EPH, EPV GPS values

pordipatrik · June 23, 2023, 10:54am

Can somebody tell me what do eph and epv s mean?
Here: https://danielcjacobs.com/uploads/ECHO_Memos/ECHO029_Logged_GPS_Data_Explanation.pdf
They considered as standard deviation.
While here:
SensorGps (UORB message) | PX4 User Guide
Accuracy, but it is not detailed.

Also, since eph and epv are used for failsafe:Parameter Reference | PX4 User Guide
, I want to use them instead of multiplying the measurement resolution with hdop and vdop.
Any feedback on the questions above would be appreciated.

JulianOes · June 24, 2023, 4:12am

For uBlox the value comes from here:

github.com

PX4/PX4-GPSDrivers/blob/1f850dea2ca80f98a91d96013e2b6cc7da2365de/src/ubx.cpp#L1890


      
          			_gps_position->vel_ned_valid = false;
          		}
          
          
		_gps_position->satellites_used	= _buf.payload_rx_nav_pvt.numSV;
          
          
		_gps_position->lat		= _buf.payload_rx_nav_pvt.lat;
          		_gps_position->lon		= _buf.payload_rx_nav_pvt.lon;
          		_gps_position->alt		= _buf.payload_rx_nav_pvt.hMSL;
          		_gps_position->alt_ellipsoid	= _buf.payload_rx_nav_pvt.height;
          
          
		_gps_position->eph		= static_cast<float>(_buf.payload_rx_nav_pvt.hAcc) * 1e-3f;
          		_gps_position->epv		= static_cast<float>(_buf.payload_rx_nav_pvt.vAcc) * 1e-3f;
          		_gps_position->s_variance_m_s	= static_cast<float>(_buf.payload_rx_nav_pvt.sAcc) * 1e-3f;
          
          
		_gps_position->vel_m_s		= static_cast<float>(_buf.payload_rx_nav_pvt.gSpeed) * 1e-3f;
          
          
		_gps_position->vel_n_m_s	= static_cast<float>(_buf.payload_rx_nav_pvt.velN) * 1e-3f;
          		_gps_position->vel_e_m_s	= static_cast<float>(_buf.payload_rx_nav_pvt.velE) * 1e-3f;
          		_gps_position->vel_d_m_s	= static_cast<float>(_buf.payload_rx_nav_pvt.velD) * 1e-3f;
          
          
		_gps_position->cog_rad		= static_cast<float>(_buf.payload_rx_nav_pvt.headMot) * M_DEG_TO_RAD_F * 1e-5f;

So it’s UBX-NAV-PVT.hAcc which is described as “Horizontal accuracy estimate” in mm.

See: https://content.u-blox.com/sites/default/files/products/documents/u-blox8-M8_ReceiverDescrProtSpec_UBX-13003221.pdf

pordipatrik · June 26, 2023, 7:36am

Thank you!
My problem with this is that accuracy is not defined here neither. In some places it is listed as standard deviation.

It is a bit confusing. Could you clarify this to me?

Many thanks,
Patrik

JulianOes · June 26, 2023, 8:51pm

My assumption is that it’s a standard deviation but if you want to be sure you might have to talk to uBlox and ask them.