Ground proximity warning systems (GPWS) are carried by aircraft to prevent controlled flight into terrain accidents (CFIT). GPWS uses radar altimeters to monitor the aircraft’s altitude and detect dangerous situations, providing visual and audible warnings to the pilot. The Enhanced Ground Proximity Warning System (EGPWS) uses GPS to provide accurate terrain data beyond the range of radar. Military aircraft use a more sophisticated range of equipment, including digital terrain maps and GPS, to project their flight path and give pilots advanced warning of potential collisions.
A ground proximity warning system, or GPWS, is a type of equipment carried by aircraft to warn pilots if they are dangerously low altitude and in danger of crashing. The primary goal of these systems is to prevent what is called a controlled flight into terrain accident, or CFIT, an accident in which an aircraft crashes into the ground, into water, or into an obstacle such as a mountain or building despite to be duly manned and in flight condition. This may be the result of factors such as navigation errors, pilot fatigue or disorientation, or reduced visibility due to weather conditions. CFIT incidents have become dramatically less frequent since ground proximity warning systems came into widespread use in the 1970s. The use of a GPWS on large aircraft is required by law in many countries.
The aircraft’s altitude is monitored by a ground proximity warning system with a radar altimeter, which transmits radio waves downward from the aircraft to determine how far the ground is. Most commercial aircraft-borne radar altimeters are short-range devices with ranges of less than one mile (approximately 1.6 km). Radar information is monitored and analyzed by a computer that can identify dangerous situations and trends in the data, such as dangerously fast rate of descent, dangerously close terrain, or unexpected loss of altitude. If dangerous conditions are detected, the ground proximity warning system provides visual and audible warning signals to the pilot.
The main limitation of standard ground proximity warning system designs is that it is only visible directly below the aircraft. It can detect when the plane is too low or losing altitude, but if the terrain itself rises steeply, the GPWS won’t be able to inform the pilot until the plane is already over rising terrain. Such a warning may come too late, especially if darkness or weather conditions have darkened the soil. In civil aircraft, this weakness was addressed by the development of the Enhanced Ground Proximity Warning System (EGPWS), a breakthrough made possible by the creation of the Global Positioning System (GPS). The EGPWS contains an electronic database of terrain and maintains constant contact with GPS to track the aircraft’s own location, enabling it to provide pilots with accurate terrain data beyond the range of their own radar.
The limitations of a conventional GPWS become a much more serious problem in a high-speed, low-altitude military aircraft. If such an aircraft is approaching terrain that presents a risk of collision, such as a hill or mountain, any warning of the distance of the aircraft from the ground that does not arrive until the aircraft is actually on the upslope will go to only fractions of second before the collision Some modern military aircraft, such as the American F-16 Fighting Falcon, the French Mirage 2000 and the Eurofighter Typhoon, are therefore equipped with a more sophisticated range of equipment combining radar altimeter, maps digital terrain and GPS link used in an EPGWS, with additional data from aircraft flight control and inertial navigation systems. This allows the aircraft to project its current flight path miles ahead and compare it to its elevation maps to give the pilot more advanced warning of potential collisions.
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