LC29HEA: Doppler velocity in RMC and High Precision NMEA

IoT Modules GNSS Module
1

Hi Support Team,

I am evaluating the LC29HEA for a project involving IMU sensor fusion. I have two technical questions regarding the NMEA output:

  1. Doppler Velocity: Does the LC29HEA support Doppler-based velocity in the RMC message? We require velocity derived directly from Doppler measurements rather than position-calculated or smoothed velocity.

  2. High Precision GGA: Does this module support high-precision NMEA (extra decimal places for Latitude/Longitude) in GGA and RMC sentences to support RTK accuracy? If so, is there a specific $PAIR or $PQTM command to enable it?

Thanks in advance for your help!

2

Hi @Yahya,

1. Doppler Velocity Support

The LC29H (EA) utilizes the Airoha AG3335 chipset, which typically derives velocity from Doppler measurements for instantaneous accuracy rather than simple position differentiation.

  • RMC Message: The standard $GNRMC message outputs Speed Over Ground (SOG) and Course Over Ground (COG) . While the RMC synopsis does not explicitly label these as Doppler-derived, the chipset’s performance characteristics (Velocity Accuracy of 0.1 m/s) are consistent with Doppler-based calculations.* Recommended Message for Fusion: For projects involving IMU sensor fusion, it is highly recommended to use the proprietary $PQTMVEL message instead of standard RMC.
    • Synopsis: $PQTMVEL,<MsgVer>,<Time>,<VelN>,<VelE>,<VelD>,<GrdSpd>,<Spd>,<COG>,<GrdSpdAcc>,<SpdAcc>,<HeadingAcc>*<Checksum> . * Advantages: It provides granular North, East, and Down (NED) velocity components in m/s, along with 2D/3D speed accuracy estimates and heading accuracy, which are critical inputs for a Kalman filter or similar fusion algorithms.

2. High-Precision NMEA (Extra Decimal Places)

Yes, the module supports high-precision NMEA output with extra decimal places for Latitude and Longitude to represent RTK-level accuracy.

  • Enabling Command: Use the proprietary command $PQTMCFGNMEADP to configure the number of decimal places.* Command Syntax: $PQTMCFGNMEADP,W,<UTC_DP>,<POS_DP>,<ALT_DP>,<DOP_DP>,<SPD_DP>,<COG_DP>*<Checksum> .* Parameter for Lat/Lon: The second data field, <POS_DP> , controls the number of decimal places for position in standard NMEA messages.
    • Range: 0–8 decimal places (the default is 6).* Persisting the Change: After sending the configuration command, you must execute the save command $PQTMSAVEPAR and then restart the module for the new precision settings to take effect.

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Example Command: To set 8 decimal places for position fixes and 3 for speed (the default for the EA variant), you would send: $PQTMCFGNMEADP,W,3,8,3,2,3,2*<Checksum> .

Note:

  1. LC29H (EA) at its default 10 Hz RTK rate , ensure your UART baud rate is set to 460800 bps or higher .
  2. At frequencies greater than 1 Hz, the module is optimized to output only RMC and GGA (and any enabled PQTM messages) at the high frequency, while satellite-specific messages like GSA and GSV will remain at 1 Hz to prevent data loss.
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