Earlier quoted context omitted.
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. https://www.e-education.psu.edu/natureofgeoinfo/book/export/... I think the US Government can also shut it off at a moments notice.
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
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-...