A New MIMU/GNSS Ultra-Tightly Coupled Integration Architecture for Mitigating Abrupt Changes of Frequency Tracking Errors  
[PERSON]  
1 School of Automation, Northwestern Polytechnical University, Xi'an 710072, China; [EMAIL_ADDRESS] (S.L. ); [EMAIL_ADDRESS] (Q.F. )1 School of Automation, Northwestern Polytechnical University, Xi'an 710072, China; [EMAIL_ADDRESS] (S.L. ); [EMAIL_ADDRESS] (Q.F. )1 School of Automation, Northwestern Polytechnical University, Xi'an 710072, China; [EMAIL_ADDRESS] (S.L. ); [EMAIL_ADDRESS] (Q.F. )1 School of Automation, Northwestern Polytechnical University, Xi'an 710072, China; [EMAIL_ADDRESS] (Q.F. )1 School of Automation, Northwestern Polytechnical University, Xi'an 710072, China; [EMAIL_ADDRESS] (S.L. ); [EMAIL_ADDRESS] (Q.F. )1 School of Automation, Northwestern Polytechnical University, Xi'an 710072, China; [EMAIL_ADDRESS] (Q.F. )1 School of Automation, Northwestern Polytechnical University, Xi'an 710072, China; lishai@nwpu. One of the triggers is the crystal oscillator of the receiver. \"Micro-jump\" events, which refer to step changes in the local oscillation frequency, are well-known defects of crystal oscillators [8; 9]. A \"micro-jump\" event is disruptive to the frequency tracking loops, which causes the local replica carrier frequency to deviate from its nominal value and results in a large jump in the frequency tracking error. In some extreme cases, a micro jump of as little as 20 parts per billion (ppb) can be sufficient to cause a loss of tracking [9]. The acceleration sensitivity or g-sensitivity of crystal oscillators will also induce a large frequency offset error [10; 11]. A \"2 g-tipover\" test is a basic way to assess the g-sensitivity of crystal oscillators. The acceleration sensitivity can be compensated using IMU measurements to reduce the negative impacts on the GNSS receiver [12]. This method has better compensation effects in pedestrian and vehicle users, but its applicability in highly dynamic environments needs to be further evaluated. MIMU is another component that induces large changes in frequency tracking errors. A low-cost MIMU is more prone to failure in highly dynamic environments. Some failures can make MIMU measurements unavailable. However, under multiple failure conditions, the MIMU is still accessible, but the measurement errors will exhibit step changes [13]. After integration operations, large specific force measurement errors result in large velocity errors. Particularly for highly dynamic vehicles, velocity errors induced will be more significant. In an UTC integration system, the velocity solutions of the INS are fed back to numerically-controlled oscillator (NCO) command generators. Since velocity errors increase, the local replica carrier frequency will deviate from the true received carrier frequency. Therefore, large MIMU measurement errors will result in large jumps in frequency errors. Large frequency tracking errors will cause the attenuation of coherent integration values. Further, they cause a sudden decrease in the estimated carrier-to-noise ratio (\(C/N_{0}\)) and an increase in the code tracking error. In highly dynamic environments, if the large frequency tracking error cannot be mitigated in time, it may even result in loss of the tracking lock. Several previous methods were developed to cope with large frequency offsets. When the oscillator frequency jumps significantly, the apparent \(C/N_{0}\) will suddenly drop for all tracking channels [9]. For this reason, the estimated \(C/N_{0}\) is used as an indicator to detect large frequency errors in [9]. The optimal frequency offset compensation is obtained through a search algorithm. This method is very effective for the large frequency error caused by the crystal oscillator. However, the frequency error caused by MIMU measurement errors depends on the line-of-sight direction of the satellite being tracked. The estimated \(C/N_{0}\) of each tracking channel may not drop suddenly at the same time.