RTCM3 Knowledge Patch

RTCM SC-104 standard for differential GNSS corrections. Current version: RTCM 10403.4 (December 2023). This patch covers protocol details needed to parse RTCM3 frames, handle message types, and implement NTRIP client/server connections.

Source: RTCM Special Committee 104 standards, IGS documentation, u-blox integration guides

Quick Reference

Area	What it covers	Details
Message types	Version 3.1-3.4 message number ranges, constellation coverage	Message Types and SSR
SSR corrections	Extended SSR (1240-1270), SSR phases 1-3, PPP-RTK	Message Types and SSR
MSM vs Legacy	Resolution limits, timestamp alignment, format selection	MSM vs Legacy
NTRIP protocol	v1/v2 client connections, base station upload, VRS	NTRIP Protocol

Key Pragmas

• Use MSM messages (1071+) for all new implementations — legacy (1001-1012) only covers GPS/GLONASS.
• Use MSM4 for RTK corrections, MSM7 for full observables with Doppler.
• Default to NTRIP v1 for maximum compatibility (RTKLIB and most low-cost devices only support v1).
• When parsing RTCM3, the frame starts with preamble 0xD3, followed by 6 reserved bits, 10-bit length, the message payload, and a 24-bit CRC (CRC-24Q).
• VRS casters require sending NMEA GGA before corrections flow — always implement GGA feedback in NTRIP clients.
• Watch for the 1 ms timestamp problem: receivers that don't align to round milliseconds can cause RTK failures. Compensate pseudorange/phase when adjusting timestamps.

Frame Format

┌──────────┬──────┬──────────┬─────────────────┬──────────┐
│ Preamble │ Res. │ Length   │ Payload         │ CRC-24Q  │
│ 0xD3     │ 6b   │ 10 bits │ 0-1023 bytes    │ 3 bytes  │
│ (1 byte) │      │          │ (starts with    │          │
│          │      │          │  12-bit msg ID) │          │
└──────────┴──────┴──────────┴─────────────────┴──────────┘

Total overhead: 6 bytes per frame (3 header + 3 CRC). Max payload: 1023 bytes. First 12 bits of payload are always the message type number.

Header Byte Layout

Byte 0:    0xD3 (preamble)
Byte 1:    00LLLLLL  (6 reserved bits = 0, upper 6 bits of length)
Byte 2:    LLLLLLLL  (lower 8 bits of length, giving 10-bit length total)
Bytes 3+:  payload (length bytes)
Last 3:    CRC-24Q

Length field = payload size only (excludes header and CRC).

CRC-24Q

Polynomial: 0x1864CFB (CRC-24 Qualcomm). Computed over bytes 0 through 2+length (header + payload). The CRC is big-endian in the stream.

def crc24q(data: bytes) -> int:
    crc = 0
    for byte in data:
        crc ^= byte << 16
        for _ in range(8):
            crc <<= 1
            if crc & 0x1000000:
                crc ^= 0x1864CFB
    return crc & 0xFFFFFF

Parsing a Frame from a Byte Stream

def read_rtcm3_frame(stream: bytes, offset: int) -> tuple[int, bytes] | None:
    """Returns (message_type, payload) or None if no valid frame found."""
    while offset < len(stream) - 5:
        if stream[offset] != 0xD3:
            offset += 1
            continue
        # Extract 10-bit length from bytes 1-2
        length = ((stream[offset + 1] & 0x03) << 8) | stream[offset + 2]
        frame_end = offset + 3 + length + 3  # header + payload + CRC
        if frame_end > len(stream):
            return None  # incomplete frame
        frame = stream[offset:offset + 3 + length]
        crc_recv = int.from_bytes(stream[offset + 3 + length:frame_end], 'big')
        if crc24q(frame) != crc_recv:
            offset += 1
            continue
        payload = stream[offset + 3:offset + 3 + length]
        msg_type = (payload[0] << 4) | (payload[1] >> 4)
        return msg_type, payload
    return None

Common Message Type Quick Look

Msg	Description	Notes
1005	Station coordinates (ARP, no antenna height)	Minimal base position
1006	Station coordinates + antenna height	Preferred over 1005
1033	Receiver/antenna descriptor strings	Informational
1077	GPS MSM7 (full observables)	Most common GPS message
1087	GLONASS MSM7	Most common GLO message
1097	Galileo MSM7
1127	BDS MSM7
1230	GLONASS code-phase bias	Required for GLO ambiguity resolution
1019	GPS ephemeris
1020	GLONASS ephemeris
1042	BDS ephemeris
1045/1046	Galileo F/NAV / I/NAV ephemeris

SSR Phase Summary

Phase	Components	Use case
Phase 1	Orbit + clock + code bias	PPP (e.g. IGS real-time service)
Phase 2	+ phase bias + vertical iono	PPP-RTK (requires denser networks)
Phase 3	+ slant iono + tropo	Full PPP-RTK

Legacy vs MSM Resolution

Field	Legacy (1002/1010)	MSM (1077/1087)
Pseudorange	0.02 m	Higher resolution
Carrier phase	0.5 mm	Higher resolution
Doppler	Not supported	Supported (MSM5/7)
Half-cycle invalid	Not supported	Supported
Constellations	GPS + GLONASS only	All constellations

NTRIP Connection (v1)

GET /mountPt HTTP/1.0
User-Agent: NTRIP client/1.0
Authorization: Basic dXNlcjpwYXNzd29yZA==

# Success: ICY 200 OK
# Then raw RTCM3 binary stream follows

Auth is Base64-encoded user:password. Default caster port: 2101. For VRS, send $GPGGA sentences on the same connection before corrections arrive.

Reference Index

• Message Types and SSR — Version 3.1-3.4 progression, message number ranges, MSM subtypes, extended SSR (1240-1270), SSR phases
• MSM vs Legacy — Resolution comparison table, timestamp alignment problem, format selection guide
• NTRIP Protocol — v1 vs v2 differences, client/server connection examples, VRS, base station upload