Marconi Astronics' Autonomous Landing Guidance (ALG) system enables pilots to see in low-visibility conditions, allowing them to land aircraft safely in many adverse weather conditions.
ALG is an "enhanced vision system" that uses millimeter-wave radar technology to provide a clear image of the runway and environs to the pilot on a head-up display (HUD). It works by sending out a high-fidelity signal that cuts through fog, snow, rain, dust, and smoke. That signal returns to the aircraft and is used to create an image that provides a visual representation of what is ahead. This image appears on the pilot's raster-capable HUD. The symbology from the aircraft's instruments is also added to the display, and what was hidden from view is now very visible, allowing the pilot to land in some of the worst conditions - regardless of any limited ground-based guidance capabilities on the runway.
The HUD image shows areas of high radar reflectivity - such as buildings, approach lights, rocks, and rough dirt - in green. Areas of low reflectivity, such as the runway, are blank.
The enabling technology of the ALG system is a 94 GHz imaging radar, which, due to its high frequency, is capable of penetrating low-visibility conditions. The radar features a low-power (300 mW), frequency-modulated-continuous-wave (FMCW) architecture, with the sensitivity and resolution that clearly identifies visual cues, giving pilots sufficient confidence to continue approach.
Traditionally, the response to the pilot's need for visual assistance to perform an approach and landing has been to automate the process as much as possible. However, the ALG system allows pilots to use existing navigational equipment such as GPS to navigate from departure to a point on the glideslope identified as the ALG transition point (typically 500-600 ft above ground level, 2 mi from touchdown). When a pilot makes a Category I precision approach and reaches the 200-ft decision height, it must maintain sufficient visual contact for the remainder of the approach to touchdown. ALG extends the pilot's visibility to "see through weather," beginning at altitudes more than twice the established Category I minimum of 200 ft and continuing through the visual transition point. Category III runway lighting and markers, Category III certified ILS/MLS equipment on the aircraft and ground, and pilot training in use of these equipment, procedures, and operations are no longer required.
Conceived by Marconi Astronics (formerly Lear Astronics), ALG originated from missile-seeker technology. In February 1994, an alliance of manufacturers, airlines, and government agencies led by Lear was awarded a multimillion dollar defense-conversion contract by the Defense Advanced Research Projects Agency (DARPA) to develop ALG.
An initial two-year Proof-of-Concept Phase included ground testing and extensive flight testing. Large platform testing on a United Airlines' Boeing 727-200 and the U.S. Air Force C-135 demonstrated the viability of the ALG system for both commercial and military applications.
In flight testing that began in September 1998, test pilots at the 418th Flight Test Squadron at Edwards Air Force Base, CA, used the ALG system during testing to successfully land a C-130H Hercules transport aircraft in actual visibility and ceiling conditions below the U.S. Air Force's minimum-allowed levels. The system was also used to land in simulated zero-visibility conditions.
To simulate zero visibility, flight test engineers covered the aircraft's cockpit windows with a translucent, orange plastic lining. While in flight, pilots wore a blue visor that prevented them from seeing through the orange plastic, but allowed them to see inside the cockpit in order to land the aircraft using the ALG system.
Flight testing consisted of about 35 flights, with the system totaling more than 130 flight hours. About 90% of the flights included simulating zero-visibility conditions. These tests culminated with landing in actual limited-visibility conditions at an airfield in Fresno, CA. Conditions at the airfield were foggy and below minimums (the restriction for C-130s flying with typical landing systems is a ceiling of 200 ft and a runway visual range of about 2600 ft). During this landing, the ceiling was 50 ft and the runway visual range was about 700 ft.
Frank Bokulich
