Not all EGPWSs use radar altitude.
Mode 4 does, use RADALT
for the rest EGPWS uses geomatric altitude:
Geometric Altitude is a computed aircraft altitude designed to help ensure optimal operation of the EGPWS Terrain Awareness and Display functions through all phases of flight and atmospheric conditions. Geometric Altitude uses an improved pressure altitude calculation, GPS Altitude, Radio Altitude, and Terrain and Runway elevation data to reduce or eliminate errors potentially induced in Corrected Barometric Altitude by temperature extremes, non-standard altitude conditions, and altimeter miss-sets.
Geometric Altitude also allows continuous EGPWS operations in QFE environments without custom inputs or special procedures by the flight crew when operating in a QFE environment. Required Inputs for Geometric Altitude The Geometric Altitude computation requires GPS Altitude with Vertical Figure of Merit (VFOM) and RAIM failure indication along with Standard (Uncorrected) Altitude and Radio Altitude. Ground Speed, Roll Angle, and Position (Latitude and Longitude) are used indirectly and are also required. Additionally, Corrected Barometric Altitude, Static Air Temperature (SAT), GPS Operational Mode and the Number of
Satellites Tracked are used if available. The required GPS signals can be provided directly from an external ARINC 743 / 743A receiver or from
the optional internal EGPWS Xpress GPS Receiver card. Standard Altitude, Corrected Barometric Altitude, and Static Air Temperature (SAT) are provided directly from the ADC. If SAT is not available, geometric altitude is computed using Standard Altitude with a corresponding reduction in accuracy.
Remember: The whole point of EGPWS and GPWS is based on warning the pilot for the mistakes a pilot made!!
SOAB