This doesn't really make sense. GNSS is simply a reference to a receiver's ability to access any navigation constellation, that is, GPS, plus GLONASS, Beidou, and Galileo, operated by Russia, China, and the EU respectively.

Just about any modern GPS receiver in a dedicated device has this capability. This may not be the case for mobile devices for cost reasons. The Ublox NEO-M8N, for example can concurrently receive signals from 3 GNSS constellations and has a single unit price around $15.

The thing is, all those satellites don't necessarily help precision as they tend to cover different geographic regions. There aren't a lot of Beidou sats passing over the US, for example.

What does help are augmentation services like WAAS, SBAS, or EGNOS which provide regional GPS correction data originating from known, fixed points in the region.

Differential GPS works similarly and can offer greater precision, but requires a different receiver in a whole other part of the RF spectrum.

Finally, all this precision comes at a cost, that being time. If you want centimeter level accuracy it can take a fair bit of time to get a good fix. In my experience, a couple minutes with a good modern receiver.

That's very cool - thanks for sharing!