The MSAW implemented in the US is in the ARTS terminal automation system. It is there because a TWA 707 hit a mountain on approach to IAD some years ago.. .. .The MSAW works by taking the target file information and comparing it to a grid of terrain elevations and obstructions. When the two get too close the MSAW activates a display blink and audio warning. It is not coupled in any way to the a/c radar altimeter or anything.. .. .Problems with it have been too many false alerts and the software must be maintained. Oft times it isn't, and since ARTS will be replaced by STARS any year now nobody wants to put the work into it.. .. .It was a KAL flight on Guam went into Nimitz Hill by the VOR. NTSB report has all kinds of details, not all pleasant.