Characterization of GNSS Signals Tracked by the Igmas Network Considering Recent BDS-3 Satellites

remote sensing

Article Characterization of GNSS Signals Tracked by the iGMAS Network Considering Recent BDS-3 Satellites

Xin Xie 1, Rongxin Fang 1,* , Tao Geng 1 , Guangxing Wang 1,2, Qile Zhao 1 and Jingnan Liu 1 1 GNSS Research Center, Wuhan University, Wuhan 430079, China; [email protected] (X.X.); [email protected] (T.G.); [email protected] (G.W.); [email protected] (Q.Z.); [email protected] (J.L.) 2 Faculty of Information Engineering, China University of Geosciences, Wuhan 430074, China * Correspondence: [email protected]; Tel.: +86-027-6877-7328

Received: 19 September 2018; Accepted: 31 October 2018; Published: 3 November 2018

Abstract: The international GNSS monitoring and assessment system (iGMAS) tracking network has been established by China to track multi-GNSS satellites. A key feature of iGMAS stations is the capability to fully track new navigation signals from the recently deployed BDS-3 satellites. In addition to the B1I and B3I signals inherited from BDS-2 satellites, the BDS-3 satellites are capable of transmitting new open service signals, including B1C at 1575.42 MHz, B2a at 1176.45 MHz, and B2b at 1207.14 MHz. In this contribution, we present a comprehensive analysis and characterization of GNSS signals tracked by different receivers and antennas equipped in the iGMAS network, especially as they relate to BDS-3 signals. Signal characteristics are analyzed in terms of the carrier-to-noise density ratio for the different signals as measured by the receiver, as well as pseudo-range noise and multipath. Special attention is given to discussion of the satellite-induced code bias, which has been identiﬁed to exist in the code observations of BDS-2, and the inter-frequency clock bias (IFCB), which has been observed in the triple-frequency carrier phase combinations of GPS Block IIF and BDS-2 satellites. The results indicate that the satellite-induced code bias is negligible for all signals of BDS-3 satellites, while small IFCB variations with peak amplitudes of about 1 cm can be recognized in BDS-3 triple-carrier combinations.

Keywords: iGMAS; BDS-3; satellite-induced code bias; inter-frequency clock bias; multipath combination; triple-frequency carrier phase combination

1. Introduction With the rapid development of the global navigation satellite system (GNSS), the China Satellite Navigation Office has initiated the international GNSS Monitoring and Assessment System (iGMAS) to monitor and assess the performance and operational status of the Chinese BeiDou Navigation Satellite System (BDS), as well as to promote compatibility and interoperability among different GNSS constellations [1,2]. As a backbone of iGMAS, a network of multi-GNSS monitoring stations has been set up around the globe to track multi-GNSS satellites. Several research institutions have been selected as analysis centers to process collected GNSS observation data and provide satellite orbits, clocks, and other kinds of information. There are currently 23 stations in the iGMAS network. A key feature of iGMAS stations is the capability to track new BDS signals from recently deployed global system satellites. The development of BDS is divided into three phases: the demonstration system (BDS-1), the regional system (BDS-2), and the global system (BDS-3) [3,4]. BDS-1 was completed with two geostationary (GEO) satellites and a backup satellite in 2003. BDS-2 consists of five GEO, five inclined

Remote Sens. 2018, 10, 1736; doi:10.3390/rs10111736 www.mdpi.com/journal/remotesensing Remote Sens. 2018, 10, 1736 2 of 14 geostationary orbit (IGSO) satellites, and four medium earth orbit (MEO) satellites, and has been providing official positioning, navigation and timing services over the Asia-Pacific area since December 27, 2012. Each of the BDS-2 satellites transmits signals in quadrature phase-shift keying (QPSK) modulation on three frequency bands, namely B1I at 1561.098 MHz, B2I at 1207.14 MHz, and B3I at 1268.52 MHz. BDS-3 is expected to provide greatly improved services to global users with 3 GEO, 3 IGSO, and 24 MEO satellites by 2020. By February 2016, five experimental BDS-3 satellites (hereafter, abbreviated to BDS-3e), including two IGSO and three MEO satellites, had completed deployment with the task of testing various key technologies for upcoming BDS-3, including new types of navigation signals, inter-satellite links, and onboard frequency standards. During the past two years, many studies have been carried out to investigate various aspects of these BDS-3e satellites, including signal quality analysis [5], precise orbit determination (POD) [6], performance of satellite clocks [7], precise point positioning (PPP) [8], and relative positioning [5]. Since the iGMAS stations support BDS-3 signal tracking, the majority of studies of the BDS-3e satellites were based on the data collected by the iGMAS tracking network. Tan et al. [9] conducted initial POD of BDS-3e satellites using observations of nine iGMAS stations, and the orbit overlap precision for IGSO and MEO satellites is approximately 10 cm and 40 cm, respectively, in the radial component. Zhang et al. [10] comprehensively analyzed the satellite-induced code bias for BDS-3e satellites using data from iGMAS stations. Li et al. [11] estimated the differential code biases (DCBs) of BDS-3e using iGMAS observations and evaluated the performance of both satellite and receiver DCBs, with results indicating that the estimated DCBs of BDS-3e satellites are more stable than those of BDS-2. In terms of satellite signals, Xiao et al. [12] analyzed the navigation signals transmitted by the M2-S satellite with the help of a 7.3-m high-gain antenna. Zhang et al. [5] performed quality assessment for four BDS-3e satellites using tracking data of only one station, including old B1I and B3I signals and new open service signals, namely B1C, B2a and B2b. In Yang et al. [13], B2a+b and S-band Bs signals transmitted by BDS-3e satellites were also included in quality analysis, which indicated that the B2a+b signal has the best performance. It is worth noting that the BDS-3e satellites would not serve as a part of the BDS-3 constellation, but as a complement to the BDS-2 constellation with the termination of the validation task. At the end of 2017, China launched two BDS-3 satellites into space. The satellites were aboard a Long March-3B carrier rocket which launched from Xichang Satellite Launch Center, China. Up to June 2018, eight BDS-3 MEO satellites (excluding experimental satellites) have been successfully launched followed by extensive testing and evaluation. These satellites are manufactured by the China Academy of Space Technology (CAST) and the China Academy of Science (CAS). Some details of these eight BDS-3 satellites are listed in Table1. It is noted that the PRN numbers of some satellites may be changed in the future. The ranging performance of a GNSS mainly depends on the measurement quality of the transmitted satellite navigation signals. Analysis of signal quality, measurement noise and multipath is critical for navigation systems [14]. Up to the present, only BDS-3e satellites have been analyzed. There has been a lack of reports conducted on the analysis of authentic BDS-3 satellite signals. In this study, we focus on the assessment and characterization of GNSS signals tracked by the iGMAS network, especially for the signals from the eight recently deployed BDS-3 satellites. The paper is organized as follows. First, an overview of the iGMAS network with BDS-3 tracking capability is described. Then, the tracking characteristics of BDS-3, BDS-2, GPS, and Galileo signals are compared and discussed, including the carrier-to-noise density ratio and the pseudo-range multipath and errors. In the following section, we analyze the satellite-induced code biases for all the available signals of BDS-3. Finally, we present the results of triple-frequency carrier-phase combinations to investigate the inter-frequency clock biases. Remote Sens. 2018, 10, 1736 3 of 14

Table 1. Status of the BDS-3 satellites (June 2018).

