The system needs to transmit a CW signal.
This signal is then reflected back the Rx via an active tag at the far end in a controlled TDD method
The Received phase is then compared to the transmitted phase.
Thus the Tx and Rx PLLs need to be phase locked with each other and on during the complete cycle.
Any phase drift or movement from the Tx synth WRT to the Rx synth will result in a measurement error.