**1. Introduction**

Real-time kinematic (RTK) and precise point positioning (PPP) are usually conducted to satisfy the high-precision positioning requirements of autonomous cars and unmanned aerial vehicles. RTK technology can archive centimeter-level positioning accuracy with short initialization time [1,2], but it needs regional corrections from a reference station or reference network. The concept of PPP technology was proposed in 1997 [3,4], and it could obtain decimeter- even to centimeter-level positioning accuracy using standalone global navigation satellite system (GNSS) equipment [5]. Over the decades, PPP has become a widely used high-precision GNSS positioning technology. Precise GNSS satellite products are indispensable for the implementation of PPP. Motivated by the requirements of GNSS real-time applications, the international GNSS service (IGS) has been providing realtime service (RTS) through Internet communication since 2013 [6,7]. The RTS corrections, including precise orbit correction, clock offset correction, and code biases, are sent to users based on the Networked Transport of RTCM via Internet Protocol (NTRIP) [8]. By applying

**Citation:** Xu, X.; Nie, Z.; Wang, Z.; Wang, B.; Du, Q. Performance Assessment of BDS-3 PPP-B2b/INS Loosely Coupled Integration. *Remote Sens.* **2022**, *14*, 2957. https:// doi.org/10.3390/rs14132957

Academic Editors: Chuang Shi, Shengfeng Gu, Yidong Lou and Xiaopeng Gong

Received: 27 May 2022 Accepted: 19 June 2022 Published: 21 June 2022

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these corrections, the users can compute precise satellite orbits and clocks, and then carry out real-time PPP. However, RTS may be interrupted when the Internet is unavailable [9].

The commissioning of the BeiDou global navigation satellite system (BDS-3) was announced on 31 July 2020 [10,11]. Several featured services are provided by BDS-3, including short message communication, international search and rescue, and the PPP-B2b service, in addition to global positioning, navigation and time (PNT) services [12]. The BDS-3 PPP-B2b service can support real-time PPP based on PPP-B2b corrections broadcast by 3 BDS-3 geostationary earth orbit (GEO) satellites [13,14]. The PPP-B2b corrections include satellite orbit correction, clock correction, and code bias correction. At present, the BDS-3 PPP-B2b service only provides corrections for BDS-3/GPS satellites. The BDS-3 PPP-B2b service can be obtained free of charge. In addition, unlike IGS-RTS, Internet communication is not required since the BDS-3 PPP-B2b service is a satellitebased service. Therefore, the PPP-B2b service has great potential for real-time GNSS applications. Recently, publications have presented the precision of PPP-B2b corrections and positioning performance based on the PPP-B2b service [15–19]. These results indicate that the signal-in-space accuracy of precise ephemeris calculated with PPP-B2b corrections is at the decimeter- even to centimeter-level. In term of positioning performance, centimeterlevel accuracy in the static mode and decimeter-level in the kinematic mode could be achieved. However, positioning performance based on the BDS-3 PPP-B2b service would be significantly degraded in severe environments, due to GNSS signal blockages. To overcome this weakness, GNSS should be integrated with other sensors [20]. The inertial navigation system (INS) is an autonomous and spontaneous navigation system, which has potential to overcome degradation of GNSS positioning performance when GNSS signal outage occurs. Many valuable studies in PPP/INS integration have been published in the past decades, including loosely coupled (LC) model and tightly coupled (TC) integration [21–25]. In these publications, the precise ephemeris derived from IGS final precise products or from the IGS RTS service is adopted for PPP/INS integration processing. However, there is a considerable delay obtaining IGS final precise products. As for the IGS RTS service, the PPP/INS integration is restricted when there is a lack of Internet communication. It is worth mentioned that the above disadvantages can be overcome when the BDS-3 PPP-B2b service has implemented PPP/INS integration.

To our knowledge, there has until now been no published research on BDS-3 PPP-B2b/INS integration. In this paper, we focus on performance assessment of BDS-3 PPP-B2b/INS loosely coupled integration. Two vehicle kinematic experiments were carried out to assess the performance of PPP-B2b/INS loosely coupled integration. The rest of the paper is organized as follows. In Section 2, the methodology is introduced, including PPP positioning based on PPP-B2b corrections, the INS model, and their loosely coupled integration. In Section 3, the experimental set-ups and data processing strategies are presented in detail. Section 4 presents the performance of PPP-B2b/INS loosely coupled integration in urban environments. Finally, conclusions and perspectives are illustrated in Section 5.
