Complex sentence: In the summer of 1942 the 4th Air Army in which Pokrishin's was a part of received the first mobile radar stations which were tested in aiding over-water interceptions of German and Romanian aircraft, that proved highly successful.
Split into simpler sentences: In the summer of 1942 the 4th Air Army which Pokryshkin was a part of received the first mobile radar stations. They were tested in aiding over-water interceptions of German and Romanian aircraft, and they proved highly successful.