          # Comprehensive test suite
└── examples/
    ├── maritime_epirb.sh       # Example: encode maritime EPIRB
    ├── aviation_elt.sh         # Example: encode aviation ELT
    └── plb_with_position.sh    # Example: encode PLB with coordinates

Technical Notes for SDR Use

Recording beacon signals:

1. Tune SDR to 406.028 MHz
2. Use 200 kHz+ bandwidth (BPSK is narrow)
3. Record as WAV (mono, 48 kHz or higher)
4. Decode with sarsat_decode.py

Generating test signals:

1. Use sarsat_encode.py to create synthetic beacon WAV files
2. Useful for testing decoder workflows without actual beacons
3. Can simulate multiple bursts for realistic recordings

Real-world beacon activation:

• Never activate emergency beacons unless in genuine distress
• Emergency beacon networks actively monitor 406 MHz
• False alarms trigger expensive search operations
• Penalties can include fines and legal liability

References

• COSPAS-SARSAT System: https://www.cospas-sarsat.int/
• 406 MHz Beacon Technical Standards: IMO, ICAO, and ITU specifications
• LEOSAR and MEOSAR satellite coverage
• Local User Terminal (LUT) networks worldwide