Microwave landing system

Microwave landing system. It is an advanced, all-weather precision approach and landing guidance system that was developed to replace the older Instrument Landing System (ILS). Utilizing microwave transmissions in the C band and Ku band, it provided greater flexibility, accuracy, and a wider selection of approach paths compared to its predecessor. The system was standardized internationally through the efforts of organizations like the International Civil Aviation Organization and saw significant development and testing, particularly in the United States and the United Kingdom, during the 1970s and 1980s.

Overview

The primary purpose was to overcome the limitations inherent in the widely used Instrument Landing System, which operates on VHF and UHF frequencies. Unlike ILS, which offers a single, fixed glide path, this system provided multiple approach angles, including steeper glide slopes beneficial for aircraft such as VTOL types and operations at airports with obstacle clearance challenges. Key operational advantages included a wider coverage sector, reduced susceptibility to multipath interference from buildings or terrain, and the ability to support curved and segmented approach procedures. This flexibility was intended to enhance airspace efficiency and safety at major hubs like Heathrow Airport and O'Hare International Airport.

Technical description

The ground installation consisted of an Azimuth Station providing lateral guidance and an Elevation Station for vertical guidance, both transmitting time-referenced scanning beams across the coverage sector. These transmissions were in the C band (5000-5250 MHz) for the azimuth and Ku band (15.4-15.7 GHz) for elevation. An aircraft's onboard receiver, part of its avionics suite, measured the time difference between the scanning beams to compute precise azimuth and elevation angle data relative to the runway. This data was then displayed on the pilot's flight director or could be coupled to the autopilot for a fully automated approach. The system also included a high-integrity data link for transmitting information such as runway identifier and wind shear alerts.

Development and history

Development was driven by the need for a more capable landing system to support the increasing density and performance of jet aircraft in the 1970s. In the United States, the Federal Aviation Administration and NASA led significant research, with competing prototypes from companies like Texas Instruments and Hazeltine Corporation. Parallel development occurred in the United Kingdom under the Royal Aircraft Establishment and in Australia. A major international collaboration through the International Civil Aviation Organization culminated in the selection of the Time-Reference Scanning Beam (TRSB) technique as the worldwide standard in 1978, a decision heavily influenced by work from Boeing and the MIT Lincoln Laboratory.

Comparison with ILS

Compared to the Instrument Landing System, it offered superior performance in several key areas. It was largely immune to the site sensitivity and signal reflections that plagued ILS installations near large structures like the Empire State Building. The availability of multiple glide paths, from 0 to 30 degrees, provided operational flexibility that the single, typically 3-degree ILS glide path could not. Furthermore, its use of microwave frequencies allowed for smaller antenna arrays and avoided congestion in the increasingly crowded VHF navigation band. However, these advantages came with significantly higher complexity and cost for both ground infrastructure and aircraft avionics.

Implementation and deployment

Following ICAO standardization, implementation programs began in several nations. The Federal Aviation Administration initiated an ambitious deployment plan across the United States, while the UK Civil Aviation Authority planned installations at major airports. Countries like Australia, Canada, and Japan also conducted trials. The system was installed at locations such as Washington Dulles International Airport and Toronto Pearson International Airport. Military applications were also pursued, with the United States Navy and Royal Air Force evaluating it for use on aircraft like the F/A-18 Hornet and Panavia Tornado.

Decline and legacy

Despite its technical promise, widespread adoption was curtailed by the rapid development and global acceptance of the Global Positioning System-based Satellite navigation. Systems like the Wide Area Augmentation System and Local Area Augmentation System, developed by the Federal Aviation Administration, provided similar precision approach capabilities without requiring expensive ground-based infrastructure at each airport. By the 1990s, most implementation programs, including the FAA's national plan, were canceled or scaled back. Its legacy lies in advancing the technology of precision approach systems, influencing later ground-based augmentation system designs, and demonstrating the operational value of flexible approach paths, a concept now realized through Performance-Based Navigation and Required Navigation Performance procedures. Category:Aviation navigation systems Category:Avionics Category:Microwave technology