Who is actually obstructing aircrafts’ GPS in the Baltic?

.ON APRIL 29TH Finnair, Finland’s nationwide airline company, said that it would put on hold tours to the far eastern Estonian city of Tartu for a month. Recently pair of airplanes were actually pushed to reverse after their GPS signals were interfered with. GPS jamming has boosted sharply in the Baltic location over the past 2 years.

Who lags it and just how harmful could it be?