Meaning of GPS status messages

We have several outputs in the GPS message and in the Status word.


GPS message:

  • GPSFixType
  • GPSFixStatusFlags
  • PositionAccuracyEst
  • PositionDOP



  • GPSFix
  • MTiGMode


The GPS message that contains the GPSFixType, GPSFixStatusFlags, PositionAccuracyEst and PositionDOP is directly from the uBlox receiver (NAV-SOL:




  • GPSFix can be 2D or 3D. In most cases, we see 3D.
  • GPSFixStatusFlags provide information whether the time has been applied (e.g. time of week known; last bit is the GPS-fix indicator)
  • PositionAccuracyEstimate is in cm (our exporter converts this to meters). This value will be somewhere between 2 and several hundreds of meter. We reject the GPS when the PositionAccuracyEstimate is higher than 25 meter.
  • PositionDOP is the Dillution of Precision and is solely based on the GPS satellites in view. A ‘wide’ geometry, where satellites can be seen on the horizon and high in the sky gives the best value.


The StatusWord has two bits that apply to GPS:

  • GPSFix is copied directly from bit 0 in the GPSFixStatusFlags in the uBlox NAV-SOL message.
  • MTi-G Mode is the way the GPS data is used in the sensor fusion algorithm.
    • A “3” means that the MTi-G is using the GPS data.
    • A “1” means that the MTi-G was using GPS data and is now coasting/dead-reckoning the position based on the inertial sensors (the MTi-G is not using GPS data in this mode). This is done for 45 seconds, before the MTi-G Mode drops to “0”.
    • A “0” means that the MTi-G doesn’t use GPS data and also that it doesn’t output position based on the inertial sensors.
  • MTi-G Mode can discard GPS messages, even if the GPS Fix is valid. The MTi-G performs three checks on the GPS data to ensure robustness:
    • Is there a GPS Fix, provided in the GPSFixStatusFlags?
    • Is the PositionAccuracyEstimate better than a certain value (in most cases 25m)
    • Is the latency less than 0.29s? This is difficult to notice, as the GPS data is still coming in, but the time stamp of the GPS messages is incorrect.



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