Long term lurk here but this might be of use.
ADSBExchange has a flag where it can show a/c reporting degraded GPS on ADSB.
globe. adsbexchange. com/?badgps (not sure i can post active links but basically ADSB Exchange site with the flag ?badgps after it).
Obviously a single aircraft in a place is likely just something unrelated but whole groups of aircraft showing it in a cluster is highly suggestive. Just needs nearby traffic in an area to report it.
Kaliningrad showed this last night and the well known cluster out of Syria, Kirkuk along with Ankara, Istanbul and Moscow show up every day.