Abstract: An apparatus and method are provided for Doppler estimation of a signal received over a time-varying transmission medium, the method including receiving the signal over a channel of the time-varying transmission medium; sampling a magnitude of the received signal repeatedly over time; identifying unidirectional threshold crossings of the sampled magnitude about a threshold magnitude relative to a mean magnitude; measuring differences between the sampled magnitude and the threshold magnitude for the identified crossings; analyzing the measured differences over time; estimating the time-varying nature of the channel based on the analyzed differences; and transmitting a signal indicative of the estimated time-varying nature.