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

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.