This work describes a technique for improving the signal tracking performance of stationary GPS receivers that are vulnerable to wideband radio frequency interference (RFI). The method is directly applicable to ground-based reference receivers for differential GPS systems such as the Ground Based and Space Based Augmentation Systems (GBAS and SBAS), as well as other ground-based receivers that require high continuity of service. The high stability of atomic clocks (relative to crystal oscillators) is leveraged to significantly reduce the bandwidth of the receiver phase lock loop (PLL), thereby reducing the impact of wideband interference. The main contribution of this paper is a rigorous theoretical computation and experimental validation of the minimum achievable PLL bandwidth, considering all relevant contributions to phase jitter. The results show that using a rubidium oscillator PLL bandwidths as low as 0.25 Hz are achievable, resulting in reliable tracking at C/No levels as low as 25 dB-Hz. This is 7 dB lower than the 32 dB-Hz standard for GBAS ground stations . Experimental results are then presented to validate the theoretical analysis. The experiments are performed using an RF signal generator to directly simulate a GPS signal with wideband interference, which is realized as a deteriorated carrier-to-noise ratio. The signal generator also reproduces the effects of satellite motion and ionospheric dynamics. The simulated signal enters into a GPS RF front-end kit, which utilizes an external clock input from a commercially available rubidium clock. A software receiver is used for the tracking tests.