Satellite SVN Int. Sat. ID Manuf. PRN Notes Slot B-7; launched 5 Nov. 2017 BDS-3 MEO-1 C201 2017-069A CAST C19 PRN C19 used until 12 Jun. 2018 BDS-3 MEO-2 C202 2017-069B CAST C20 Slot B-8; launched 5 Nov. 2017 BDS-3 MEO-3 C206 2018-018B CAST C21 Slot B-5; launched 12 Feb. 2018 BDS-3 MEO-4 C205 2018-018A CAST C22 Slot B-6; launched 12 Feb. 2018 BDS-3 MEO-7 C203 2018-003A CAS C27 Slot A-4; launched 11 Jan. 2018 BDS-3 MEO-8 C204 2018-003B CAS C28 Slot A-5; launched 11 Jan. 2018 BDS-3 MEO-9 C207 2018-029A CAS C29 Slot A-2; launched 29 Mar. 2018 BDS-3 MEO-10 C208 2018-029B CAS C30 Slot A-3; launched 29 Mar. 2018

2. Datasets For BDS-3 satellites, signal design has been upgraded compared with BDS-2. In addition to the old B1I and B3I signals inherited from the BDS-2 satellites, several new open service signals are also transmitted by the BDS-3 satellites, namely, B1C at 1575.42 MHz, B2a at 1176.45 MHz, and B2b at 1207.14 MHz [12]. The B1C signal is compatible with GPS L1 and Galileo E1 signals; the B2a signal shares the same frequency as GPS L5 and Galileo E5a signals; and the B2b signal matches with the Galileo E5b signal. In addition, the BDS-3 B2b signal shares the same frequency as the BDS-2 B2 signal, but with a different modulation type. The frequencies, wavelengths and chip rates of BDS-2 and BDS-3 open service signals are listed in Table2.

Table 2. Open service signals of BDS-2 and BDS-3.

System Signal Frequency (MHz) Wavelength (cm) Chip Rate (Mcps) B1I 1561.098 19.20 2.046 BDS-2 B2I 1207.140 24.83 2.046 B3I 1268.520 23.63 10.23 B1I 1561.098 19.20 2.046 B1C 1575.420 19.03 1.023 BDS-3 B2a 1176.450 25.48 10.23 B2b 1207.140 24.83 10.23 B3I 1268.520 23.63 10.23

The iGMAS tracking network was developed by the China Satellite Navigation Ofﬁce to track multi-GNSS observations and provide satellite orbits and clocks, station coordinates and many other kinds of information for users. At the time of this study, the iGMAS network consisted of 23 stations, 17 of which can track multi-GNSS satellites, including the recently deployed BDS-3 satellites. The distribution of these 17 stations is shown in Figure1. Table3 lists the conﬁguration of these stations, which includes three different types of receivers and four different types of antennas. All receivers can simultaneously track GPS L1/L2/L5, Galileo E1/E5a/E5b, BDS-2 B1I/B3I, and BDS-3 B1I/B1C/B2a/B3I signals, and the GNSS_GGR receiver can additionally track BDS-3 B2b signals. Ten-day observation data collected by these stations from 28 May 2018 (DOY 148) to 6 June 2018 (DOY 157) are used for this study. Remote Sens. 2018, 10, x FOR PEER REVIEW 3 of 13

The paper is organized as follows. First, an overview of the iGMAS network with BDS-3 tracking capability is described. Then, the tracking characteristics of BDS-3, BDS-2, GPS, and Galileo signals are compared and discussed, including the carrier-to-noise density ratio and the pseudo-range multipath and errors. In the following section, we analyze the satellite-induced code biases for all the available signals of BDS-3. Finally, we present the results of triple-frequency carrier-phase combinations to investigate the inter-frequency clock biases.

Table 2. Open service signals of BDS-2 and BDS-3.

The iGMAS tracking network was developed by the China Satellite Navigation Office to track multi-GNSS observations and provide satellite orbits and clocks, station coordinates and many other kinds of information for users. At the time of this study, the iGMAS network consisted of 23 stations, 17 of which can track multi-GNSS satellites, including the recently deployed BDS-3 satellites. The distribution of these 17 stations is shown in Figure 1. Table 3 lists the configuration of these stations, which includes three different types of receivers and four different types of antennas. All receivers can simultaneously track GPS L1/L2/L5, Galileo E1/E5a/E5b, BDS-2 B1I/B3I, and BDS-3 B1I/B1C/B2a/B3I signals, and the GNSS_GGR receiver can additionally track BDS-3 B2b signals. Ten- day observation data collected by these stations from 28 May 2018 (DOY 148) to 6 June 2018 (DOY 157) are Remoteused Sens.for2018 this, 10 study., 1736 4 of 14

Figure 1. Distribution of selected stations in the iGMAS network.

Table 3. Conﬁguration of selected iGMAS stations.

Tracked BDS-3 Abb. Location Receiver Antenna Signals ABJA Abuja GNSS_GGR RINT-8CH CETD B1I, B3I, B1C, B2a, B2b CHU1 Changchun GNSS_GGR RINT-8CH CETD B1I, B3I, B1C, B2a, B2b GUA1 Urumqi GNSS_GGR RINT-8CH CETD B1I, B3I, B1C, B2a, B2b HMNS Hermanus GNSS_GGR RINT-8CH CETD B1I, B3I, B1C, B2a, B2b THAT Tahiti GNSS_GGR RINT-8CH CETD B1I, B3I, B1C, B2a, B2b XIA1 Xi’an GNSS_GGR RINT-8CH CETD B1I, B3I, B1C, B2a, B2b CANB Canberra CETC-54-GMR-4011 GNSS-750 B1I, B3I, B1C, B2a DWIN Darwin CETC-54-GMR-4011 GNSS-750 B1I, B3I, B1C, B2a PETH Perth CETC-54-GMR-4011 GNSS-750 B1I, B3I, B1C, B2a ZHON Antarctica CETC-54 GMR-4011 GNSS-750 B1I, B3I, B1C, B2a BJF1 Beijing CETC-54-GMR-4016 LEIAR25.R4 LEIT B1I, B3I, B1C, B2a CLGY Calgary CETC-54-GMR-4016 LEIAR25.R4 LEIT B1I, B3I, B1C, B2a WUH1 Wuhan CETC-54-GMR-4016 LEIAR25.R4 LEIT B1I, B3I, B1C, B2a BRCH Braunschweig CETC-54-GMR-4016 NOV750.R4 NOVS B1I, B3I, B1C, B2a LHA1 Lhasa CETC-54-GMR-4016 NOV750.R4 NOVS B1I, B3I, B1C, B2a ICUK London CETC-54-GMR-4016 NOV750.R4 NOVS B1I, B3I, B1C, B2a KNDY Kandy CETC-54-GMR-4016 GNSS-750 B1I, B3I, B1C, B2a

