Originally Posted by IO540
GPS altitude is the only way to make anticollision work, because one cannot rely on the pilot setting (or even having) the right QNH.
For ACAS equipment, such as the TAS600, to work it's only necessary for mode C transponders to be correctly calibrated. The common reference pressure setting is then 1013mb, so vertical distances between aircraft can be easily calculated relative to each other, rather than using height above MSL.