A double stage NCO (numerically controlled oscillator)-based tracking loop with a new frequency discriminator is proposed to track the carrier frequency of the GNSS signal in signal degraded environments. The structure can be easily implemented and modified with conventional GNSS receivers, and the new frequency discriminator has absolute linearity in a wide pull in range. The principle and performance of the method are described and analyzed in detail. Simulation and field test with a software receiver show that this method can effectively track a weak signal compared with other presented methods in an open loop.