# Integrity Monitoring of PPP-RTK Positioning: Part II: LEO Augmentation
## 1 Introduction  
Low Earth orbit (LEO) satellites have been frequently discussed for the positioning, navigation and timing (PNT) service in recent years. Compared with the medium Earth orbit (MEO) and geostationary (GEO) satellites of different global navigation satellite systems (GNSSs), LEO satellites exhibit their advantages in their large number, low altitude, fast speed and strong signal strength [1]. The increased number of satellites improves their geometry and the redundancy of observations, which benefits positioning and timing precision in general [2]. The low altitude and the corresponding high speed of LEO satellites enable a much faster geometry change and hereby significantly shorten the convergence time for precise point positioning (PPP) [3, 4, 5]. The fast speed of the LEO satellites, in addition, whitens the multipath effects that have been a bottleneck for GNSS positioning in complicated measurement environments [6]. Compared to the GNSS satellites, LEO satellites' much stronger signal strength allows their signals to penetrate obstacles, which enlarges the limited satellite visibility in areas surrounded by high buildings, or even indoors [7]. Nevertheless, the LEO satellites also experience quick and short passes compared to the GNSS satellites [8], and suffer from lower elevation angles and smaller footprints [9]. The benefits of LEO augmentation have been frequently discussed to improve positioning performance. Its benefits in the integrity monitoring (IM) of the positional results, which finds more and more applications nowadays in the intelligent transport systems (ITSs), have been rarely discussed. In [10], LEO augmentation is shown to be able to improve the IM performance of the Beidou navigation satellite system (BDS). In [11], the IM of LEO precise orbit determination (POD) results were discussed. In this study, the focus is on the IM service for ground-based users, who will need the integration of LEO signals in their positioning activities in the near future. Compared to the PPP method that has been a hot spot in many studies of the LEO-augmented PNT service, precise point positioning--real-time kinematic (PPP-RTK) positioning combines the advantage of the independency in the PPP and that of the integer ambiguity resolution (IAR) of the RTK [12; 13]. It is thus considered as IAR-enabled PPP. In the first part of this series of papers [14], an IM procedure was proposed for GNSS-based PPP-RTK positioning. A pessimistic horizontal positional error bound, namely the horizontal protection level (HPL), was estimated under a given horizontal probability of misleading information, denoted as \(\text{PMI}_{\text{H}}\). The HPL was shown to be closely related to the ionosphere and multipath conditions. A low HPL tends to provide high IM availability, as the system becomes unavailable when HPL exceeds a pre-defined horizontal alert limit (HAL). In this contribution, the influences of the LEO augmentation will be discussed based on the proposed IM procedure. Using the planned navigation-oriented LEO constellation of the CentiSpace in China [15], the changes brought by the LEO augmentation in the measurement geometry, the PPP-RTK positioning and the HPLs are discussed for both the ambiguity-fixed and -float cases. The analysis is performed when LEO satellites augment single-, dual- and triple-GNSS-constellations, i.e., the global positioning system (GPS), the Galileo and the BDS under the L1/L5 dual-frequency scenario. Different characteristics of LEO satellites are utilized to exhibit their influences on positioning and IM performances using the PPP-RTK method. This paper starts with an introduction to the changes of the measurement geometry under the tested LEO augmentation. The processing strategy that was explained in the first part of this series of papers is shortly reviewed. Afterward, different issues for LEO-augmented PPP-RTK positioning are discussed in detail, with the focus put on the changes in the formal horizontal precision and the HPL. The conclusion is given at the end of the paper.