# Detection of Induced GNSS Spoofing Using S-Curve-Bias
## 1 Introduction
Subsequently, the spoofing slowly increases the power beyond that of the authentic signal, pulls off the authentic signal towards right until the receiver is completely controlled by the spoofing as shown in Figure 1d-f. In Figure 1f, it is about two chips code phase ahead of the authentic signal. In fact, the spoofing only needs a slightly higher power to assure a successful locking of the victim receiver. Since the tracking loop maintains lock in this spoofing process, it would lead to an incorrect position or timing for the victim receiver, while no loss of lock will be detected by a conventional GNSS receiver. Figure 1: Schematic diagram of the induced spoofing attack process. In (**a**), the spoofing lags the authentic signal with a lower power. From (**a**) to (**c**), the spoofing gradually approaches the authentic signal. Subsequently, the power of spoofing is greater than that of the authentic signal and the receiver is gradually controlled by the spoofing in (**d–f**). The existing anti-spoofing methods can be roughly divided into two categories, namely, detection techniques and suppression techniques. Among them, the detection techniques are designed to identify whether the receiver has been subjected to a spoofing attack, and the purpose of suppression techniques is to help the victim receiver restore its positioning and navigation capability. Here, the existing spoofing detection techniques are briefly reviewed, including spatial-based detection techniques, signal power detection techniques, navigation information detection techniques, integrated navigation detection techniques, encryption authentication techniques, and signal quality monitoring techniques. Spatially-based detection techniques utilize the characteristics of the spoofer, which transmits multiple satellite signals with the same antenna. On the contrary, the authentic satellite signals come from different directions. Therefore, the spoofing signals are spatially coherent and the spoofing can be identified by determining the correlation degree among satellite signals through a spatial processing technique [15; 16; 17]. Such techniques are typically costly to implement, since they require the use of multi-antenna receivers and large observation intervals. For signal power detection techniques, the receivers continuously monitor the power related parameters and declare a spoofing attack when there is an outlier. The parameters related to power include carrier-to-noise ratio [18; 19], absolute power, and distribution checks of correlator outputs [20]. These techniques require the receiver to have a high accuracy in measuring parameters of the received signal, and the hardware complexity of the receiver will also increase correspondingly. Navigation information detection techniques detect signal code rate and phase rate. For the real satellite signals, the Doppler frequency and code rate are generated by the relative motion between the GNSS satellites and the receiver, thus the two have consistency [21; 22]. Simple spoofing can not keep the consistency of Doppler frequency and code rate. When there is inconsistency between them, it is decided to suffer from a spoofing attack. This method is invalid for intermediate and sophisticated spoofing attacks because they overcome such consistency checks. Integrated navigation detection techniques involve combinations of GNSS signals and other navigation devices, which assist the receiver to identify spoofing effectively [23]. This techniques increase the complexity of GNSS receiver's hardware and software. Most encryption authentication techniques need to change the structure of GNSS signals and this techniques can not be applied in a short time [24; 25; 26]. Signal quality monitoring techniques determine whether there is interference by monitoring the correlation distortion of the tracking loop according to the pseudo-code's auto-correlation property [7; 24; 26] or even pre-correlation signal quality [27; 28]. Several metrics are also proposed in literature [29; 30; 31].