3. Methodology and Results

3.1. Carrier-To-Noise Density Ratio

The carrier-to-noise density ratio (C/N0) and its variation with elevation provides a key figure of merit for the characterization of the wide variety of signals accessible with modern receivers. The observed C/N0 values of GPS, Galileo (GAL), and BDS-2 and BDS-3 signals at CHU1, CANB, and ICUK stations are depicted as a function of satellite elevation in Figure2. These three stations are equipped with different receivers (Table3). The CHU1 station can track BDS-3 B1I/B1C/B2a/B2b/B3I signals, while the CANB and ICUK stations are unable to track BDS-3 B2b signals. The GPS is restricted to Block IIF satellites that can simultaneously transmit L1/L2/L5 signals; the BDS-2 is confined to only MEO satellites (C11, C12, and C14) for comparison; and the BDS-3 involves the eight satellites listed in Table1 and examined in this study. The tracking data for a period of ten days from DOY 148 to DOY 157 (2018) provide the basis for the analysis. For each station, we first group all C/N0 values according to their corresponding elevations separately for each signal of the selected GPS, Galileo, Remote Sens. 2018, 10, 1736 5 of 14

◦ BDS-2, and BDS-3 satellites. Then, the C/N0 values within an elevation range of 5 are classiﬁed into one group. Finally, a mean value of C/N0 is computed and shown for each elevation group. For GPS and Galileo in the top panels of Figure2, it can be clearly seen that the GPS L5 signals exhibit the highest C/N0 values over the entire elevation range for each of the three stations. The CHU1 station shows distinctly different signal characteristics than the other two stations. On the whole, the Galileo E1 signal has the lowest power level at CHU1, while the GPS L2 signals have the lowest power levels at the CANB and ICUK stations. At the CANB and ICUK stations, the C/N0 values for all GPS and Galileo signals start at 34–40 dB-Hz at low elevation angles and reach 48–54 dB-Hz at zenith, while at the CHU1 station, the C/N0 values start at 32–38 dB-Hz at low elevation angles and reach 50–57 dB-Hz at zenith, and increase more quickly as elevation angle increases. For BDS-2 and BDS-3 in the bottom panels of Figure2, it is clear that BDS-3 B1C signals exhibit the lowest C/N 0 values for theRemote three Sens. stations, 2018, 10, x especially FOR PEER REVIEW CHU1 and ICUK, which is consistent with the results of BDS-3e satellite5 of 13 analysis in Zhang et al. [5] and Yang et al. [13]. For compatible signals (B1I/B3I) of BDS-2 and BDS-3, theBDS B1I-2 for signals all three of BDS-3 stations, are approximatelywhile the B3I signals 3 dB-Hz show more almost powerful the same than thoseC/N0 values of BDS-2 for for BDS all-2 three and stations,BDS-3. while the B3I signals show almost the same C/N0 values for BDS-2 and BDS-3.

CHU1 CANB ICUK 60 60 60 (a) (b) (c) 55 55 55

50 50 50

45 45 45

40 40 40

C/N0(dB-Hz) 35 35 35 GPS L1 GAL E1 GPS L1 GAL E1 GPS L1 GAL E1 30 GPS L2 GAL E5b 30 GPS L2 GAL E5b 30 GPS L2 GAL E5b GPS L5 GAL E5a GPS L5 GAL E5a GPS L5 GAL E5a 25 25 25 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90

60 60 60 (d) (e) (f) 55 55 55

50 50 50

45 45 45 BDS-2 B1I 40 BDS-2 B3I 40 40 BDS-3 B1I C/N0(dB-Hz) 35 BDS-3 B3I 35 BDS-2 B1I BDS-3 B1I 35 BDS-2 B1I BDS-3 B1I BDS-3 B1C BDS-2 B3I BDS-3 B3I BDS-2 B3I BDS-3 B3I 30 BDS-3 B2a 30 BDS-3 B1C 30 BDS-3 B1C BDS-3 B2b BDS-3 B2a BDS-3 B2a 25 25 25 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 Elevation [deg] Elevation [deg] Elevation [deg]

Figure 2.2. Mean C/N00 valuesvalues as as a a function function of of satellite satellite elevation elevation for all the available signals of GPS and Galileo (a–cc)) andand BDS-2BDS-2 andand BDS-3BDS-3 (d–ff)) atat stationstation CHU1CHU1 equippedequipped with GNSS_GGR receiver ( a,d);); stationstation CANBCANB equippedequipped withwith CETC-54-GMR-4011CETC-54-GMR-4011 receiver (b,,e);); and stationstation ICUKICUK equippedequipped withwith CETC-54-GMR-4016CETC-54-GMR-4016 receiverreceiver ((cc,,ff),), respectively.respectively.

The new new signals signals of of BDS BDS-3-3 share share the the same same frequencies frequencies as some as some of GPS of and GPS Galileo and Galileo signals. signals. Figure Figure3 shows3 shows C/N0 values C/N 0 values of these of signals these signals for BDS for-3, BDS-3, GPS, GPS,and Galileo and Galileo observed observed at ABJA at ABJA station. station. For ForB1C/L1/E1 B1C/L1/E1 signals, signals, the C/N the0 C/Nvalues0 values of GPS of L1 GPS signal L1ssignals are about are 3 about dB-Hz 3 higher dB-Hz compared higher compared with Galileo with GalileoE1, and E1, about and 6 about dB-Hz 6 dB-Hz higher higher compared compared with with the BDS the- BDS-33 B1C B1C signal. signal. For For B2a/L5/E5a B2a/L5/E5a signals, signals, the thedifferences differences of their of their C/N C/N0 values0 values are within are within 2 dB 2-H dB-Hz.z. The TheGPS GPSL5 signal L5 signal is more is more powerful powerful than than the theBDS BDS-3-3 B2a B2asignal, signal, and andthe Galileo the Galileo E5a signal E5a signal is less is powerful less powerful than the than BDS the-3 BDS-3B2a. For B2a. B2b/E5b For B2b/E5b signals, signals,the C/N0 the values C/N of0 valuesthe Galileo of the E5b Galileo signal E5b are signal1-2 dB are-Hz 1-2 higher dB-Hz than higher the BDS than-3 theB2b BDS-3 signal. B2b There signal. are Therealso some are also differences some differences for reported for reported C/N0 values C/N 0 betweenvalues between stations stations (ABJA, (ABJA, CHU1) CHU1) with the with same the samereceiver receiver and antenna, and antenna, which which is inevitable is inevitable as thisas this is is related related to to the the environment, environment, although signalsignal strengthstrength isis moremore affectedaffected byby thethe hardwarehardware ofof thethesatellite satellite and and receiver. receiver. In In addition, addition, the the observed observed C/N C/N00 values could be affected by the tracking mode, and thus are not necessarily representative of the power level actually received from the satellites.

