EGPWS??
As grounded has said. Rad Alt is used for more than a landing aid.
An a/c can be clean and at a considerable altitude and still be in proximity to terrain. The crew need to know this. I've been in an a/c when the GPWS has triggered (Mode 2 on initial approach into KTM) and I can tell you we were doing more than 195kts at the time (ground speed and TAS are a lot higher at altitude)
Sorry I don't have a reference but you may find one on the net.