**1. Introduction**

The accurate positioning, navigation, and timing (PNT) services can be provided by the Global Navigation Satellite System (GNSS), and it has been widely used in many areas such as agriculture [1], weather monitoring [2], time and frequency transfer [3], and disaster monitoring [4]. As of the end of April 2022, there are about 120 GNSS satellites, including GPS, GLONASS, Galileo, BDS-2, BDS-3, and QZSS systems, which can provide PNT services to users around the world [5–7]. The available satellites, frequency, and PRN

**Citation:** Li, M.; Huang, G.; Wang, L.; Xie, W.; Yue, F. Performance of Multi-GNSS in the Asia-Pacific Region: Signal Quality, Broadcast Ephemeris and Precise Point Positioning (PPP). *Remote Sens.* **2022**, *14*, 3028. https://doi.org/ 10.3390/rs14133028

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

Received: 16 May 2022 Accepted: 22 June 2022 Published: 24 June 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

for each system are listed in Table 1. For GNSS, the observation data quality on the receiver side directly determines the result of precise data processing at the GNSS server and user end. The positioning performance at the GNSS user side plays a key role in the quality of GNSS service, and the broadcast ephemeris directly affects the performance of real-time navigation and positioning. The stations located in the Asia-Pacific region can receive the signals broadcasted by five navigation satellite systems at the same time. Therefore, it is worthy to investigate and evaluate the performance of GNSS observation data quality, broadcast ephemeris, and positioning performance in the Asia-Pacific region.


**Table 1.** Available satellites of each system at the end of April 2022.

For the observation data quality of GNSS: the BDS-3 satellites signals were evaluated by Zhang et al.; the B1I/B2I/B3I signals of the BDS-2 satellites, L1/L2/L5 of the GPS Block IIF satellites, and E1/E5a/E5b of the Galileo satellites are also evaluated for comparison in their study [8]. The observation data quality of the BDS-3 signal was studied and analyzed by Yang et al. [9]. The signal quality of BDS-2 and BDS-3 satellites was evaluated by Yan et al. [10]. The observation data quality and positioning performance of BDS/QZSS satellites were studied by Bu et al. [11]. The data quality of BDS/GPS/GLONASS satellites was investigated by Zuo et al. [12]. The signal quality of Galileo/BDS/GPS satellites was evaluated by Tian et al. [13]. It can be seen that previous studies about signal quality mainly focus on single and dual systems; the investigation and comparison of observation data quality among five GNSS systems in Asia-Pacific regions are limited and still needs further study.

In the context of GNSS broadcast ephemeris accuracy assessment, the GPS/GLONASS/BDS/Galileo/QZSS broadcast ephemeris was evaluated by Montenbruck et al. [14], but the BDS-3 satellite constellation had not been built at that time. After the BDS-3 satellites were launched, their broadcast ephemeris orbit and clock offset accuracy were evaluated by many scholars [15,16], and the comparison of broadcast ephemeris orbit and clock offset accuracy between BDS-2 and BDS-3 was also conducted [17]. However, with the modernization of GNSS satellites, the performance of broadcast ephemeris orbit and clock offset accuracy of five GNSS systems satellites is still unknown and unrevealed, it is urgent to conduct a comprehensive evaluation and comparison of the broadcast ephemeris performance for five GNSS systems satellites.

In terms of multi-GNSS positioning, the contribution of QZSS to the single-frequency PPP of GPS/BDS/GLONASS/Galileo satellites was studied by Hong et al. [18]. The positioning performance of BDS/QZSS in the Asia-Pacific region was evaluated by Bu et al. [11]. The precise positioning performance of QZSS and GPS in the Asia-Pacific region was investigated by Li [19]. The positioning performance of BDS-2/BDS-3 in the Asia-Pacific region was analyzed by Cao et al. [20]. It can be seen that the previous studies about PPP in the Asia-Pacific region are mainly focused on single or dual systems, while the PPP performance of multi-GNSS in the Asia-Pacific region is still limited.

In this contribution, multi-GNSS (GPS/GLONASS/Galileo/BDS-2/BDS-3/QZSS) observation data quality, broadcast ephemeris orbit, and clock offset performance and PPP performance in the Asia-Pacific region are investigated in detail. Based on the observation data from 10 Asia-Pacific MGEX stations, multi-GNSS broadcast ephemeris and precise satellite orbit and clock offset products, the observation data quality, broadcast ephemeris orbit, clock offset performance, and PPP performance in the Asia-Pacific region are evaluated and compared from DOY 283 to 289, in 2021. This paper is organized as follows: after this introduction, the observation data quality of five GNSS systems from 10 stations located in the Asia-Pacific region are investigated and analyzed in terms of carrier-to-noise-density ratio(C/N0) and pseudorange multipath in Section 2. The accuracy of the broadcast ephemeris orbit and clock offset and signal-in-space ranging errors (SISRE) of the five systems is investigated, evaluated, and compared in Section 3. The static and kinematic PPP performance of five GNSS systems in the Asia-Pacific region is evaluated and compared from convergence time and positioning accuracy in Section 4. Finally, the conclusions are presented in Section 5.

