[Antenna hardware]: unknown position of the Antenna phase center (APC)
Antenna phase center is unknown for each antenna and needs to be calibrated to get the exact value. The build in antenna is an OEM model from Tallysman and phase offset could be uniq for each model, antenna phase offset highly depends on it surroundings. But its to be assumed phase center is close to ARP. A test could reveal that.
On my unites it varies between few mm to almost 4cm (reach with tin foil) and varies over time. Phase offset also changes over time.
https://www2.unb.ca/gge/Resources/gpsworld.february05.pdf
cited from TB_RTK