60 60 60 (a) (b) (c) 55 55 55

50 50 50

45 45 45

40 40 40

C/N0 (dB-Hz) C/N0 35 BDS-3 B1C 35 BDS-3 B2a 35 GPS L1 BDS-3 B2b 30 30 GPS L5 30 GAL E1 GAL E5a GAL E5b 25 25 25 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90

Elevation [deg] Elevation [deg] Elevation [deg]

Remote Sens. 2018, 10, x FOR PEER REVIEW 5 of 13

BDS-2 for all three stations, while the B3I signals show almost the same C/N0 values for BDS-2 and BDS-3.

CHU1 CANB ICUK 60 60 60 (a) (b) (c) 55 55 55

50 50 50

45 45 45

40 40 40

60 60 60 (d) (e) (f) 55 55 55

50 50 50

Figure 2. Mean C/N0 values as a function of satellite elevation for all the available signals of GPS and Galileo (a–c) and BDS-2 and BDS-3 (d–f) at station CHU1 equipped with GNSS_GGR receiver (a,d); station CANB equipped with CETC-54-GMR-4011 receiver (b,e); and station ICUK equipped with CETC-54-GMR-4016 receiver (c,f), respectively.

The new signals of BDS-3 share the same frequencies as some of GPS and Galileo signals. Figure 3 shows C/N0 values of these signals for BDS-3, GPS, and Galileo observed at ABJA station. For B1C/L1/E1 signals, the C/N0 values of GPS L1 signals are about 3 dB-Hz higher compared with Galileo E1, and about 6 dB-Hz higher compared with the BDS-3 B1C signal. For B2a/L5/E5a signals, the differences of their C/N0 values are within 2 dB-Hz. The GPS L5 signal is more powerful than the BDS-3 B2a signal, and the Galileo E5a signal is less powerful than the BDS-3 B2a. For B2b/E5b signals, the C/N0 values of the Galileo E5b signal are 1-2 dB-Hz higher than the BDS-3 B2b signal. There are also some differences for reported C/N0 values between stations (ABJA, CHU1) with the same Remote Sens. 2018, 10, 1736 6 of 14 receiver and antenna, which is inevitable as this is related to the environment, although signal strength is more affected by the hardware of the satellite and receiver. In addition, the observed C/N0 values couldcould bebe affected affected by by the the tracking tracking mode, mode, and and thus thus are notare necessarilynot necessarily representative representative of the powerof the powerlevel actually level actually received received from the from satellites. the satellites.

60 60 60 (a) (b) (c) 55 55 55

50 50 50

45 45 45

40 40 40

C/N0 (dB-Hz) C/N0 35 BDS-3 B1C 35 BDS-3 B2a 35 GPS L1 BDS-3 B2b 30 30 GPS L5 30 GAL E1 GAL E5a GAL E5b 25 25 25 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90

Elevation [deg] Elevation [deg] Elevation [deg]

Figure 3. Mean C/N0 values as a function of elevation for the signals with overlapping frequencies of BDS-3, GPS, and Galileo at ABJA station. The signals include B1C/L1/E1 (a); B2a/L5/E5a (b);

and B2b/E5b (c).

3.2. Pseudo-range Noise and Multipath In order to analyze the pseudo-range noise and multipath errors, the multipath (MP) combination is computed as follows [15,16]:

MPi = Pi − (a + 1)·Li + a·Lj + B 2 2 2 (1) with a = 2· fj /( fi − fj ) where the subscripts i and j indicate the carrier phase frequencies, MPi is the code multipath of the signal i, Pi is the pseudo-range observation of signal i, Li and Lj are carrier phase observations (in meters), and B is a bias that can be considered a constant if no cycle slip occurs on phase observations. The corresponding frequencies are fi and fj. This combination eliminates all errors except for receiver noise, multipath errors and bias variations in the pseudo-range observation. Results of the MP combination for all available GPS, Galileo, BDS-2, and BDS-3 signals at HMNS, PETH, and LHA1 stations equipped with different receivers are illustrated in Figure4. The root mean square (RMS) of the MP combinations by satellite elevation is plotted. The used satellites and test period are the same as for the aforementioned C/N0 analysis. For GPS and Galileo signals in the top panels of Figure4, we can see that signals observed at low elevation angles are affected more severely by multipath conditions. At the station HMNS, the GPS L5 and Galileo E5a/E5b signals exhibit smaller noise and multipath errors, which are less than 0.3 m over the entire elevation range. This superior performance could be explained by the fact that the L5/E5a/E5b signals have higher chip rates of 10.23 Mbps and are therefore less susceptible to multipath errors. For PETH and LHA1 stations, only the Galileo E1 signal has obviously larger noise and multipath errors, and GPS L1 and L2 signals do not exhibit this phenomenon, which could be due to the receiver firmware. It should be noted, however, that multipath errors are largely affected by the specific choice of bandwidth, which may differ among receivers and tracked signals. For BDS-2 and BDS-3 signals in the bottom panels of Figure4, we can see that the variations of noise and multipath errors for BDS-2 B1I/B3I signals are obviously different from the GPS and Galileo signals when the elevation angles are higher than about 50◦, especially for the B1I signal. This is due to BDS-2 satellite-induced code biases, which are discussed in the next section. For the same signals of B1I and B3I, BDS-3 shows a better performance compared with BDS-2 for all stations. For all signals of BDS-3, as expected, the B1C signal exhibits the largest noise and multipath errors for both stations due to a lower chipping rate of 1.023 Mcps. For the station HMNS, the errors of BDS-3 B1I/B3I/B2a/B2b Remote Sens. 2018, 10, 1736 7 of 14 signals are less than 0.3 m at low elevation angles and about 0.05 m at zenith, while for PETH and LHA1 stations, the B2a signal shows an abnormal performance with larger errors. Figure5 shows the RMS of receiver noise and multipath errors of BDS-3, GPS, and Galileo signals with overlapping frequencies, which are B1C/L1/E1, B2a/L5/E5a, and B2b/E5b. It can be seen that the differences of the RMS values are insignificant for each group of signals, and the pseudo-range errors of BDS-3 signals are comparable to those of GPS and Galileo. For B1C/L1/E1, the RMS values of the receiver noise and multipath errors are 0.1–0.2 m close to the zenith and increase to 0.4–0.6 m at low elevation angles, while for B2a/L5/E5a and B2b/E5b, the RMS values are less than 0.3 m over the entireRemote elevationSens. 2018, 10 range,, x FOR and PEER less REVIEW than 0.1 m at high elevation angles. 7 of 13

