What the "S" is going on at LHR?

[Departures Beckham]

I know that TA stands for Traffic Alert, but what does RA stand for? Also, does a TCAS get data relating to the position of other aircraft from equipment aboard the other aircraft (e.g. altimemter) or does the TCAS calculate the aircrafts position itself (in a similar way to primary radar).

Thanks in advance.