# On the Use of a Feedback Tracking Architecture for Satellite Navigation Spoofing Detection
## 1 Introduction  
The increasing concern for security in all electronic and telecommunication systems has affected many different sectors of today's society, one of them being the Global Navigation Satellite Systems (GNSS). Modern society strongly relies on GNSS, for a constantly increasing number of applications and services. However, the issues related to the security of such systems is sometimes underestimated. This is the case of some services relying on GNSS civil signals. In fact, the menace of intentional radio-frequency interference, such as jamming or spoofing attacks, is gaining momentum, and discussions are being held all over the world trying to find ways to protect GNSS civil users from these attacks. Nowadays, the effects of these intentional interferences, which are able to compromise the correct working of the GNSS receivers, are well known [1; 2; 3; 4; 5], and the need for improving the security of the receiver has been demonstrated [6; 7], especially in the case of applications whose malfunctioning would put people's safety at risk. Not only navigation and transportation companies often rely on the position obtained from the GNSS signals, but also power grids, telecommunications towers, banking procedures, etc., use GNSS to synchronize many different processes [3]. There are different types of intentional interference [5; 8] that affect the correct functioning of GNSS receivers [3; 5; 9; 10]. The most subtle and dangerous type of intentional interference is the spoofing attack, which consists of the transmission of GNSS-like signals, aiming at taking control of the receiver. If the spoofer takes control of the receiver, it is able to freely change its position and timing solutions without the receiver noticing any unexpected effect [3; 9]. Spoofing attacks have been demonstrated on several occasions [6; 11], and this has raised an alarm throughout the GNSS community. Different kinds of implementations of the spoofing attacks will be presented in Section 2. In order to design a countermeasure to spoofing attacks, this paper revisits and improves a tracking technique known as Extended Coupled Amplitude Delay Lock Loop (ECADLL), originally presented in [12; 13]. The goal of the work is to tailor the architecture to spoofing detection, making it also able to mitigate the effects created by spoofing attacks. Moreover, an effective algorithmfor spoofer detection with the capability to distinguish between multipath and a spoofing attack is defined. The ECADLL was tested against realistic spoofing scenarios, and the performance was compared against another recently proposed anti-spoofing technique, known as the Signal Quality Monitoring Technique (SQMT) [14]. With the introduction of a ratio metric detection algorithm in the monitoring block, the detection latency was reduced, and the computational load of the algorithm was decreased. The ECADLL consists of a feedback tracking architecture that was proposed as a multipath mitigation technique [12; 13] able to remove multipath signals from the tracking channels. Given the similar effects that spoofing signals and multipath signals have when observed in the correlation domain, the ECADLL was later proposed as a spoofing detection technique [15; 16]. However, a proper detection algorithm was never been detailed and tested in realistic scenarios. In the present study, we address these two unresolved aspects. The paper is organized as follows: first, in Section 2, a review of the types of spoofing attacks and a recall of the most common anti-spoofing techniques are presented. In Section 3, the theoretical description of the working procedure of the ECADLL is revisited. Section 4 defines the spoofing detection algorithm, and an analytic explanation on how to better exploit the information provided by the ECADLL is provided. In Section 5, the experimental setup and the different scenarios used to obtain the results are described.