When I was a civil engineer last century gps accuracy was an issue because people wanted to use gps for surveying. They came up with a system that would use 2 receivers and a radio between them to get much higher accuracies.
There are two frequencies (sometimes three) transmitted by the satellites. Using two frequencies allows certain atmospheric delays to be accurately estimated. Trouble is only one code is available for civilian applications. The trick with two receivers is to solve for the phase of secret signal (rather than decode it) by solving an integer least squares problem. This allows accuracy of the order of +/- 5cm
There are multiple civilian frequencies available now, and cheap receivers for 'em. I have a pair of F9P's running at present, and just received some GT-U12's for testing.
Surprisingly the answer is most often no, however in poor signal areas like in a city or under canopy the answer is yes as you are more likely to get a signal from the remaining bits of sky view.
The other benefit is when doing PPP the convergence time is dramatically shorter with multi constellation.
When doing RTK most receivers will use only GPS, as it is generally the most accurate, but it can and will use GLONASS occasionally. I have never seen one use beidu or Galileo
I briefely looked through the pdf, and probally missed it?
I live in the Bay Area, and homeowners are concerned over a few inches.
(There's a huge need for cheap surveys. Until GPS gets it to under a inch, traditional surveys will off pipes, and landmarks, is here to stay? Or, am I wrong?)
Civil and mining engineers have been making use of centimeter-accurate GNSS configurations for years now. The equipment necessary costs a few thousand dollars, more if you need even more precision. Most major civil and mining engineering projects use self-guided earth-moving equipment, not feasible without this technology.
The problem for homeowners is that surveys aren't enough. The vast majority of land titles are not registered in precise coordinates; they're registered in terms of landmarks, benchmarks, and so forth. This is changing at a glacial pace, but for now, the tech isn't the problem.
https://www.e-education.psu.edu/natureofgeoinfo/book/export/...
I think the US Government can also shut it off at a moments notice.