Abstract: Calibration apparatus for an antenna/receiver system having, in combination, means for receiving one or more electromagnetic data encoded signals from a transmitter and producing in response to each data signal two signals having a phase relationship, the receiving means comprising an antenna; signal processing means for using the phase relationship to determine information related to the data signals; and means for coupling a calibration signal into the antenna to produce two calibration signals having a known phase relationship; the signal processing means using the known phase relationship to compensate for phase errors when processing the data signals.