There’s a reasonably cheap and easy answer for most of us (i.e. the US civil aviation sector). I read a paper a few months back on the possibility of using the ADS-B ground stations as a backup RNAV source. If I recall correctly, the ground station signals included the necessary data to do an accurate time-of-flight calculation. You need have to have an accurate timing source onboard (not an atomic clock of your own, just a good digital clock), at least three towers in view, and a modest microcontroller to crunch the numbers.
The team that did the study were able to triangulate a position +/- 20 meters. I’m a bit surprised it hasn’t simply become a standard feature on ADS-B receivers. Here’s a link to the paper: https://navi.ion.org/content/68/2/293