How does RTKANTENNA work?

RTKANTENNA applies the user defined values from

*THISANTENNAPCO = to offset position from L1 phase centre to antenna reference point
*THISANTENNAPCV = to take into account phase centre variations

However, this only works if the RTK filter is running (won't work in single point positioning). 
These offsets will also be applied to BESTPOS and BESTGNSSPOS messages when running SPAN.