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

Mobile phones use dual frequencies now. Some use the Broadcom BCM47755 chip, but the most common ones are various Qualcomm Snapdragon SoCs.

https://www.broadcom.com/products/wireless/gnss-gps-socs/bcm...

https://www.euspa.europa.eu/newsroom/news/qualcomm-launches-...

https://www.gpsworld.com/qualcomm-launches-3-dual-frequency-...

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.

They could, but multiple governments now publicly broadcast from similar constellations, so it wouldn't have nearly as much benefit for them to do so.

Modern dirt cheap receivers can pick up European, Chinese, Russian, etc. constellations for location just as well as the American one.

Always wondered about this, can overlapping systems be used simultaneously for higher reliability or precision?

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

Those two radio systems are still used. Just that many states have CORS sites that post correction information.

What is the accuracy of GPS surveying?

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.

RTK has an accuracy of around 1cm. Its widely used by surveyors (and other things like agriculture, construction machine control, etc).