Refer to figures.
The Ground Proximity Warning System (GPWS) generates advisory Alerts and mandatory response Warnings to the flight crew in respect of their proximity to terrain.

Typically, it takes into account the aircraft’s height as well as its descent rate and configuration. It has no terrain display or map, no predictive capability, and is radio altimeter based (unlike TAWS and HTAWS, which also use a 3D database

Specifically, the GPWS get its information from the:

• Radio altimeter;

• the Air Data Computer (ADC);

• NAV/ILS receiver (Glideslope);

• Gear position (plus flap position for aeroplanes)

as modified for helicopters, over a range of 50-2450 feet.

The GPWS will warn the crew in case of (amongst others):

• dangerous proximity to the ground

• loss of altitude during take-off or missed approach

• wrong landing configuration

• descent below the glide path

The basic installation has five modes of operation:

• Mode 1 - excessive (barometric) rate of descent

• Mode 2 - excessive closure rate, from a reducing radio altitude.

• Mode 3 - negative climb rate (for radio altitude) or sinking after take-off or go-around.

• Mode 4 - approaching too close to the ground with gear up.

• Mode 5 - going too far below the glideslope (more than 1.3 dots), assuming that the pilot has tuned the correct ILS frequency.

• Mode 6 - Miscellaneous stuff like automatic height callout and bank angles, usually company-specific.

• Mode 7 - Windshear Mode. Windshear warning results in the illumination of red Windshear Warning lights and an aural siren followed by the message "WINDSHEAR, WINDSHEAR, WINDSHEAR".

Finally, the manufacturers of GPWS equipment draw a distinction between the more and the less urgent warnings by calling:

• The LESS urgent “ALERTS” which require a corrective response.

• The MORE urgent “WARNINGS” which require an immediate climb manoeuvre.

For these two urgent cases, GPWS must have an aural warning signal and may have a visible signal.