Protocol Overview

Technical reference for the NMEA 0183, PAIR, PQTM, and RTCM3 protocols as implemented by the Quectel LC29H GNSS module family. Protocol constants and internal identifiers are sourced from QNMEA.dll decompilation and the Quectel LC29H GNSS Protocol Specification V1.1.

Applies to: LC29H(AA), LC29H(BA), LC29H(CA), LC29H(DA), LC29H(BS)

NMEA Sentence Format

Every NMEA 0183 sentence follows this wire format:

$TTSSS,field1,field2,...,fieldN*XX\r\n

Component	Bytes	Description
$	1	Start delimiter (0x24)
TT	2	Talker ID
SSS	3	Sentence ID (e.g. GGA, RMC)
,	1 each	Field separator (0x2C)
*	1	Checksum delimiter (0x2A)
XX	2	Checksum as uppercase hex
\r\n	2	Line terminator (0x0D 0x0A)

Protocol Constants

From QNMEA.dll decompilation:

Constant	Value	Notes
TALKERID_LEN	2	Characters before sentence ID
SENTENCEID_LEN	3	Characters after talker ID
CHECKSUM_LEN	2	Hex digits after *
MIN_NMEA_LEN	12	Shortest valid sentence
MAX_NMEA_LEN	82	Longest valid sentence (NMEA spec limit)

Checksum Calculation

The checksum is the XOR of every byte between $ and *, exclusive of both delimiters:

checksum = 0
for each byte between '$' and '*':
    checksum = checksum XOR byte
format as "%02X"

Example — $GNRMC,010555.000,A,2232.4682,N,11404.6748,E,0.00,125.29,230822,,,D*71:

XOR all bytes in GNRMC,010555.000,A,2232.4682,N,11404.6748,E,0.00,125.29,230822,,,D yields 0x71.

Coordinate Encoding

Field	Format	Example	Decimal
Latitude	DDMM.MMMMM	2232.4682	22 + 32.4682/60 = 22.54114
Longitude	DDDMM.MMMMM	11404.6748	114 + 04.6748/60 = 114.07791

Hemisphere indicators N/S and E/W follow the numeric field. South and West are negative in decimal degrees.

Talker IDs

The LC29H actively uses a subset of 24 registered talker IDs depending on which constellations are enabled.

Talker ID	System	Notes
GP	GPS (USA)	L1 C/A, L5
GL	GLONASS (Russia)	L1
GA	Galileo (EU)	E1, E5a
GB	BeiDou (China)	B1I, B2a (preferred BeiDou talker)
BD	BeiDou (China)	Alternate BeiDou talker ID
QZ	QZSS (Japan)	L1 C/A, L5
GN	Multi-GNSS combined	Used when fix includes multiple constellations
GI	NavIC / IRNSS (India)	Registered but not tracked by LC29H
PQ	Proprietary Quectel	Used in $PQ prefixed proprietary sentences

Primary talker IDs in output: GP, GL, GA, GB, QZ, and GN.

Sentence ID Index

From QNMEA.dll, sentence types are assigned numeric indices used by PAIR062 (set output rate) and PAIR063 (query output rate).

Index	Sentence	Full Name
0	GGA	Global Positioning System Fix Data
1	GLL	Geographic Position — Latitude/Longitude
2	GSA	GNSS DOP and Active Satellites
3	GSV	GNSS Satellites in View
4	RMC	Recommended Minimum Specific GNSS Data
5	VTG	Course Over Ground and Ground Speed
6	ZDA	Time and Date
7	GRS	GNSS Range Residuals
8	GST	GNSS Pseudorange Error Statistics

Note

The LC29H(BA, CA, DA, EA) variants support indices 0–5 only. The GNS sentence is available via PAIR076 but does not appear in the QNMEA index table.

Proprietary Sentence Prefixes

PAIR — Quectel/MediaTek Proprietary Commands

$PAIR<NNN>,<param1>,<param2>,...*XX\r\n

ID Range	Category	Examples
001	ACK/NACK	Response to all PAIR set commands
003–007	System restart	Cold/hot/warm start, factory reset
021	Version query	Firmware version
050	Fix rate	Position fix interval (100–10000 ms)
051	Baud rate	UART1 baud rate
066–067	Constellation	Enable/disable GPS, GLONASS, Galileo, BeiDou, QZSS
070–076	NMEA output	Per-sentence output rate
080–081	Navigation mode	Normal, fitness, stationary, drone
382–383	SBAS	SBAS augmentation (AA variant)
432–437	RTCM output	RTCM mode, antenna ref, ephemeris (DA/BS)
470–472	EPO	Extended Prediction Orbit (AA variant)
490–491	EASY	Self-generated ephemeris (AA variant)
511, 513	NVRAM	Save navigation data / save config to flash
650	Backup mode	Ultra-low-power RTC backup (~7 uA)
690–691	Periodic mode	Power-saving duty cycle
752	PPS	Pulse-per-second output
830–831	Raw measurement	Raw GNSS observation output
864–867	Port config	Per-port baud rate and flow control

See PAIR Commands for the complete reference.

