A ground proximity warning system, or GPWS, is a type of equipment carried by airplanes to warn pilots if they are at a dangerously low altitude and in danger of crashing. The main purpose of these systems is to prevent what is called a controlled flight into terrain crash, or CFIT, an accident in which an aircraft crashes into the ground, the water, or an obstacle such as a mountain or building despite being properly crewed and airworthy. This can be the result of factors such as navigation errors, pilot fatigue or disorientation, or reduced visibility owing to weather conditions. CFIT incidents have become dramatically less frequent since ground proximity warning systems entered widespread use in the 1970s. The use of a GPWS in 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 plane to determine how far away the ground is. Most radar altimeters carried by commercial aircraft are short-range devices with ranges of less than a mile (about 1.6 km). The information from the radar is monitored and analyzed by a computer that can identify hazardous situations and trends in the data, such as a dangerously rapid rate of descent, dangerously close ground during, or unexpected loss of altitude. If hazardous conditions are detected, the ground proximity warning system gives visual and audio warning signals to the pilot.
The primary limitation of standard ground proximity warning system designs is that it looks only directly below the aircraft. It can detect when the airplane 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 aircraft is already over the rising terrain. Such a warning may come too late, especially if darkness or weather conditions have obscured the ground. In civilian aircraft, this weakness was addressed by the development of the enhanced ground proximity warning system (EGPWS), an advance made possible by the creation of the global positioning system (GPS). The EGPWS contains an electronic terrain database and maintains constant contact with the GPS to track the aircraft's own location, allowing it to provide pilots with accurate data on terrain beyond the range of its own radar.
The limitations of a conventional GPWS become a far graver problem in a high-speed military aircraft flying at low altitudes. If such an aircraft is approaching terrain that presents a collision hazard, such as a hill or mountain, any warning of the aircraft's distance from the ground that doesn't come until the plane is actually over the rising slope will arrive mere fractions of a second before collision. Thus, some modern military aircraft, such as the the American F-16 Fighting Falcon, the French Mirage 2000, and the Eurofighter Typhoon, are equipped with a more sophisticated array of equipment that combines the radar altimeter, digital terrain maps, and GPS link used in an EPGWS, with additional data from the aircraft's flight control and inertial navigation systems. This allows the aircraft to project its current flight path miles ahead and check it against its elevation maps to give the pilot more advanced warning of possible collisions.