Remote Sens. 2018, 10HMNS, x FOR PEER REVIEW PETH LHA1 7 of 13 0.8 0.8 0.8 (a) GPS L1 GAL E1 GPS L1 GAL E1 (c) GPS L1 GAL E1 (b) GPSHMNS L2 GAL E5b GPSPETH L2 GAL E5b GPSLHA1 L2 GAL E5b 0.8 0.8 0.8 GPS L5 GAL E5a GPS L5 GAL E5a GPS L5 GAL E5a 0.6 (a) GPS L1 GAL E1 0.6 GPS L1 GAL E1 0.6 (c) GPS L1 GAL E1 (b) GPS L2 GAL E5b GPS L2 GAL E5b GPS L2 GAL E5b GPS L5 GAL E5a GPS L5 GAL E5a GPS L5 GAL E5a 0.6 0.6 0.6

0.4 0.4 0.4 MP (m) MP 0.4 0.4 0.4

MP (m) MP 0.2 0.2 0.2

0.2 0.2 0.2 0.0 0.0 0.0 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 0.0 0.0 0.0 0.8 0 10 20 30 40 50 60 70 80 90 0.8 0 10 20 30 40 50 60 70 80 90 0.80 10 20 30 40 50 60 70 80 90 (d) BDS-2 B1I (e) BDS-2 B1I (f) BDS-2 B1I 0.8 BDS-2 B3I 0.8 BDS-2 B3I 0.8 BDS-2 B3I (d) BDS-3 BDS-2 B1I B1I (e) BDS-2BDS-3 B1IB1I (f) BDS-2 BDS-3 B1I B1I 0.6 0.6 0.6 BDS-3 BDS-2 B3I B3I BDS-2BDS-3 B3IB3I BDS-2 BDS-3 B3I B3I BDS-3 BDS-3 B1C B1I BDS-3 B1I BDS-3 BDS-3 B1I B1C 0.6 0.6 BDS-3 B1C 0.6 BDS-3 BDS-3 B2a B3I BDS-3BDS-3 B3IB2a BDS-3 BDS-3 B3I B2a 0.4 BDS-3 BDS-3 B2b B1C 0.4 BDS-3 B1C 0.4 BDS-3 B1C

MP (m) MP BDS-3 B2a BDS-3 B2a BDS-3 B2a

0.4 BDS-3 B2b 0.4 0.4 MP (m) MP 0.2 0.2 0.2

0.2 0.2 0.2

0.0 0.0 0.0 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 0.0 0.0 0.0 0 10 20Elevation30 40 50[deg]60 70 80 90 0 10 20 Elevation30 40 50 [deg]60 70 80 90 0 10 20 30Elevation40 50 [deg]60 70 80 90 Elevation [deg] Elevation [deg] Elevation [deg] FigureFigure 4.4. RMSRMS of of the the noise noise and and multipath multipath errors errors as a function as a function of satellite of satelliteelevation elevation for all the foravailable all the availablesignalsFigure of4. signals RMS GPS of of and the GPS Galileonoise and and Galileo (a multipath–c) and (a–c )BDS anderrors- BDS-22 andas a function and BDS BDS-3-3 (ofd– satellitef (d) – atf) atstation elevation station HMNS HMNS for all equipped the equipped available with with GNSS_GGRGNSS_GGRsignals of receiver GPS receiver and (a , (Galileoda);,d station); station (a– PETHc) and PETH equippedBDS equipped-2 and with BDS with CETC-54-GMR-4011-3 (CETCd–f) at-54 station-GMR- 4011 receiver HMNS receiver equipped (b,e); and(b,e ) with station; and LHA1stationGNSS_GGR equipped LHA1 equipped receiver with CETC-54-GMR-4016 ( awith,d); stationCETC- 54 PETH-GMR receiver equipped-4016 receiver (c ,f with), respectively. (CETCc,f), respectively.-54-GMR-4011 receiver (b,e); and station LHA1 equipped with CETC-54-GMR-4016 receiver (c,f), respectively. 0.7 0.7 0.7 0.7 BDS-3 B1C 0.7 BDS-3 B2a 0.7 BDS-3 B2b 0.6 0.6 0.6 GPS BDS-3 L1 B1C BDS-3GPS L5 B2a BDS-3 GAL E5bB2b 0.6 0.6 0.6 0.5 GAL GPS E1 L1 0.5 GPSGAL L5E5a 0.5 GAL E5b 0.5 GAL E1 0.5 GAL E5a 0.5 0.4 0.4 0.4 0.4 0.4 0.4 0.3 0.3 0.3 MP (m) 0.3 0.3 0.3

0.2MP (m) 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.0 0.0 0.0 0.00 10 20 30 40 50 60 70 80 90 0.0 0 10 20 30 40 50 60 70 80 900.0 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 Elevation [deg] Elevation [deg] Elevation [deg] Elevation [deg] Elevation [deg] Elevation [deg] Figure 5. RMS of the noise and multipath errors as a function of satellite elevation for the signals with FigureFigure 5. 5.RMS RMS of of the the noise noise and and multipathmultipath errorserrors as a function function of of satellite satellite elevation elevation for for the the signals signals with with overlapping frequencies of BDS-3, GPS, and Galileo at HMNS station. The signals include B1C/L1/E1 overlappingoverlapping frequencies frequencies of of BDS-3, BDS-3, GPS, GPS and, and Galileo Galileo atat HMNSHMNS station. The signals signals include include B1C/L1/E1 B1C/L1/E1 (a); B2a/L5/E5a (b); and B2b/E5b (c). (a);(a B2a/L5/E5a); B2a/L5/E5a ((bb);); andand B2b/E5bB2b/E5b (c (c).).

3.3.3.3. Satellite Satellite-Induced-Induced Code Code Bias Bias SatelliteSatellite-i-nducedinduced code code bias,bias, whichwhich isis absent from otherother GNSSGNSS satellitessatellites, ,has has been been identified identified to to existexist in in BDS BDS-2-2 code code observations observations andand reportedreported in many studiesstudies [17[17––1919]]. .In In our our experiment, experiment, it itwas was furfurtherther confirmed confirmed using using observations observations fromfrom the iGMAS station,station, asas shownshown in in Figure Figure 6. 6. Since Since such such an an obviousobvious bias bias was was consistently consistently observed observed with various receivers receivers and and antennas, antennas, it it cancan likely likely be be attributedattributed to to the the transmitting transmitting satellites satellites [20][20].. InIn addition, thisthis ssatelliteatellite--iinducednduced code code bias bias is is frequency frequency- - andand elevation elevation-dependent-dependent, ,and and can can vvaryary moremore than 1 m from horizonhorizon to to zenith, zenith, which which would would severely severely affectaffect BDS BDS-2- 2precise precise application applicationss involvinvolveded with code observations [21[21––2323].] .Therefore, Therefore, the the question question of of whetherwhether this this bias bias continues continues to to existexist inin BDSBDS--3 code observations deservesdeserves our our attention. attention.

