Phase Center Offset (PCO) is a key concept in satellite navigation and precise GNSS positioning. It refers to the difference between the physical antenna reference point and the actual phase center. The phase center is the point where GNSS signals are effectively received. However, this point does not remain fixed for all frequencies or satellite geometries.
First, every GNSS antenna has its own unique phase center location. This location varies depending on signal frequency and signal direction. For example, the L1 and L2 signals may have different phase centers in the same antenna. Next, PCO must be carefully measured and corrected. If ignored, it introduces systematic errors in GNSS measurements. These errors degrade position accuracy, especially in high-precision applications.
Manufacturers usually provide PCO values for their antennas. These are defined in a standardized coordinate system. Survey and geodetic applications use this information to correct position estimates. Then, GNSS processing software applies PCO corrections. These corrections align the measured signal phase with the true geometric point. Without this step, baseline solutions and height estimations could be biased.
Differences between PCO and PCV
Moreover, PCO is different from phase center variation (PCV). PCO refers to a fixed offset, while PCV describes signal variation around the phase center. Both must be modeled for centimeter-level precision.
Antenna calibration helps determine accurate PCO and PCV values. This process can be absolute or relative. Absolute calibration is more precise and uses controlled test environments. Additionally, satellite antennas also have PCO. These must be considered in precise orbit and clock products. International GNSS services publish satellite PCO values for public use.
In real-time applications, ignoring PCO leads to errors in differential positioning. Therefore, correction models are critical for applications like RTK and PPP.
Finally, modern multi-frequency antennas often include detailed calibration files. These allow high-precision receivers to apply frequency-specific PCO corrections. This improves both horizontal and vertical accuracy.