Dual GPS - What if one of them get "Glitched"

I have 2 identical F9P GPS module working as dual GPS for AC4.0 and I have some questions, hope that someone could help:

  1. How to verify/check that how much better dual GPS comparing to single GPS. My idea is setting waypoint and analyze the dataflash log. However, I expected GPSB (yellow) will appear but it did not:

  1. If one of the GPS gets wrong (cable connection, hardware problem…) or be glitched, how Ardupilot will behave? (continue to calculate blending GPS by doing the weighted average or ignore the bad one)

Thanks in advance!

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Are you using them in “best” mode or Blended mode?

GPS_AUTO_SWITCH = 2 / “Blend”. Alternatively set to 1 / “UseBest” to only use the better GPS. The better GPS is decided based on the GPS’s self reported accuracy.

Pretty sure that I am using the Blend mode (GPS_AUTO_SWITCH = 2). I have the log attached, please have a look if it helps.

The log viewer in MP does not show the yellow blend line but in file export to Matlab, there were some label related to these: