I bought a second NEO-M8T to test and installed it on Hermione with a custom dome acrylic case I made:
I took her for a couple of live flights, using my now standardized 2m square flight plan.
Here’s what the ground facing video saw…
and here’s the NEO-M8T equivalent:
On the plus side, it did get updates throughout the flight, unlike any of the previous ones I’ve tried, so it seems the DroTek EMI filter is working well.
On the far more significant downside however, although the GPS could see 10 satellites, it was using just 4 of them. In comparison, Hermione’s previous mode found 14 and used 10 of this.
Bit stuck now until DroTek get back to me with suggestions; the only suspicion I can see is their choice of GPS aerial to use with the NEO-M8T.