Not a TC expert, but the following might be relevant.
1. Mode 'S' (and plain vanilla Mode 'C') will always transmit an actual level using the transponder with reference to the Standard Pressure Setting. It does not care what the pilot has set as the pressure datum on the aircraft altimeter.
2. The Selected Flight Level is what the pilot has set in his FMS or Mode Control Panel, or whatever piece of kit gives him an interface to his autopilot and management systems. It has no datum attached to it, except that the pilot (or system) will be using whatever setting is on the altimeter to monitor or achieve level compliance.
3. STCA processing is carried out on the 'raw' Mode 'C' or 'S' actual level information. In otherwords, using the reported Flight Levels from the aircrafts' transponders. An entirely separate piece of software carries out the altitude/Flight Level conversion for display to the ATCO. Thus, regardless of the displayed level datum being shown to the controller, STCA will always be working to a common value when deciding whether to give an alert or not.
Last edited by PPRuNe Radar; 10th Mar 2006 at 10:27.