Performance Analysis and Experimental Validation of Broadband Interference Mitigation Using an Atomic Clock-Aided GPS Receiver

Ground Based Augmentation System (GBAS) ground installations have proven to be vulnerable to Radio Frequency Interference (RFI), mainly caused by the illegal use of personal privacy devices (PPD). Recent research has demonstrated typical properties of popular PPDs found in the market and has concluded that the effect of PPD interference on the GPS signal is nearly equivalent to that of a wideband signal jammer. This threat degrades both continuity and integrity for the existing GBAS system. In this work, a baseband strategy using resources that are practically implementable at GBAS ground stations are considered. Among those resources, we focus on using atomic clocks as the main tool to mitigate broadband GNSS signal interference. For GPS receivers in general, poor stability of the temperature controlled crystal oscillator (TCXO) fundamentally limits the potential to reduce tracking loop bandwidth because of the need to maintain a minimum loop bandwidth to track the dynamics of the clock itself. This limitation becomes much less constraining when using an atomic clock at the receiver, especially in the static, vibration-free environment of a GBAS ground station. High frequency-stability atomic clocks naturally reduce the minimum required bandwidth for tracking clock errors. In this paper, analyses are conducted to theoretically obtain the minimum tracking loop bandwidths using clocks of varying quality. Carrier tracking loop performance under deteriorated C/N-0 conditions (i.e., during interference) is investigated. The tracking loop bandwidth reductions and interference attenuations that are achievable using different qualities of atomic clocks, including chip-scale atomic clocks (CSAC) are provided in this paper. In addition to the theoretical analyses, actual GPS intermediate frequency (IF) signals are sampled using a GPS RF frond-end kit connected to a commercially available atomic clock (CSAC in this work). The sampled IF data are fed into a software receiver together with and without simulated wideband interference to evaluate the performance of interference mitigation using atomic clocks. The actual tracking errors generated from real IF data are used to validate the system performance predicted by the preceding broadband interference mitigation analyses.