# Performance Assessment of Multi-GNSS PPP Ambiguity Resolution with LEO-Augmentation
## 1 Introduction  
Precise point positioning (PPP) [1] technology has been widely used in many fields, such as precision agriculture, precise mapping, deformation monitoring, water vapor inversion, precise navigation, and positioning [2], owing to its high positioning accuracy, simple structure, and flexible operation (without any reference station). However, it usually takes tens of minutes to achieve centimeter-level positioning accuracy. Reliable ambiguity resolution (AR) [3, 4] also requires a longer observation time, due to the obstacle of atmospheric errors, especially ionospheric delays. Regional augmentation precise point positioning, also known as precise point and real time kinematic positioning (PPP-RTK) [5], utilizes precise atmospheric corrections (generated using the reference network) to accelerate ambiguity resolution [6]. Unfortunately, there are places where reference stations cannot be established or available, such as in oceans, deserts, or developing areas. Many efforts have been made to improve PPP performance in terms of observations, such as combining multiple global navigation satellite systems (GNSSs) and using multifrequency signals. GPS/GLONASS combined PPP can improve the convergence speed [7], but the positioning accuracy is not as good as the GPS-only PPP when the orbit and clock offset quality of the GLONASS satellite is poor [8]. At the same time, GPS/GLONASS combined PPP can also accelerate the convergence of ambiguity parameters, thereby shortening the time to first fix (TTFF) of ambiguity [9; 10]. For GPS/BDS/GLONASS/Galileo four-system combined PPP, compared with single-system PPP, multisystem combined positioning not only converges faster but also improves the availability of PPP technology in special environments, as well as the reliability of positioning results [11; 12; 13]. In addition to multisystem fusion, multifrequency observations can improve the performance of PPP. Based on simulated GPS three-frequency data, [PERSON] et al. [14] conducted PPP fixed-solution experiments and pointed out that three-frequency PPP could achieve ambiguity-fixed solutions in a few minutes. [PERSON] et al. [15] conducted PPP float solution experiments based on real GPS three-frequency data, and the results showed that the convergence time of three-frequency PPP could be shortened by 10%. [PERSON] et al. [16] found that the convergence speed and positioning accuracy of three-frequency PPP ambiguity resolution was better than that of three-frequency PPP and dual-frequency PPP-AR in the initialization stage, through a BDS three-frequency uncombined PPP-AR experiment. [PERSON] et al. [17] evaluated the performance of a BDS/Galileo combination with three-frequency PPP-AR, and the results showed that multi-GNSS fusion could further improve the position estimation capability of a single system. Not only were the observations themselves found to be beneficial for reducing the convergence time in PPP, but so too was the incorporation of external ionospheric corrections from multiscale network configurations [18; 19]. However, due to the slow geometric change in the satellites' position relative to the ground station, PPP initialization still took approximately 10 min. In addition to multi-GNSS and multifrequency satellites, low Earth orbit (LEO) satellites are another way to improve PPP performance. In recent years, commercial companies have proposed their own LEO constellation, consisting of hundreds to thousands of LEO satellites that would provide broadband Internet services worldwide. [PERSON] et al. [20] demonstrated the potential of the LEO constellation as a navigation system, which was also confirmed in the first generation of the Iridium constellation. This constellation was the first low Earth orbit satellite constellation to integrate communication and navigation functions. It comprises 66 satellites distributed in six orbital planes, with an orbital altitude of 781 km and an orbital inclination of 86.4\({}^{\circ}\).