# An Adaptive INS-Aided PLL Tracking Method for GNSS Receivers in Harsh Environments
## 1 Introduction  
With the availability of accurate position, velocity and timing information that enables many applications we use in our daily lives, the global navigation satellite system (GNSS) has become nearly ubiquitous for both civil and military users [1; 2]. However, compared with delay-locked loop (DLL), carrier tracking loop is commonly regarded as the weak link in GNSS signal processing. This fragility results from the larger carrier dynamics caused by line-of-sight (LOS) motion and the reduction of signal strength. In a highly dynamic environment, traditional phase-locked loop (PLL) can easily lose lock due to the rapid Doppler frequency changes. The GNSS signal is also easily attenuated below the normal tracking threshold due to the operating environment-induced interferences [3]. Usually, the most effective way to improve dynamic performance is to broaden the noise bandwidth and reduce the coherent integration time. However, a narrower noise bandwidth and longer integration time are required to decrease the thermal noise and improve the tracking sensitivity. For these two constraints, certain scenarios with high dynamic and signal attenuation will make it more difficult for GNSS signal to remain locked [4; 5; 6]. In general, the inertial navigation system (INS)-aided tracking method is an effective solution to this problem, since much of the vehicle dynamics is compensated by the inertial data, and the PLL bandwidth can be reduced [7; 8]. The performance improvement of INS-aided GNSS tracking in highly dynamic applications with GNSS signal attenuation has been verified [9; 10; 11; 12]. However, the loop noise bandwidth is constrained by the navigation solution error of the GNSS/INS and the receiver's oscillator errors [13]. For civil and military vehicles with high dynamics, _i.e._, urban unmanned aerial vehicles (UAVs), aeronautic/astronautic aircrafts and precision-guided weapons [3], when they are under harsh environment conditions such as severe ionospheric scintillation, radio frequency interference or jamming [13], the navigation solution error varies with signal dynamics and attenuation, which means that the INS-aided PLL with constant loop parameters cannot optimally accommodate the harsh environment. To further improve the tracking performance, the PLL capable of adjusting its bandwidth has been put forward to achieve more tracking robustness. For example, in [14], the adaptive bandwidth algorithm for the PLL without external aid works well in the environment with high carrier to noise density ratio (\(C/N_{0}\)) and less random dynamic variations. [PERSON] _et al._[15] used a reduced inertial measurement unit (IMU) to aid a GPS receiver and designed an adaptive PLL loop filter capable of adjusting its bandwidth, which can improve both of the navigation performance and PLL tracking ability. A fuzzy logic controller (FLC) was also applied in [8] to choose an adaptive bandwidth for INS-aided PLL, and the improvement of tracking performance was demonstrated by simulation results. In addition to the noise bandwidth, the coherent integration time of the tracking loop is also one of the major factors impacting tracking performance [10; 16; 17], especially for harsh environments. The changing integration time implies a need to adjust the loop update rate. By analyzing the Kalman filter-based carrier-phase tracking loop, [PERSON] [6] drew the conclusion that there is a trade-off between epoch-by-epoch detectability and the deleterious effect of reducing the loop update rate, and proposed a method to choose the optimal integration time for the carrier loop without INS aid based on a given condition. Hence, an adaptive INS-aided PLL tracking method capable of adjusting both its noise bandwidth and integration time could achieve better performance than the method of only adjusting the noise bandwidth in the harsh environment.