#### **2. Data Quality**

The observation data of 10 stations (CEDU, DARW, JFNG, MIZU, NNOR, PIMO, SIN1, USUD, YARR) from the MGEX network located in the Asia-Pacific region from day of year (DOY) 283 to 289 in 2021 are applied. The distribution of these stations is shown in Figure 1, and the latitude, longitude, receiver type, and antenna type of each station are listed in Table 2. These 10 stations are evenly distributed in different latitudes and longitudes around the Asia-Pacific region, and all frequencies of GPS, BDS-2, BDS-3, GLONASS, Galileo, and QZSS satellites can be received by these stations, which can better reflect the observation data quality and positioning performance of multi-GNSS in the Asia-Pacific region. The observation data quality was studied in the Asia-Pacific region in terms of two indicators: C/N0 and pseudorange multipath, in which the C/N0 can reflect the ability of the signal strength from satellite to receiver, whereas the impact of the satellite signal due to ground interference on the receiver can be reflected in pseudorange multipath. The C/N0 and pseudorange multipath are important indicators in the observation data quality assessment, and their performance directly affects the performance of PNT services.

**Table 2.** Receiver information.



**Table 2.** *Cont.*

**Figure 1.** Distribution of selected stations.

#### *2.1. Carrier-to-Noise-Density Ratio*

The C/N0 is the ratio of the carrier signal to noise, which can reflect the signal strength of GNSS observations on the receiver side. The larger the C/N0, the smaller the noise and the better the signal quality, and vice versa. The C/N0 of every satellite at each epoch can be directly obtained from the observation files. In this study, the relationship between the C/N0 and the elevation of the GNSS satellite signal is investigated and analyzed. When obtaining the C/N0, the sampling interval of the observation data is set as 30 s and the elevation mask is set to 0◦. All C/N0 values within 5 degrees of elevation angle are grouped into one group, and then the average of C/N0 within each group of elevation angle is calculated [21].

The average C/N0 corresponding to elevation for BDS-2 MEO, BDS-2 IGSO, BDS-3 MEO, BDS-3 IGSO, Galileo, GPS, GLONASS, and QZSS satellite is calculated, and one typical satellite of each system was shown in Figure 2, respectively. For the BDS-2 MEO satellite, the C/N0 of the B1I signal for the C12 satellite is slightly worse than that of B3I and B2I; although the signal strength is different, the C/N0 variation of these three frequencies shows the same variation trend. The C/N0 of B1I, B3I, and B2I signals have comparable performance for all elevation angles in terms of BDS-2 IGSO satellites. If these three signals can be selected for users, the difference in C/N0 does not need to be considered. In terms of the BDS-3 satellites, the C/N0 of the B2b signal is worse than that of B1C, B1I, B2a, and B3I whether for the IGSO or MEO satellites. It can be clearly seen that the C/N0 of BDS-3 satellites is better than that of BDS-2, which may be that the Binary Offset carrier (BOC) and Quadrature Multiplexed Composite Binary Offset Carrier (QMBOC) signal design is applied to BDS-3 satellites. For MEO satellites, the C/N0 of BDS-3 is higher than that of BDS-2 with 1–2 dB-Hz; while these values are 2–3 dB-Hz for IGSO satellites. The C/N0 of the Galileo E5 signal shows the best performance among the five Galileo frequency bands. The C/N0 of the L2 Z-tracking signal channel for GPS satellites is poorer than that of other frequencies. Moreover, the C/N0 for the L1 C/A signal is slightly poorer than that of L1C, L2C, and L5 I + Q. The C/N0 of GLONASS G1 frequency is better than that of G2, which can be attributed to the lower frequency value of the G2 signal. The C/N0 values for five QZSS frequencies present a similar performance at the different elevation angles. In terms of the five systems: the GPS and Galileo satellites show the best performance, and the C/N0 value can reach 55 dB-Hz when the elevation is nearly 90 degrees, both the BDS and GLONASS are poorer than that of GPS and Galileo, and the QZSS presents the worst performance among five GNSS systems.

**Figure 2.** The average C/N0 versus elevation angle of BDS-2/ BDS-3/GPS/QZSS/GLONASS/Galileo from DOY 283 to 289, 2021.
