The Instrument Landing System (ILS) was introduced in the post-war era based on the SCS 51 concepts. This used two separate radio signals, one for lateral guidance, the "localizer" and the other for vertical guidance, the "glideslope". They both worked on the same basic principle; each of the signals was broadcast on a separate carrier frequency with a fixed relationship so that the glideslope frequency was always a fixed amount away from the localizer.