Differential Doppler is a common technique for emitter localization in which the signal of a stationary emitter is intercepted by at least two moving receivers. The frequency difference between the receivers is measured at several locations along their trajectories and the emitter's position is then estimated based on these measurements. This two-step approach is suboptimal since each measurement is performed independently, although all measurements correspond to the same position. Instead, a single step maximum likelihood approach is proposed here for known and unknown waveforms. The position is determined directly from all the observations by a search in the position space. Simulations show that the proposed method outperforms the differential Doppler method for weak signals while both methods converge to the Cramer Rao bound for strong signals. Also, in some cases of interest the proposed method inherently selects reliable observations while ignoring unreliable data.