PQTM — Quectel Platform Commands

$PQTM<NAME>,<params>...*XX\r\n

Command	Direction	Description
PQTMVERNO	Query/Response	Firmware version, hardware, SDK
PQTMSAVEPAR	Set	Save all parameters to flash
PQTMRESTOREPAR	Set	Restore factory defaults
PQTMGNSSRESTART	Set	Restart GNSS engine
PQTMCFGPORT	Read/Write	Port protocol and baud rate
PQTMCFGNMEAMSG	Read/Write	Per-port NMEA message rate
PQTMCFGEAMASK	Read/Write	Satellite elevation mask
PQTMCFGDRMODE	Read/Write	Dead reckoning mode (BA/CA)
PQTMCFGSVIN	Read/Write	Survey-in configuration (BS)

See PQTM Commands for the complete reference.

ACK/NACK Protocol

Every PAIR set command generates an acknowledgement response:

$PAIR001,<CommandID>,<Result>*XX\r\n

Result Codes

Code	Name	Description
0	Success	Command accepted and executed
1	Fail	Command execution failed
2	Not Supported	Command ID not recognized or not available
3	Invalid Parameter	Parameter out of range or wrong type
4	Timeout	Command timed out during execution

Example Exchange

Host  -> $PAIR066,1,1,1,1,0*3A     (enable GPS+GLONASS+Galileo+BeiDou, disable QZSS)
Module<- $PAIR001,066,0*3B          (success)

Host  -> $PAIR050,500*21            (set fix rate to 500ms / 2Hz)
Module<- $PAIR001,050,0*36          (success)

Host  -> $PAIR050,50*14             (set fix rate to 50ms -- out of range)
Module<- $PAIR001,050,3*35          (invalid parameter)

Tip

ACKs arrive asynchronously among NMEA position output. Implementations should match the CommandID field to correlate requests and responses.

Fix Quality Indicators

GGA field 5 encodes the GNSS fix quality:

Value	Name	Description
0	No Fix	Position not available
1	GPS Fix	Standard GNSS autonomous fix (SPS)
2	DGPS Fix	Differential correction applied (SBAS or RTCM)
3	PPS Fix	Precise Positioning Service (military GPS)
4	RTK Fixed	Carrier-phase ambiguities resolved; centimeter accuracy
5	RTK Float	Carrier-phase ambiguities not resolved; decimeter accuracy
6	Estimated	Dead reckoning or estimated position
7	Manual	Position entered manually
8	Simulation	Simulator mode

For RTK operation, the typical progression is: 0 (no fix) → 1 (autonomous) → 5 (RTK float) → 4 (RTK fixed).

Default Output Configuration

Out of the box, the LC29H outputs these sentences at 1 Hz on UART1 (115200 baud, 8N1):

Sentence	Default Rate	PAIR Command
GGA	1 (every fix)	PAIR073
GLL	0 (disabled)	PAIR070
GSA	1 (every fix)	PAIR074
GSV	1 (every fix)	PAIR075
RMC	1 (every fix)	PAIR071
VTG	1 (every fix)	PAIR072
GNS	0 (disabled)	PAIR076

Rate values: 0 = disabled, 1 = every position fix, 2 = every second fix, up to 20.

Variant Feature Matrix

Feature	AA	BA	CA	DA	BS
Standard NMEA (GGA/RMC/GSA/GSV/VTG/GLL)	Yes	Yes	Yes	Yes	Yes
GNS output	Yes	Yes	Yes	Yes	Yes
SBAS augmentation (PAIR382)	Yes	—	—	—	—
EASY ephemeris prediction (PAIR490)	Yes	—	—	—	—
Constellation selection (PAIR066)	Yes	—	—	—	—
RTK rover (RTCM3 input)	—	Yes	—	Yes	—
RTK base (RTCM3 output)	—	—	—	—	Yes
Dead reckoning (PQTMCFGDRMODE)	—	Yes	Yes	—	—
CAN bus interface (PQTMCFGCAN)	—	Yes	—	—	—
Survey-in (PQTMCFGSVIN)	—	—	—	—	Yes
Raw measurement output (PAIR830)	—	—	—	—	Yes
Navigation mode (PAIR080)	Yes	Yes	Yes	Yes	Yes
Power management (PAIR650/690)	Yes	Yes	Yes	Yes	Yes
PPS output (PAIR752)	Yes	Yes	Yes	Yes	Yes