After reading everything available on several units, he selected a model from the General Post Office (GPO) that worked at what was at that time very high frequencies. As part of their tests of this system, in June 1932 the GPO published a report, No. 232 Interference by Aeroplanes. The report recounted the GPO testing team's observation that aircraft flying near the receiver caused the signal to change its intensity, an annoying effect known as fading.[15]