Remote Sens. 2018, 10, 1736 8 of 14

3.3. Satellite-Induced Code Bias Satellite-induced code bias, which is absent from other GNSS satellites, has been identiﬁed to exist in BDS-2 code observations and reported in many studies [17–19]. In our experiment, it was further conﬁrmed using observations from the iGMAS station, as shown in Figure6. Since such an obvious bias was consistently observed with various receivers and antennas, it can likely be attributed to the transmitting satellites [20]. In addition, this satellite-induced code bias is frequency- and elevation-dependent, and can vary more than 1 m from horizon to zenith, which would severely affect BDS-2 precise applications involved with code observations [21–23]. Therefore, the question of Remote Sens. 2018, 10, x FOR PEER REVIEW 8 of 13 whether this bias continues to exist in BDS-3 code observations deserves our attention.

BDS-2 GPS Galileo 2 2 2 B1I L1 E1 B3I L2 E5a 1 1 L5 1 E5b

0 0 0 MP (m) MP

-1 -1 -1

-2 -2 -2 0 30 60 90 0 30 60 90 0 30 60 90 Elevation [deg] Figure 6.Figure Multipath 6. Multipath (MP) (MP) series series of oftracked tracked signals of of BDS-2 BDS (B1I/B3I),-2 (B1I/B3I), GPS (L1/L2/L5),GPS (L1/L2/L5), and Galileo and Galileo (E1/E5a/E5b)(E1/E5a/E5b) satellites satellites as a function as a function of satellite of satellite elevation elevation at WUH1WUH1 station. station. Figure7 shows the MP series for ﬁve available BDS-3 signals observed from the station FigureHMNS 7 shows equipped the MP with series GNSS_GGR for five receiver available and B RINT-8CHDS-3 signals CETD observed antenna, from the stationthe station BJF1 HMNS equippedwith with CETC-54-GMR-4016 GNSS_GGR receiver receiver and and RINT LEIAR25.R4-8CH CETD LEIT antenna, and the thestation station BJF1 PETH with with CETC-54- GMR-4016CETC-54-GMR-4011 receiver and LEIAR25.R4 receiver and GNSS-750LEIT antenna, antenna, and as listedthe station in Table PETH3. The BDS-3with satellitesCETC-54 used-GMR-4011 in this study can be divided into four groups (C19/C20, C21/C22, C27/C28, C29/C30) according to receiver and GNSS-750 antenna, as listed in Table 3. The BDS-3 satellites used in this study can be the launch status as shown in Table1. We selected the results of one satellite from each group for divided illustrationinto four ingroups Figure 7(C19/C20,, and results C21/C22, of other BDS-3 C27/C28, satellites C29/C30) are similar. according It should beto noted the launch that only status as shown inthe Table RINT-8CH 1. We CETD select antennaed the isresults capable of of one receiving satellite all ﬁve from of the each BDS-3 group signals, for andillustrat othersion cannot in Figure 7, and resultsreceive of other B2b signals BDS in-3 thesatellites iGMAS are network similar. (Table It3 ).should From Figure be noted7, we canthat see only that the the MPRINT values-8CH CETD antenna becomeis capable more of stable receiving and gradually all five approach of the BDS zero as-3 thesignals, elevation and angles others increase. cannot As receive was also seenB2b insignals in Figure4, it is clear that the B1C signal is affected more seriously by multipath and noise for all selected the iGMAS network (Table 3). From Figure 7, we can see that the MP values become more stable and satellites and stations due to a lower chip rate. Most notably, the satellite-induced elevation-dependent graduallycode approach biases of allzero the as available the elevation BDS-3 signals angles observed increas bye different. As was GNSS also receivers seen in and Figure antennas 4, it were is clear that the B1C signaldramatically is affected reduced more and considered seriously to by be multipath inconsequential and relativenoise for to those all selected of BDS-2 satellites in Figure6 .and stations due to a lower chip rate. Most notably, the satellite-induced elevation-dependent code biases of all the available BDS-3 signals observed by different GNSS receivers and antennas were dramatically reduced and considered to be inconsequential relative to those of BDS-2 in Figure 6.

3.4. Inter-Frequency Clock Bias With the availability of three-frequency signals for GPS Block IIF satellites, inter-frequency clock bias (IFCB) was found, which was defined as the difference of the satellite clock offsets determined from two different ionosphere-free (IF) linear combinations of L1/L2 and L1/L5 carrier phase observations, and could most likely be attributed to inconsistency among frequency-dependent phase biases within a satellite [24,25]. According to previous studies, the IFCB between the BDS-2 B1/B2 and B1/B3 ionosphere-free satellite clocks showed a peak-to-peak amplitude of about 4 cm and, more noticeably, 10–40 cm for GPS Block IIF satellites [26,27]. In contrast, no apparent IFCB variations could be identified for the three signals of Galileo and QZSS satellites [28]. The presence of such IFCB makes it inappropriate to use one set of satellite clock products for data processing across all frequency bands, which complicates the implementation of triple-frequency PPP [29,30]. Thus, some approaches have been developed to estimate IFCBs [24,31]. The estimated IFCBs exhibit a time dependency and can be modeled. Li et al. [31] and Pan et al. [32] investigated the modeling of IFCBs for describing their characterization and predicting their values in advance. As discussed in previous publications, the IFCB is directly observable from the triple-frequency carrier phase combination, which is the difference of two different carrier phase IF combinations, as follows:

DIF(1,,,,  2  3) =− IF(  1  2) IF (  1  3 ) 2 2 2 2 f1 f 1  f 2 f3  (2) =2 2 − 2 2  1 − 2 2   2 +  2 2    3 f1− f 2 f 1 − f 3  f 1 − f 2  f 1 − f 3  where 1,,  2  3 are carrier phase observations (in meters) on three distinct frequencies fi (with

f1 f 2 f 3 ). In addition to observing the IFCB variations, the triple-frequency carrier phase

Remote Sens. 2018, 10, x FOR PEER REVIEW 9 of 13 combination is usually used to assess carrier phase errors, since it is both geometry-free and ionosphere-free and reflects a weighted sum of the carrier phase multipath and noise on the respective frequencies after removing the constant ambiguity. Numerical values of the coefficients for several triple-frequency carrier phase combinations shown in this study are listed in Table 4. For BDS-2 satellites, the iGMAS stations can only receive double-frequency signals of B1I and B3I, and thus cannot form the combination. For BDS-3, some stations are capable of receiving five frequencies of signals, which provide more choices to form the triple-frequency carrier phase combination. In this study,Remote Sens. (B12018I, B3I,, 10 , B2b)1736 and (B1C, B2b, B2a) triple-frequency combinations of BDS-3 are chosen9 of as 14 typical examples for analysis.

