MSAW is one of a family of ground-based safety nets designed to provide warnings to air traffic controllers.
There is not a standard or performance specification for MSAW, so the features of each system varies depending on the system manufacturer, influenced sometimes by the needs expressed by the ANSP (Air Navigation Service Provider).
Many modern MSAW systems store the alerting surface as a grid of elevation values which represent the terrain (i.e. a terrain model). A margin above the terrain (typically a few hundred feet) is often included in the terrain model. Almost invariably, MSAW applies a simple linear prediction to determine whether each aircraft under ATC will be lower than the terrain model within a specified warning time or a 'look-ahead time'.
Some older MSAW systems which have no prediction are probably still in operational use - in these cases the MSAW surface may be set slightly higher, although the balance between warning time and nuisance alert rate may be harder to get right.
MSAW inhibition areas are defined to encompass the final approach paths to runways which allows aircraft to land at airports/airfields without triggering an MSAW alert. If APM (Approach Path Monitor) is available, then it often makes sense for the APM approach funnel and the MSAW inhibition area to have the same dimensions so that there is no gap in the protection afforded by the ground-based safety nets.