## 1 Introduction  
Due to the high satellite orbit, the Global Navigation Satellite System (GNSS) has low signal landing power and is susceptible to interference. Many works investigate various signal-processing methods to acquire and track weak signals. For weak GNSS signal acquisition, a block averaging pre-processing (BAP) approach that coherently accumulates signal blocks to improve the signal power is introduced [1]. To solve the bit transition problem in coherent integration, block accumulating semi-coherent integration of correlations (BASIC) was proposed in 2008 [2]. In addition to acquisition, weak signal tracking is also important, for the tracking loop is necessary to extract the observables. To track a signal under dynamic stress and Radio Frequency interference (RFI), [PERSON] proposed an FLL-assisted-PLL tracking method, which integrates both the frequency-locked loop (FLL) and the phase-locked loop (PLL) characteristics and obtains the best overall performance [3]. [PERSON] [4] puts forward a Kalman filter-based tracking algorithm and applies a wavelet de-noising technique in the PLL, which demonstrates that the strategies work well under low signal-to-noise ratio (SNR) and high dynamic applications. For a high sensitivity receiver, no matter whether in acquisition or tracking, the key problem is to extend the coherent time [5]. To further improve the availability and sensitivity of the Global Navigation Satellite System, augmentation from the other system is always used, including internet ephemeris and receiver Doppler information. If the receiver can get the ephemeris information in another way, FLL can be used to keep the loop on track and extract the observables, which has a higher sensitivity than PLL. [PERSON] [6] proposes a new architecture for Global Positioning System (GPS) weak signal frequency tracking, which uses squaring and block processing to track signals with a carrier-to-noise ratio (C/N\({}_{0}\)) as low as 14.5 dB-Hz under static conditions. [PERSON] designed a Doppler-aided GPS navigation system for weak signals in his dissertation, which mainly uses the FLL to track the signal under strong ionospheric scintillation [7]. [PERSON] gives a comprehensive lecture on how an FLL works in the weak signal environments of high sensitivity tracking and compares the performance of different FLL tracking methods [8]. Our previous work [9] designs a double-stage Numerically Controlled Oscillator (NCO)-based tracking loop with a Differential Amplitude (DA) frequency discriminator, which has absolute linearity in a wide pull-in range and can improve the carrier tracking sensitivity of the GNSS receiver in a weak signal environment. However, the A-GNSS system has its limitations in the forest area, since the internet is not available. Moreover, it cannot provide the receiver status to help receive the GNSS signals in harsh environments. As for the high dynamic condition, the most widely used technology is GNSS/Inertial Navigation System (INS) deep integration, which could eliminate the Doppler in the tracking loop with receiver speed estimated from the Inertial Measurement Unit (IMU) and satellite speed obtained from ephemeris [10; 11]. This integration method is very suitable for high dynamic or strong interference conditions [12]. The main shortcomings are that the IMU could not provide a good Doppler estimation for the receiver when it is static [13]. Neither Assisted-GNSS (A-GNSS) nor a GNSS/INS coupled system can provide the clock information, which may become the main error factor in a long coherent integration [14; 15]. Low Earth Orbit (LEO) satellite systems, most of which are designed for communicating, have higher received power and faster-moving velocity than GNSS, for they are running at the lower orbit. Due to these characteristics, LEO has been recently used for positioning or augmenting the GNSS system [16; 17]. However, these augmentations are still on the system or observable level, which could not improve the GNSS signal received ability, especially under harsh conditions.