RINT-8CH CETD LEIAR25.R4 GNSS-750 C28 3 C19 3 C21 3 3 C30 2 2 2 2 1 1 1 1 0 0 0 0

B1I B1I (m) -1 -1 -1 -1 -2 -2 -2 -2 -3 -3 -3 -3 0 30 60 90 0 30 60 90 0 30 60 90 0 30 60 90 3 3 3 3 2 2 2 2 1 1 1 1 0 0 0 0

B3I B3I (m) -1 -1 -1 -1 -2 -2 -2 -2 -3 -3 -3 -3 0 30 60 90 0 30 60 90 0 30 60 90 0 30 60 90 3 3 3 3 2 2 2 2 1 1 1 1 0 0 0 0 -1 -1 -1 -1 B1C (m)B1C -2 -2 -2 -2 -3 -3 -3 0 30 60 90 0 30 60 90 0 30 60 90 0 30 60 90 3 3 3 3 2 2 2 2 1 1 1 1 0 0 0 0

B2a B2a (m) -1 -1 -1 -1 -2 -2 -2 -2 -3 -3 -3 -3 0 30 60 90 0 30 60 90 0 30 60 90 0 30 60 90 3 3 3 3 2 2 2 2 1 1 1 1 0 0 0 0 -1 -1 -1 -1 B2b B2b (m) -2 -2 -2 -2 -3 -3 -3 -3 0 30 60 90 0 30 60 90 0 30 60 90 0 30 60 90 Elevation [deg] Figure 7. MPMP combinations combinations as as a afunction function of of elevation elevation for for four four BDS BDS-3-3 satellites satellites on onfive five frequencies frequencies of signalsof signals observed observed at at three three iGMAS iGMAS stations stations equipped equipped with with different different antennas antennas (RINT(RINT-8CH-8CH CETD/LEIAR25.R4 LEIT/GNSS LEIT/GNSS-750)-750) during during DOY DOY 150 150-152,-152, 2018. 2018. The The five five rows rows from from top top to to bottom representrepresent five five BDS BDS-3-3 signals: B1I, B3I, B1C, B1C, B2a, B2a, and and B2b; B2b; the the four four columns columns from from left left to to right right represent represent foufourr selected selected BDS BDS-3-3 satellites: C19, C19, C21, C28, and C30.

3.4. Inter-FrequencyTable 4. Triple-frequency Clock Bias carrier phase combinations for analysis of carrier phase noise and multipath errors,With the as well availability as the inter of- three-frequencyfrequency clock bias signals (IFCB for) variations. GPS Block IIF satellites, inter-frequency clock bias (IFCB)GNSS was System found, which Signals was deﬁned as the differenceCombination of the satellite clock offsets determined from two differentGPS ionosphere-free(L1, (IF) L2, linear L5) combinations+0.285 of L1 L1/L2 − 1.546 and L2 L1/L5 + 1.261 carrier L5 phase observations, and couldGalileo most likely be attributed(E1, E5b, toE5a) inconsistency +0. among161 E1 − frequency-dependent 1.422 E5b + 1.261 E5a phase biases within a satellite [24,25]. According to previous studies, the IFCB between the BDS-2 B1/B2 and B1/B3 (B1I, B3I, B2b) +0.457 B1I − 1.944 B3I + 1.487 B2b ionosphere-freeBDS-3 satellite clocks showed a peak-to-peak amplitude of about 4 cm and, more noticeably, (B1C, B2b, B2a) +0.161 B1C − 1.422 B2b + 1.261 B2a 10–40 cm for GPS Block IIF satellites [26,27]. In contrast, no apparent IFCB variations could be identiﬁed for the three signals of Galileo and QZSS satellites [28]. The presence of such IFCB makes it inappropriate to use one set of satellite clock products for data processing across all frequency bands, which complicates the implementation of triple-frequency PPP [29,30]. Thus, some approaches have been developed to estimate IFCBs [24,31]. The estimated IFCBs exhibit a time dependency and can be modeled. Li et al. [31] and Pan et al. [32] investigated the modeling of IFCBs for describing their characterization and predicting their values in advance. Remote Sens. 2018, 10, 1736 10 of 14

As discussed in previous publications, the IFCB is directly observable from the triple-frequency carrier phase combination, which is the difference of two different carrier phase IF combinations, as follows:

DIF(ϕ1, ϕ2, ϕ3) = IF(ϕ1, ϕ2) − IF(ϕ1, ϕ3) 2 2 2 2 f1 f1 f2 f3 (2) = 2 2 − 2 2 ·ϕ1 − 2 2 ·ϕ2 + 2 2 ·ϕ3 f1 − f2 f1 − f3 f1 − f2 f1 − f3 where ϕ1, ϕ2, ϕ3 are carrier phase observations (in meters) on three distinct frequencies fi (with f1 > f2 > f3). In addition to observing the IFCB variations, the triple-frequency carrier phase combination is usually used to assess carrier phase errors, since it is both geometry-free and ionosphere-free and reflects a weighted sum of the carrier phase multipath and noise on the respective frequencies after removing the constant ambiguity. Numerical values of the coefficients for several triple-frequency carrier phase combinations shown in this study are listed in Table4. For BDS-2 satellites, the iGMAS stations can only receive double-frequency signals of B1I and B3I, and thus cannot form the combination. For BDS-3, some stations are capable of receiving five frequencies of signals, which provide more choices to form the triple-frequency carrier phase combination. In this study, (B1I, B3I, B2b) and (B1C, B2b, B2a) triple-frequency combinations of BDS-3 are chosen as typical examples for analysis.

Table 4. Triple-frequency carrier phase combinations for analysis of carrier phase noise and multipath errors, as well as the inter-frequency clock bias (IFCB) variations.

