The traditional PPP/INS system is still not used as widely as the DGNSS/INS system in precise applications, although no local reference stations are required. The main reason that prevents its use is that the traditional PPP/ INS system is based on the float ambiguity solution, which leads to long convergence period and unstable positioning accuracy. We propose a tightly coupled ambiguity-fixed PPP/INS integration. First, the derivation of the observation model of the ambiguity-fixed PPP at the single-difference level using integer phase clock products from Center National d'Etudes Spatiales is presented in detail. Then the inertial navigation system model is presented. With these two models, the tightly coupled model of the PPP/INS integration is established. Finally, two carborne tests are used to evaluate the performance of the tight integration of ambiguity-fixed PPP and INS. Experimental results indicate that the proposed ambiguity-fixed PPP/INS integration is able to reach stable centimeter-level positioning after the first-fixed solution and its overall performance is comparable to that of the DGNSS/INS integration, and rapid re-convergence and re-fixing are achievable after a short period of GNSS outage for the PPP/INS integration.

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As Global Navigation Satellite System (GNSS) spoofing techniques are highly stealthy and pose a tremendous risk to targets using GNSS technology, studies on GNSS spoofing techniques have been in the spotlight. If the accurate position and velocity of the target receiver can be obtained, the target receiver can be covertly spoofed during the signal tracking stage using synchronous lift-off spoofing. However, it is often difficult to accurately obtain the position and velocity of a target in real GNSS spoofing scenarios. To address this problem, To study the effects of spoofing signals' power (relative to the real signal), code pulling rate, carrier Doppler shift, initial code phase difference, and carrier phase difference on the efficacy of spoofing, the intrusion of receiver's signal tracking loop by spoofing signals is mathematically modeled. Based on the model, an asynchronous lift-off spoofing for GNSS receivers in the signal tracking stage is proposed. Theoretical analysis and experimental results show that the new method resulted in stable Doppler frequency variations, short fluctuations in carrier-to-noise ratio (C/N) and signal lock time, and gentle changes to the receiver's 3D Earth-Centered Earth Fixed (ECEF) coordinates, when the target's position and velocity were approximately known during the intrusion period. The proposed spoofing method is highly feasible and could expand the scope of applicability of lift-off spoofing.

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Satellite navigation spoofing has become a central issue of jamming technology research because of its serious threat and ability to conceal itself. Increasingly, targets are equipped with more robust GNSS/IMU systems and normalized innovation squared (NIS) is used to detect interference. Therefore, it is harder to implement covert trajectory spoofing on a GNSS/IMU system than a GNSS-only target. In practice, spoofing is needed to control unknown targets. Therefore, covert trajectory spoofing for GNSS/IMU targets is an important issue. Hence, using the information fusion of a GNSS/IMU system, the influence of spoofing on loosely coupled GNSS/IMU positioning is derived. To avoid ill-posed equations when introducing a measurement deviation, a Kalman gain matrix local regularization method is proposed to accurately determine the measurement deviation. To avoid triggering the NIS detection alarm, the range that enables the introduced measurement deviation to remain concealed is calculated. Then, a two-step trajectory guidance algorithm is proposed to quickly guide the target onto the spoofing trajectory. The simulation results show that the proposed trajectory spoofing algorithm can guide a loosely coupled GNSS/IMU target along a spoofing trajectory without triggering the NIS detection alarm. The proposed method can remain concealed and has good theoretical and practical application value. INDEX TERMS Spoofing interference, trajectory spoofing, local regularization method, two-step trajectory guidance algorithm, concealment, GNSS/IMU, NIS.