There is / was a working solution to this called LORAN-C that is being phased out. The accuracy isn't all that great (60 ft / 18 metres at best) but can be used to provide a 'disagree' alert to navigation systems. ( <a href="https://en.wikipedia.org/wiki/LORAN-C#The_future_of_LORAN" rel="nofollow">https://en.wikipedia.org/wiki/LORAN-C#The_future_of_LORAN</a> )
I'd think the circular pattern comes from slight timing differences (they look like a regular sine-wave-like oscillation) between the multiple satellites being spoofed as if the transmitters were not completely in sync.<p>Disclaimer: it's a guess - this is <i>NOT</i> my expertise.
perhaps they need to start using something like the Airborne Collision Avoidance System<p><a href="https://en.m.wikipedia.org/wiki/Airborne_collision_avoidance_system" rel="nofollow">https://en.m.wikipedia.org/wiki/Airborne_collision_avoidance...</a>