Re-calculating S-parameter error terms after modification of a calibrated port

A two-port S-parameter calibration between a first port and a second port of a test system having a multi-port vector network analyzer is performed to provide a first S-parameter calibration of the test system. A transfer device is connected between the first and second ports of the test system. A port of the test system is changed to provide a second state of the test system, and a plurality of ratioed un-corrected parameters of the transfer device are measured with the test system in the second state. A second S-parameter calibration of the test system in the second state is determined using the ratioed un-corrected parameters and S-parameter data.