venus/axiom-nmea/debug/field_mapping.py

#!/usr/bin/env python3
"""
Extract and display the field mapping for sensor data.
Based on protobuf structure analysis.
"""

import struct

def read_pcap(filename):
    packets = []
    with open(filename, 'rb') as f:
        header = f.read(24)
        magic = struct.unpack('<I', header[0:4])[0]
        swapped = magic == 0xd4c3b2a1
        endian = '>' if swapped else '<'
        while True:
            pkt_header = f.read(16)
            if len(pkt_header) < 16:
                break
            ts_sec, ts_usec, incl_len, orig_len = struct.unpack(f'{endian}IIII', pkt_header)
            pkt_data = f.read(incl_len)
            if len(pkt_data) < incl_len:
                break
            if len(pkt_data) > 42 and pkt_data[12:14] == b'\x08\x00':
                ip_header_len = (pkt_data[14] & 0x0F) * 4
                payload_start = 14 + ip_header_len + 8
                if payload_start < len(pkt_data):
                    packets.append(pkt_data[payload_start:])
    return packets

# Based on protobuf analysis, here's the structure:
STRUCTURE = """
================================================================================
RAYMARINE PACKET STRUCTURE (from protobuf analysis)
================================================================================

FIXED HEADER (20 bytes @ 0x0000-0x0013):
  0x0000-0x0007: Packet identifier (00 00 00 00 00 00 00 01)
  0x0008-0x000B: Source ID
  0x000C-0x000F: Message type indicator
  0x0010-0x0013: Payload length

PROTOBUF MESSAGE (starts @ 0x0014):

  Field 1 (length-delim): Device Info
    └─ Field 1: Device name ("AXIOM 12")
    └─ Field 2: Serial number info

  Field 2 (length-delim): GPS/Position Data
    ├─ Field 1 (fixed64/double): LATITUDE
    ├─ Field 2 (fixed64/double): LONGITUDE
    ├─ Field 3 (fixed64/double): (unknown, often NaN)
    ├─ Field 4 (fixed64/double): (altitude or similar)
    ├─ Field 5 (fixed64/double): (timestamp or distance)
    └─ Field 6 (fixed64/double): (timestamp or distance)

  Field 3 (length-delim): Heading Block
    ├─ Field 1 (fixed32/float): Heading value 1 (radians)
    └─ Field 2 (fixed32/float): HEADING (radians) ← ~31-33°

  Field 6 (length-delim): [only in 446+ byte packets]
    └─ Field 1 (fixed32/float): (unknown angle)

  Field 8 (length-delim): Rate/Motion Data
    ├─ Field 1 (fixed32/float): Rate of turn?
    └─ Field 2 (fixed32/float): (often NaN)

  Field 13 (length-delim): WIND/NAVIGATION DATA ← Main sensor block
    ├─ Field 1 (fixed32/float): Heading copy (radians)
    ├─ Field 2 (fixed32/float): Heading smoothed (radians)
    ├─ Field 3 (fixed32/float): (small angle, ~0°)
    ├─ Field 4 (fixed32/float): TRUE WIND DIRECTION (radians) ← ~62-70°
    ├─ Field 5 (fixed32/float): WIND SPEED (m/s) ← ~7.26 = 14.1 kts
    ├─ Field 6 (fixed32/float): Wind speed (different value)
    ├─ Field 7 (fixed32/float): (large angle ~245°)
    ├─ Field 8-13: Duplicates/smoothed values of above

  Field 14 (length-delim): Additional sensor data (multiple instances)
  Field 21 (length-delim): More angles
  Field 38, 41: Empty/reserved

================================================================================
KEY SENSOR MAPPINGS
================================================================================

SENSOR              PARENT FIELD    CHILD FIELD    UNIT
------------------  -------------   -----------    --------
Latitude            Field 2         Field 1        degrees (double)
Longitude           Field 2         Field 2        degrees (double)
Heading             Field 3         Field 2        radians (float)
True Wind Dir       Field 13        Field 4        radians (float)
Wind Speed          Field 13        Field 5        m/s (float)

================================================================================
"""

print(STRUCTURE)

# Now verify with actual data
print("VERIFICATION WITH ACTUAL DATA:")
print("=" * 70)

packets = read_pcap("raymarine_sample_TWD_62-70_HDG_29-35.pcap")

# Get a 344-byte packet
for pkt in packets:
    if len(pkt) == 344:
        # Skip 20-byte header, protobuf starts at 0x14
        proto = pkt[0x14:]

        print(f"\n344-byte packet analysis:")
        print(f"  Expected: TWD 62-70°, Heading 29-35°")

        # Field 3 starts around offset 0x54 in original packet = 0x40 in proto
        # But we need to navigate by protobuf structure

        # Let's use the known byte offsets and verify
        # From earlier analysis:
        # - 0x0070 had heading ~32.6°
        # - 0x00a0 had TWD ~61.7°

        def get_float(data, offset):
            if offset + 4 <= len(data):
                return struct.unpack('<f', data[offset:offset+4])[0]
            return None

        # Check field 3 (heading block) - starts around 0x54
        # Field 3, field 2 should be heading
        heading_offset = 0x0070  # From earlier analysis
        heading_val = get_float(pkt, heading_offset)
        if heading_val:
            heading_deg = heading_val * 57.2958
            print(f"  Heading @ 0x{heading_offset:04x}: {heading_val:.4f} rad = {heading_deg:.1f}°")

        # Check field 13, field 4 (TWD)
        twd_offset = 0x00a0  # From earlier analysis
        twd_val = get_float(pkt, twd_offset)
        if twd_val:
            twd_deg = twd_val * 57.2958
            print(f"  TWD @ 0x{twd_offset:04x}: {twd_val:.4f} rad = {twd_deg:.1f}°")

        # Check field 13, field 5 (wind speed)
        ws_offset = 0x00a5  # Should be right after TWD
        ws_val = get_float(pkt, ws_offset)
        if ws_val:
            ws_kts = ws_val * 1.94384
            print(f"  Wind Speed @ 0x{ws_offset:04x}: {ws_val:.2f} m/s = {ws_kts:.1f} kts")

        break

print("\n" + "=" * 70)
print("CONCLUSION: The protobuf structure explains the 'random' offsets:")
print("  - Same field numbers have different byte offsets per packet size")
print("  - This is because preceding fields vary in length")
print("  - Solution: Parse protobuf structure, not fixed byte offsets")
print("=" * 70)