GPS satelites loss midflight

We are using Cube orange, Here 3 GPS (via CAN). Arducopter 4.0.6
We are performing a long flights (tethered drone) on the same spot, with only altitude changes. (max 80m)
However yesterday we experienced a very poor GPS precision. Later in logs, we saw there was a significant loss of satelites. And you can also see HDOP went really high.

What could cause such a behaviour? I checked KP index and it didnt look that bad.
It was an open field, no trees / mountains nearby.
I never experienced having so few sattelites with here 3 before.
Logs here:

Hi @Vaclav_Demeter
First update to latest stable version of arducopter you are on a beta version of 4.0.6 (rc-1)

may thing could cause this problem like military noise , noise by other aircraft’s , signal reflection and …

but at all NSats is low and horizontal accuracy and vertical accuracy is high for a Here 3 GPS try to configure GPS_GNSS depend on your location, GPS_GNSS=67 works goo in most location for me in middle-east i can get 17 sats with a M8N gps with HDOP lower than 0.6

I second that - try GPS_GNSS=67 if you have SBAS available, or GPS_GNSS=65 if you dont have SBAS.

You probably should do some more tuning too, Attitude is not great, and motor 3 is doing significantly less work than the others. Also check MOT_BAT_MAX parameter.

I’d probably set BRD_BOOT_DELAY,5000 to reliably pick up that CAN GPS unit.

Once when I was doing a lot of development with my own designs, an onboard payload with its own GPS receiver and antenna started oscillating around the L1 frequency, and it utterly trashed the flight controller’s GPS receiver’s solution. Also, the oscillation would come and go with other activity on the payload, making the whole event frightening. While @Vaclav_Demeter’s situation may not be similar, I thought I should at least mention it as something to rule out.

Ok. Thank you all for so many good advice.
I completely forgot, that 4.0.6 when i was uploading was still in beta.
Ill do exactly as you all suggested. I know aircraft is not tune perfectly. but its quite a light aircraft with very big propellers, so it will never react as quickly as one would hope.
I am flying in central europe, so SBAS should be available