Abstract: A squaring loop for a GPS receiver locks to a carrier that is bi-phase modulated with a PRN acquisition code and a data message. The receive signal is down converted and hard limited and phase measurements are made between the rising edges of a local clock and every edge of the hard limited carrier. Phase measurements made with respect to extraneous or missing transitions resulting from the data bits that would result in phase locking instability are removed by a phase evaluation comparison. Phase measurements that are outside of limits determined in accordance with the noise characteristics of the channel are excluded thereby removing measurements that would cause the instability. The phase measurements outside the limits are resonably attributable to data bit transitions rather than to noise.