One of the features that makes PCAS superior is the patent-pending built-in altimeter, which establishes an accurate base reference for the relative altitude.

Because the altitude information is relative to your altitude, the unit must know your local altitude at all times. Portable collision avoidance avionics which display altitude information must rely on a secondary source for altitude rather than the host transponder only. Competitive units which do not incorporate an altimeter will experience erroneous reporting.