# On the Use of a Feedback Tracking Architecture for Satellite Navigation Spoofing Detection
## 2 Types of Spoofing Attacks and Countermeasures  
In recent research, the different types of spoofing attacks have been classified based on how the attack is constructed and on the difficulty of detecting it by a receiver point of view [9; 17]. A simplistic spoofing attack is based on the use of a signal simulator to build the fake signal and re-transmit it in order to fool the receiver. This type of attack is very simple to realize, but it is also expensive and easily detectable by means of trivial techniques, given that a large signal power is needed and that the fake signal is not synchronized with the constellation. The second type is known as intermediate attacks or receiver-based spoofing attacks. This type of spoofer has a built-in receiver that collects and tracks the satellite signal parameters, in order to generate a new signal that is consistent with the current constellation [11] and transmits it to the target receiver. The third type of attack is called the sophisticated receiver-based spoofer. It aims to overcome one weakness of the intermediate attack, which is that it only broadcast from a single antenna and direction, thus the sophisticated version uses several different antennas to broadcast each satellite signal in order to be undetectable through anti-spoofing techniques that rely on angle of arrival discrimination. However, these attacks have a much higher complexity level than the previous two types, making them very difficult to realize. From the three types of spoofing attacks, the intermediate spoofer is the most feasible and probable to cause damage, so is the one we will focus on throughout this article. Since the first major publication reporting the feasibility of building a spoofer with a software-defined receiver and low cost components [18], the GNSS community has been continuously developing different anti-spoofing techniques, aimed at defending against the malicious signals at different stages of the receiver. These anti-spoofing techniques have very different approaches, and they can be classified into three main groups [3; 9]:  
1. Cryptographic and authentication techniques: These methods propose to authenticate the incoming signal, using signals from different receivers or frequencies (L1/L2). Alternatively, they exploit the transmission of the P(Y)secured channel in order to identify if the received signal is authentic or not [19; 20; 21]. 2. Antenna-aided techniques: These techniques are based on the wide spatial correlation that the satellite signal has with respect to the spoofer signal during an intermediate attack. These techniques are vastly different, ranging from angle of arrival detection [22,23], to the use of synthetic arrays [24] and beam forming techniques that are able to mitigate the attacks by pointing null lobes at the spoofing signal [25,26]. 3. Receiver-level signal processing techniques: These methods are based on signal processing techniques, which focus on aspects that differ between the spoofing signal and the satellite one. Many different techniques have been proposed in this category, such as: techniques based on signal power monitoring, either monitoring the carrier to noise ratio (\(C/N_{0}\)) [27], absolute in-channel power measurements [28] or relative power variations [29]. Other signal processing techniques are based on the distribution analysis of the correlator output [30]. There exist also signal quality monitoring techniques, which aim at detecting distortions in the correlation function via the use of ratio metric tests [31, 32, 33, 34, 35]. Other techniques are based on the time of arrival and other consistency checks. Techniques based on goodness-of-fit tests have been proposed [36] along with techniques based on Receiver Autonomous Integrity Monitoring (RAIM) discrimination algorithms [29,37]. Finally, techniques based on the multiple Delay Lock Loop (DLL) used for spoofing detection and mitigation [12,13,15,16] have been proposed.