What is the "true static pressure"?
From what I can see, the "true" static pressure is the measured static pressure corrected for airspeed &amp; position (Static Source Error Correction).

In my copy of ARINC 429 (May 2004) I don't see a label definition that fits this "true static pressure". Label 171 can be used for manufacturer defined data, so I would see if you can get a better answer from the manufacturer of your data source.