In a modern production transmitter, if the power is being digitally sampled, and DSP processed, there is nothing to calibrate, i.e. there is no mechanical meter movement or component "drift" to create a skewed reading, short of failing entirely. Given the same sample, and same directional coupler, it should read the same on day 1 as in year 5. Calibration is really done when the sampling method is initially designed, using scale factors to yield results verifiable and traceable to a calorimeter. But that is not something that is adjusted on a per unit basis. That is why we, at Nautel, do not feel it is any longer necessary to have a BPME or any other external meter, as it only creates an ambiguity in readings that would cause an inspector to ask "which one is right?" A different reading on the external meter is almost inevitable given it is derived from it's own directional coupler located further down the line than the transmitter output. That said, it's still a good idea to periodically check the power output against the PA voltage, PA current, and efficiency as a sanity check.