Is there a surveyor out there who could help me understand fully how PPK in UAVS works?

I understand most of the theory but I do not understand where the GNSS receiver fits in? I am okay that a RTK system needs a GNSS receiver for real time satellite connection, but PPK does not use real time satellite connection. So what is the GNSS receiver for in that case?

I would appreciate if someone could lift the fog.

I understand most of the theory but I do not understand where the GNSS receiver fits in? I am okay that a RTK system needs a GNSS receiver for real time satellite connection, but PPK does not use real time satellite connection. So what is the GNSS receiver for in that case?

I would appreciate if someone could lift the fog.