 Many Soviet regiments underwent preliminary training at Soviet bases in the neighboring Soviet Maritime Military District.Soviet air defense troops also began to arrive along the Yalu, setting up radar installations, ground control centers, searchlights and large numbers of anti-aircraft guns to deter any attacks on the Chinese airfields.While UN pilots chafed at the restrictions imposed on attacking the MiG's Chinese airfields, it wasn't known until many years later that the MiG pilots themselves operated under tight restrictions.