Along-Track Interferometry (ATI) has been widely used for ground moving target indication (GMTI) in airborne synthetic aperture radar (SAR) systems. In ideal cases, the ATI phase obtained using two phase centers that are aligned in the along-track dimension yield clutter-only pixels with zero phase. However, the platform's motion may create a cross-track displacement between the two phase centers and in turn offset the phase centers' baseline from the along track dimension. This cross-track offset leads to non-zero phase for clutter-only pixels, necessitating calibration for accurate GMTI. This paper proposes a blind calibration method to correct the along-track baseline error in ATI-SAR systems. The success of the proposed method is shown on a set of measured data from the Gotcha sensor.