SBAS with GNSS 2

Support GNSS
1

Hello Pavel,
Does the Zubax GNSS2 support SBAS ?
The uBlox M8Q datasheet says it is supposed to works, but it doesn’t seem to be enable by default ? Maybe I’m missing something.

Thanks for your answer !

2

Yes, SBAS is supported and enabled by default. You can check that via u-blox u-center as shown here Evaluating GNSS performance with u-blox u-center - Knowledge Base - Zubax Knowledge Base

3

With the “default” Pixhawk 4 GPS running PX4, we get 5-6 satellites less, but PX4 tells us: “GPS_FIX_TYPE_DGPS” = 4 with around 9.6m H_precision.
With the newly bought GNSS2, we recieve more satelites, but have a lower accuracy (arround 1m) and we only get the FIX_TYPE: “GPS_FIX_TYPE_3D_FIX” = 3
Would the inclusion of SBAS in the calculation of the GPS_FIX be reported to PX4 in the GPS_FIX_TYPE message? Like it is with the Pixhawk GPS?

We left the GNSS 2 out for quite a while, to wait for convergence,.

4

With the newly bought GNSS2, we recieve more satelites, but have a lower accuracy (arround 1m)

Lower is better.

and we only get the FIX_TYPE: “GPS_FIX_TYPE_3D_FIX” = 3

This is because the UAVCAN driver does not propagate more detailed fix information to the upper layers. You are most likely getting an SBAS fix, but QGC won’t show you that.

5

Sorry I meant that the accuracy was worse. I will do another tests/comparison.

This is because the UAVCAN driver does not propagate more detailed fix information to the upper layers. You are most likely getting an SBAS fix, but QGC won’t show you that.

Ok, thanks for that info, then this makes sense.

6

I tested the GNSS 2 again and this time according to the RAW_GPS data it performs better in the horizontal and vertical accuracy. I also checked the reported accuracy of the Estimator over Mavlink and here it seems that even though, that the v_acc of the GPS is better the calculated vertical accuracy of the EKF is worse. I am using PX4 and would like to understand if the baro is used by the EKF in PX4. Do i need to activate the external barometer so that the EKF sees it?
See the following table:

GPS type time after StartUp (min) #sat h_acc (mm) v_acc (mm) EKF h_Acc (m) EKF v_acc (m) eph epv
normal 2 18 450 850 0.25 0.23
normal 5 18 360 660 0.25 0.23
normal 1 18 507 720 0.26 0.24
normal 5 17 400 600 0.26 0.23 64 103
zubax 1 23 350 590 0.19 0.29 108 108
zubax 1 22 306 535 0.19 0.29
zubax 4 25 320 540 0.19 0.29
zubax 6 25 320 530 0.19 0.29 97 97
zubax 8 25 240 418 0.19 0.29 100 100
1 Like
7

Yes, the barometer on Zubax GNSS 2 is disabled by default.

8

Thanks for the info,

Do you have recommendations for the EKF parameters of the PX4 for the GNSS? Like a set, which would result in better performance, due to its superiority?

9

Unfortunately, we can’t provide that information. Perhaps the PX4 forum might help.