This paper presents a new algorithm, the overlapping averging 4-frame (OAF) algorithm, which can reduce the phase error caused by phase shifter error. The relationship between the accuracy of OAF algorithm and phase shifter errors is given. Based on this relationship, it is verified that the OAF algorithm can reduce the phase error from phase shifter error. The computer simulation shows the magnitude of a calibration error and nonlinearity in the phase shifter for a number of different phase-measurement algorithms.