Effectiveness of GNSS Spoofing Countermeasure Based on Receiver CNR Measurements

A perceived emerging threat to GNSS receivers is posed by a spoofing transmitter that emulates authentic signals but with randomized code phase and Doppler over a small range. Such spoofing signals can result in large navigational solution errors that are passed onto the unsuspecting user with potentially dire consequences. In this paper, a simple and readily implementable processing rule based on CNR estimates of the correlation peaks of the despread GNSS signals is developed expressly for reducing the effectiveness of such a spoofer threat. Consequently, a comprehensive statistical analysis is given to evaluate the effectiveness of the proposed technique in various LOS and NLOS environments. It is demonstrated that the proposed receiver processing is highly effective in both line-of-sight and multipath propagation conditions. 1. Introduction GNSS satellites are approximately 20,000？km away and transmit several watts of signal power such that at the ground level, the power output of a 3-dB gain linearly polarized antenna is nominally ？130？dBm [1]. As such, a modest jammer can easily disrupt GNSS signals by increasing the noise floor, making the acquisition of GNSS signals rather difficult. A high processing gain based on a long integration time is one of the possible countermeasures to overcome a noise jammer. Nevertheless, if the GNSS receiver undergoes random motion and is subjected to multipath fading as in a typical urban environment, then the channel decorrelates quickly such that attaining such large processing gains to overcome the jamming is not feasible. However, the noise jammer is at least detectable as the spectral power in the affected GNSS receiver band will be abnormally high. Hence, the jammer can deny service but the user is aware of being jammed, limiting the damage potential of the jammer. Also the jammer is relatively easy to locate with radio direction finding and to potentially disable as its spectrum is significantly larger than the ambient noise [2, 3]. A more insidious threat is the standoff spoofer which broadcasts a set of replicas of the authentic SV signals currently visible to the mobile GNSS receiver [2]. The unaware receiver computes the navigation solution based on these counterfeit signals which are passed on to the user as being reliable with potentially damaging consequences. GNSS-based location estimates that are inaccurate but assumed to be accurate are potentially more damaging to the user than in the jamming case where at least the user knows that the service is temporarily unavailable. As the receiver processing