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. ( https://en.wikipedia.org/wiki/LORAN-C#The_future_of_LORAN )
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.
Disclaimer: it's a guess - this is NOT my expertise.
A pretty darn good guess rbanffy. The spoofed signal is a 'hacked' re-transmission of a valid GPS signal rebroadcast by local high powered ground based stations. The basic concept is that you (via a Sat dish) receive a valid GPS signal while blasting out a wide band RF signal to mask/overwhelm a local GPS receiver. 'Splatting' the airwaves and then retransmitting a delayed GPS signal is enough to flick a location by miles. Being able to decode, alter and then re-transcribe (including checksums) the GPS data stream to an over saturated local receiver is .... doable (at a nation state level). The circular patterns are a rounding of the reflection from three (possibly four) retransmitting stations.
perhaps they need to start using something like the Airborne Collision Avoidance System