GPWS is widely used in commercial aviation to warn pilots that they are flying too low and so risk a so-called controlled flight into terrain (CFIT) – that is, a crash in which an airworthy aircraft under the control of its pilot hits the ground, the water or an obstacle.
Pilots are often unaware of an impending CFIT until it is too late to take preventive action. GPWS is designed to warn them in time. It uses a radio altimeter to measure how high the plane is flying, and sounds an alarm if it descends too close to the ground. At low altitudes the system also monitors wind direction and speed, along with a number of other parameters, so as to provide the pilot with an early warning of such phenomena as wind shear.
