Global Satellite Navigation Systems (GNSS) have become the primary Position Navigation and Time (PNT) source in the maritime navigation domain, however natural and artificial interference may disrupt the GNSS signals causing performance degradation or service outages. In order to minimize the maritime navigational risk, a terrestrial backup system called Ranging Mode (R-Mode) is under development. Based on the signal of opportunity (SoOP) concept, the phase measurements of the medium frequency differential GNSS (MF DGNSS) signals can be exploited for positioning purposes. Among the possible techniques for phase estimation, the maximum likelihood (ML) and the discrete Fourier transform (DTFT) are currentlyused.