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The Low earth orbiter (LEO) based navigation signal transmitters have advantages in fast-changing geometry and low free space signal loss, which can be served as a complementary or extension of current MEO/GEO based GNSS. Broadcasting navigation signals from LEO can significantly reduce the convergence time for the long baseline RTK\precise point positioning (PPP) and improve the signal strength. Hence LEO based navigation is considered as one of the key technology of next-generation positioning, navigation and timing (PNT) systems. This study assessed the pseudorange and carrier phase measurements observed from Luojia-1A satellite with the geometry-free combination method and the zero-baseline method. The assessment results indicate that the pseudorange precision variation subject to elevation angle is caused by both signal strength variation and multipath effect. The pseudorange measurement precision reaches 0.7 m and 0.8 m respectively for the dual frequency pseudorange measurements and the carrier phase precision is 2.8 mm and 2.6 mm respectively. Based on the data collected from Luojia-1A satellite, the challenges of LEO navigation augmentation data processing were addressed. The most distinguishing features of LEO navigation signals are their large signal strength variation, large Doppler variation and large acceleration variation. All these features have adversary effect on LEO signal processing and data processing, which has not been revealed. These challenges still need to be seriously investigated to further improve the performance of the LEO based navigation augmentation system.

In this paper, the principle of vector receiver is introduced in details. According to the characteristics of real-time receiver, the real-time vector receiver architecture and real-time implementation flow are given. The vector receiver was successfully run on the hardware platform and evaluated the positioning accuracy, computational complexity and sensitivity performance. In this paper, the effect of clock drift and integration time on the tracking performance of vector receiver is given, which provides a good reference value for different series of crystal selection. Finally, for different application areas, this paper evaluates the vector receiver performance with the loop NCO update rates and the different measurement update rates, thus laying a good foundation for the subsequent implementation of GNSS/INS deep combination hardware.

There are two difficulties to obtain positioning results in dense urban without auxiliary means. Due to the signal is very weak, receiver can’t obtain positioning results because pseudo-ranges are unavailable. Though the receiver can obtain pseudo-range observations through some senior weak signal processing algorithms, the receiver still can’t achieve the required positioning results. This paper is mainly aiming at the above two problems, and proposed a two- solutions to resolve them. Solutions are as follow: the conventional receiver needs to get 4 or more satellites measurements to give a positioning result, but four satellites measurements is difficult to obtain in dense urban. This paper proposes a solution that as long as the receiver enters into the tracking mode, namely 1 millisecond measurement can be obtained, this receiver can give the positioning results. If the quality of measurements in dense urban is poor, the positioning results become the worse than in the open sky. This paper proposes another solution that using the fault diagnosis principle to improve the position result. The bad measurements can be processed through the non-linear weighting method. In order to verify the efficiency of the above algorithms, the paper processes the actual data.

A low Earth orbiter (LEO)-based navigation signal augmentation system is considered as a complementary of current global navigation satellite systems (GNSS), which can accelerate precise positioning convergence, strengthen the signal power, and improve signal quality. Wuhan University is dedicated to LEO-based navigation signal augmentation research and launched one scientific experimental satellite named Luojia-1A. The satellite is capable of broadcasting dual-frequency band ranging signals over China. The initial performance of the Luojia-1A satellite navigation augmentation system is assessed in this study. The ground tests indicate that the phase noise of the oscillator is sufficiently low to support the intended applications. The field ranging tests achieve 2.6 m and 0.013 m ranging precision for the pseudorange and carrier phase measurements, respectively. The in-orbit test shows that the internal precision of the ephemeris is approximate 0.1 m and the clock stability is 3 &times, 10&minus, 10. The pseudorange and carrier phase measurement noise evaluated from the geometry-free combination is about 3.3 m and 1.8 cm. Overall, the Luojia-1A navigation augmentation system is capable of providing useable LEO navigation augmentation signals with the empirical user equivalent ranging error (UERE) no worse than 3.6 m, which can be integrated with existing GNSS to improve the real-time navigation performance.