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Differential computation is necessary in velocity determination using carrier phase measurements from the global navigation satellite system (GNSS). A method of nonlinear tracking-differentiator is proposed to suppress both noise amplification and time delay introduced by differentiation. The carrier phase rate can be derived more precisely using the nonlinear tracking-differentiator and the time delay is compensated by second-order derivative estimation from the outputs of the differentiator. Simulation and experiments for the Gobal Positioning System (GPS) and Chinese Beidou system show that this approach can effectively improve the accuracy of velocity determination, especially under noisy or bad geometry situations.