Millimeter-wave channel sounders are much more sensitive to phase drift than their microwave counterparts by virtue of shorter wavelength. This matters when coherently combining untethered channel measurements ? scanned over multiple antennas either electronically or mechanically in seconds, minutes, or even hours ? to obtain directional information. To eliminate phase drift, a synchronization cable between the transmitter and receiver is required, limiting deployment range and flexibility indoors, and precluding most outdoor and mobile scenarios. Instead, we propose a blind technique to calibrate for phase drift by post-processing the channel measurements; the technique is referred to as blind because it requires no reference signal and, as such, works even in non-line-of-sight conditions when the (reference) direct path goes undetected. To substantiate the technique, it was tested on real measurements collected with our 60-GHz virtual phased-array channel sounder, as well as through simulation. The technique was demonstrated robust enough to deal with the most severe case of phase drift (uniformly distributed phase) and in non-line-of-sight conditions.