## 2 VFL-Assisted PLL Carrier Tracking Loop Design  
Vector tracking technology is considered to be a promising next-generation receiver technology. It couples all signal tracking channels together through a certain method and directly estimates the user's position and clock error. The estimated state information is further converted into a control quantity for feedback control of the tracking loop. By realizing information sharing among each signal channel, the vector tracking loop can provide better performance in low signal-to-noise ratios and highly dynamic environments without adding any external assistance. Although the traditional vector tracking loop couples the output of the discriminator in each channel, the measurement residuals of each signal channel are still estimated independently [19]. Based on this situation, the VFL in this paper no longer uses the frequency discriminator [24], and the coherent integration result of the I/Q branch is directly used as the observation value of the filter so that the receiver will easily operate under the condition of a low signal-to-noise ratio due to the nonlinear noise introduced by the frequency discriminator, which promotes its anti-noise performance. In addition, this paper combines the maximum likelihood estimation method with vector tracking in the VFLL and uses this VFLL to assist the PLL to reduce the tracking threshold of the carrier tracking loop. To adapt to the highly dynamic and weak-signal environment, a second order VFL-assisted third order PLL was designed with reference to the FAPLL structure. Figure 1 is a block diagram of the VFLL-assisted PLL based on the maximum likelihood estimation algorithm, and Figure 2 is a block diagram of the FAPLL based on the maximum likelihood estimation algorithm. The carrier tracking loop of the iL channel is shown in Figure 2, the digital intermediate frequency signal is first multiplied and correlated by the local signal and stripped the carrier and doppler frequency. The integral clearer integrates input correlation signals i(n) and  
Figure 1: Block diagram of the VFLL-assisted PLL structure. Figure 2: Block diagram of the FLL-assisted PLL structure. q(n) and outputs integral results I(n) and Q(n) as the observations of the navigation filter. The navigation filter uses the maximum likelihood estimation method to process the input information and outputs the estimated speed and receiver clock error. The line-of-sight projection converts the receiver speed and clock error estimates into frequency estimates using the projection relationship of the speed error and frequency error between the receiver and the satellite. Then, the frequency estimation value obtained by the VFLL and the phase estimation value output by the PLL are simultaneously input to loop filter 2, and the output result enters the NCO to obtain a local signal similar to the receiver input intermediate frequency signal. Figure 2 shows the structure of the traditional FAPLL. In the figure, the input signal in the FLL and PLL is multiplied by the local replica signal, and the output is a real baseband signal containing only data bits. To further increase the signal-to-noise ratio, the coherent integration results I(n) and Q(n) are output by the integration cleaner, whose coherent integration time is \(T_{coh}\). The phase detector/frequency detector uses the coherent integration result to estimate the phase or frequency difference. The error result is filtered by the loop filter and then fused to adjust the numerical control oscillator so that the frequency or phase of the output signal is consistent with that of the received signal. Because the FLL in the combined loop can track the higher-order components corresponding to high dynamic stress in the input signal change, the tracking and locking of the signal by the phase-locked loop in the combined loop will become relatively easy, thus improving the tracking ability of the loop for the high dynamic signal [25].