RAAS is used to prevent Runway incursions and collisions by using EGPWS system.
The inputs from the EGPWS are used to give positional awareness and avoid Runway incursions.
When A/c is taxiing and approaches the Runway hold area an audio voice stating that the A/c has approached Runway number in digits, As soon as the A/s enters the runway it again alerts that the A/c is on the Runway followed by the digits. It also calculates the distance remaining on the Take off roll and in the event of the remaining distance insufficient to complete the Take off, it gives a call including the length of remaining Runway distance available for Take off.
Since it utilizes the GPS based system it can recognize the A/c position with reference to the Runway and if the A/c attempts to Take off from a Taxiway instead of the Runway it gives Aural alerts.
While in flight the A/c position can be calculated with reference to the Runway alignment and at a specified distance as set by the manufacturer it announces the closest Runway heading, with reference to the A/c nose, A/c is going to land onto, giving available Runway length on approach as well as on roll out.
The equipment is entirely reliable on EGPWS and suffers from the errors experienced by EGPWS as well as all the errors experienced by GPS Satellite system.