Measurements of frequency and/or phase are taken at the signal ports (S1-S4) of an ISS device (10). These measurements are used to determine phase errors within the ISS device, and phase errors due to the antenna cables (C1-C4). On port, (e.g., S1) is selected as the reference port and then, based on these determined phase errors, offsetting phase errors are determined to correct the phase for the other ports (e.g., S2, S3, S4) with respect to the reference port. The signals at the antenna ports (A1-A4) are then in phase when an omnidirectional antenna pattern is desired from the TCAS antenna array (16). One in embodiment the frequency of the calibration signal is fixed; in another embodiment two different, fixed frequencies are used; and in still another embodiment the frequency is swept to achieve a predetermined measured phase difference.