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This paper addresses a probabilistic analysis of drift errors in passive remote wireless surface acoustic wave (SAW) sensing with multiple differential phase measurement (DPM). The rigorous probability density of the differential phase difference (DPD) is derived and its particular functions, all having no closed forms, are given for different signal-to-noise ratios (SNRs) in the received radio frequency (RF) pulses. Employing the von Mises/Tikhonov distribution, an efficient approximation is found via the modified Bessel functions of the first kind and zeroth order. Engineering features and small errors of the approximation are demonstrated. An analysis is given of the phase difference drift rate and error probability for the drift rate to exceed a threshold.