An all-weather aircraft landing-guidance system that operates at microwave frequencies and provides deviations from the landing runway centerline using a time-referenced scanning beam (TRSB) technique. The MLS was standardised in 1988 and approved for use in international civil aviation until at least the year 2020. MLS is used to support low-visibility instrument precision approach and landing operations in North America and Europe. In addition to the fixed-base MLS equipment design, a compact mobile microwave landing system (MMLS) equipment design exists. The instrument landing system (ILS) is also standardised internationally and approved for use indefinitely as countries implement their transition to new technologies. Standards for a third landing system, the Global Navigation Satellite System (GNSS), based primarily on Global Positioning System (GPS) technology, exist. Multimode receivers enable an aircraft to conduct an instrument approach using ILS, MLS, or GNSS.