Follow

Meaning of GPS status messages

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

 

GPS message:

  • GPSFixType
  • GPSFixStatusFlags
  • PositionAccuracyEst
  • PositionDOP

 

StatusWord

  • GPSFix
  • MTiGMode

 

The GPS message that contains the GPSFixType, GPSFixStatusFlags, PositionAccuracyEst and PositionDOP is directly from the uBlox receiver (NAV-SOL: http://www.u-blox.com/images/downloads/Product_Docs/u-blox6_ReceiverDescriptionProtocolSpec_(GPS.G6-SW-10018).pdf#page=186).

 

===

 

  • 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.

 

 

Was this article helpful?
0 out of 0 found this helpful
Do you have a question? Please post your question in our Community Forum