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This work describes a method for phase calibration of spatial interferometry (SI) systems, which have recently seen increasing use in the wind profiling community. The various available techniques are inherently dependent on any phase imbalance in the system. The imbalance is caused by a path length difference between the multiple receivers, which is typically corrected at the time of construction. Aging of the system and other environmental changes can cause a modification in the path lengths, however. An obvious method for calibrating an SI system involves examining the phase differences between the receivers for a signal produced by a source with known location. For physically small systems, this can be accomplished by pointing the antenna beam toward a man-made source, such as a simple transmitter with the same center frequency as the radar. For large MST (mesosphere/stratosphere/troposphere) radar systems, such as the Middle and Upper (MU) Atmosphere radar, this method is impractical. The present work describes the calibration of the MU radar using the radio star Cygnus A as a source of known location. As the radio star traverses the beam, the MU radar passively tracks it by measuring the phase differences of the cosmic noise signal received on spatially separated receivers. Since the location of Cygnus A is known from astronomical calculations, any phase imbalance can be compensated. Data from May 1992 show the usefulness of the method.