The Ground Proximity Warning System (GPWS) generates advisory Alerts and mandatory response Warnings to the flight crew in respect of their proximity to terrain. SOPs will state the actions required and whether there is any element of discretion in the response depending on whether it is an 'Alert' or a 'Warning' which been generated. This may therefore mean that the flight crew will take the specified action e.g. initiating a mandatory ‘terrain avoidance manoeuvre’ to climb away from the terrain. It should be noted that SOPs typically state that if the pilot has visual contact with the terrain during daylight and is assured that physical contact with the terrain is not a factor, then an 'Alert' may be ignored. However, should a 'Warning' be generated, a mandatory maximum rate climb is usually specified unless above a specified level.Such mandatory action may or may not have discretionary disregard conditions specified in the SOPs of a particular Operator, for example if it is VMC in daylight.
Large transport aeroplanes are usually equipped with a look-ahead version of GPWS generically called Terrain Avoidance and Warning System (TAWS) although this system is often also referred to be its proprietary name EGPWS. Such equipment has a real time position input which is compared with an internal terrain database. The position input is usually GPS and internal to the TAWS unit, but may alternatively be from the aeroplanes main navigation system. Controllers on fam flights may observe a map display of potential terrain hazards if this option is selected by the flight crew, which is likely when the aeroplane is in areas where terrain is significant in relation to the intended flight path.