# Spoofing Detection Using GNSS/INS/Odometer Coupling for Vehicular Navigation
## 1 Introduction  
Spoofing signals are designed to mislead GNSS receivers by generating fabricated synchronized navigation signals leading to fake navigation solutions [1; 2; 3]. Hence, detection and mitigation of spoofing attacks is critical for emerging applications such as autonomous vehicle navigation, environmental monitoring and forensic applications [4; 5]. Several related contributions have been made with focus on different layers of the receiver including antenna, IF samples, acquisition, tracking and navigation [6; 7; 8; 9; 10; 11; 12]. The spoofing detection techniques implemented in the pre-despreading and signal processing layers of a receiver are effective and can detect spoofing attacks faster than the methods implemented in the navigation layer [13]. However, these techniques require several modifications to current receiver designs. Several spoofing detection methods implemented in the measurement and navigation layers have been proposed. For example, Reference [11] implemented a position solution authenticity verification technique based on clock bias variation analysis of a moving receiver. A spoofing detection metric using carrier phase measurements with multiple receivers was implemented in [12]. In addition to the above standalone approaches, spoofing attacks can be detected by checking the consistency of the navigation solutions under test with other reference sources [13; 14]. Consistency checks can be performed in different ways including intra-system, inter-system, multi-frequency andmulti-sensor approaches. In the intra-system consistency check, the presence of spoofing signals can be detected by monitoring the consistency of the code and carrier Doppler or by monitoring the carrier-to-noise ratio [1; 13]. The emergence of different civilian GNSS constellations has led to the availability of multi-constellation receivers. Such a receiver can be designed to perform various inter-system cross-checks among different signal ensembles in order to verify the authenticity of received signal sets [14]. Modernized GNSS systems transmit civilian signals in different frequency bands. From a spoofer's viewpoint, it is considerably more difficult/costly to simultaneously spoof many frequency bands. Therefore, a multi-frequency receiver can perform some cross checks to verify the authenticity of received signal sets. Augmenting data with auxiliary devices such as IMUs can help the target receiver to discriminate against the spoofing threat [15; 16; 17; 18; 19; 20; 21; 22; 23]. In addition, a receiver can compare the solution extracted from received signals to position and navigation solutions obtained by other means, e.g., mobile networks or Wi-Fi access points [15]. Therefore, if the confidence region of different solutions does not have an intersection, there is a high likelihood of a spoofing attack. Another spoofing scenario rarely discussed in the literature is the case when the spoofer has access to the GNSS receiver antenna and may deny authentic signal reception by covering the antenna and feeding spoofed signals. In such a case, most proposed spoofing detection methods in various signal processing layers of a receiver are not functional. GNSS and inertial navigation systems (INS) have complementary error characteristics: GNSS has good long-term accuracy whereas INS has good short term accuracy. INS is self-contained, operates continuously and provides navigation solutions with low short-term noise. However, it suffers from accuracy degradation over time due to the integration of biases and drifts of the inertial measurement units (IMUs). Along with navigation solutions, attitude information can also be estimated from the INS, which is important for many applications. The complementary features of GNSS and INS make them a good choice for integration especially when GNSS observability is poor. Advantages and performance of loosely, tightly and ultra-tightly coupled GNSS/INS integration methods have been studied and reported by a number of researchers [24; 25; 26].