GNSS System Signals Combination GPS (L1, L2, L5) +0.285 L1 − 1.546 L2 + 1.261 L5 Galileo (E1, E5b, E5a) +0.161 E1 − 1.422 E5b + 1.261 E5a (B1I, B3I, B2b) +0.457 B1I − 1.944 B3I + 1.487 B2b BDS-3 (B1C, B2b, B2a) +0.161 B1C − 1.422 B2b + 1.261 B2a Remote Sens. 2018, 10, x FOR PEER REVIEW 10 of 13

The triple triple-frequency-frequency carrier carrier phase combinations for for GPS GPS Block IIF and Galileo satellites, which have been revealed in previous research,research, areare furtherfurther confirmed confirmed in in our our experiment, experiment as, as shown shown in in Figure Figure8, 8,which which plots plots the the results results of of triple-frequency triple-frequency combinations combinations for for G08G08 andand E07 satellites at station station WHU1. WHU1. As expected, obvious IFCB variations are are recognized in in the GPS Block IIF satellite and are confined confined toto peak amplitudes of about 5 5 cm, cm, while while a a good good E1/E5b/E5a E1/E5b/E5a consistency consistency and and no no significant significant bias variations can be seen in Galileo satellites. 6 90 6 90 G08 E07 4 75 4 75

2 60 2 60

0 45 0 45

-2 30 -2 30 Elevation (deg) -4 15 -4 15

-6 0 -6 0

6 8 10 14 16 18 20 22 Triple-carrier Combinations (cm) GPS time (hour) GPS time (hour) Figure 8. VariationsVariations of triple triple-frequency-frequency carrier phase combinations and and elevation elevation for for G08 G08 and E07 satellitessatellites at station WUH1 on 28 May 2018.

For BDS-3, the carrier phase observations on five frequencies from four selected satellites are used to form (B1I, B3I, B2b) and (B1C, B2b, B2a) triple-frequency combinations, as plotted in Figure 9. We can see that the time series of triple-frequency carrier phase combinations vary within ±2 cm for all the satellites. For C20 and C22 satellites manufactured by CAST, the (B1I, B3I, B2b) combination shows a good consistency and the series are dominated by the carrier phase multipath and noise with the dependence on elevation angles, while the (B1C, B2b, B2a) combination shows small IFCB variations with peak amplitudes of about 1 cm. For C27 and C29 satellites manufactured by CAS, the IFCB variations are observed in both (B1I, B3I, B2b) and (B1C, B2b, B2a) combinations and are generally confined to peak amplitudes of less than about 1 cm.

3 90 3 90 (a) C20 (b) C22 2 75 2 75 1 60 1 60 0 45 0 45 -1 30 -1

30 (deg) Elevation (B1I, B3I, B2b) -2 15 -2 (B1C, B2b, B2a) 15 -3 0 -3 13 14 15 16 17 18 19 12 13 14 15 16 17 18

3 90 3 90 (c) C27 (d) C29 2 75 2 75

1 60 1 60

Triple-carrier Combinations (cm) Combinations Triple-carrier 0 45 0 45

-1 30 -1 30 Elevation (deg) Elevation -2 15 -2 15

-3 0 -3 0 20 21 22 23 24 25 26 27 20 21 22 23 24 25 26 27 GPS time (hour) GPS time (hour) Figure 9. Variations of triple-frequency carrier phase combinations of (B1I, B3I, B2b) and (B1C, B2b, B2a) for C20, C22, C27, and C29 satellites during a pass at station HMNS.

Remote Sens. 2018, 10, x FOR PEER REVIEW 10 of 13

The triple-frequency carrier phase combinations for GPS Block IIF and Galileo satellites, which have been revealed in previous research, are further confirmed in our experiment, as shown in Figure 8, which plots the results of triple-frequency combinations for G08 and E07 satellites at station WHU1. As expected, obvious IFCB variations are recognized in the GPS Block IIF satellite and are confined to peak amplitudes of about 5 cm, while a good E1/E5b/E5a consistency and no significant bias variations can be seen in Galileo satellites. 6 90 6 90 G08 E07 4 75 4 75

2 60 2 60

0 45 0 45

-2 30 -2 30 Elevation (deg) -4 15 -4 15

-6 0 -6 0

6 8 10 14 16 18 20 22 Triple-carrier Combinations (cm) GPS time (hour) GPS time (hour)

RemoteFigure Sens. 2018 8. ,Variations10, 1736 of triple-frequency carrier phase combinations and elevation for G08 and E0711 of 14 satellites at station WUH1 on 28 May 2018.

For BDS-3,BDS-3, the carrier phase observations on five ﬁve frequencies from four selected satellites are used to formform (B1I,(B1I, B3I,B3I, B2b)B2b) andand (B1C,(B1C, B2b,B2b, B2a) B2a) triple-frequency triple-frequency combinations, combinations, as as plotted plotted in in Figure Figure9. We9. We can can see see that that the timethe time series series of triple-frequency of triple-frequency carrier carrier phase phase combinations combination varys within vary within±2 cm ± for2 cm all thefor all satellites. the satellites. For C20 For and C20 C22 and satellites C22 satellites manufactured manufactured by CAST, by CAST, the (B1I, the B3I, (B1I, B2b) B3I, combination B2b) combination shows ashows good a consistency good consistency and the and series the series are dominated are dominated by the by carrier the carrier phase phase multipath multipath and and noise noise with with the dependencethe dependence on elevation on elevation angles, angles, while the while (B1C, the B2b, (B1C, B2a) B2b, combination B2a) combination shows small shows IFCB small variations IFCB withvariations peak with amplitudes peak amplitudes of about 1of cm. about For 1 cm. C27 For and C27 C29 and satellites C29 satellites manufactured manufactured by CAS, by theCAS, IFCB the variationsIFCB variations are observed are observed in both in (B1I, both B3I, (B1I, B2b) B3I, and B2b) (B1C, and B2b, (B1C, B2a) B2b, combinations B2a) combinations and are generally and are conﬁnedgenerally toconfined peak amplitudes to peak amplitudes of less than of about less than 1 cm. about 1 cm.

3 90 3 90 (a) C20 (b) C22 2 75 2 75 1 60 1 60 0 45 0 45 -1 30 -1

30 (deg) Elevation (B1I, B3I, B2b) -2 15 -2 (B1C, B2b, B2a) 15 -3 0 -3 13 14 15 16 17 18 19 12 13 14 15 16 17 18

3 90 3 90 (c) C27 (d) C29 2 75 2 75

1 60 1 60

Triple-carrier Combinations (cm) Combinations Triple-carrier 0 45 0 45

-1 30 -1 30 Elevation (deg) Elevation -2 15 -2 15

-3 0 -3 0 20 21 22 23 24 25 26 27 20 21 22 23 24 25 26 27 GPS time (hour) GPS time (hour) Figure 9. Variations of triple-frequencytriple-frequency carrier phase combinations of (B1I, B3I, B2b) and (B1C, B2b, B2a) for C20, C22, C27, and C29 satellites during a pass at station HMNS.

4. Discussion The carrier-to-noise density ratio, pseudo-range noise and multipath, satellite-induced code bias and inter-frequency bias measured by the iGMAS stations for the different BDS-3 signals are characterized and compared with BDS-2, GPS, and Galileo. In addition to the old B1I and B3I signals inherited from the BDS-2 satellites, the BDS-3 satellites are capable of transmitting several new open service signals, namely B1C, B2a, and B2b. For the carrier-to-noise density ratio, the BDS-3 B1I signal is higher than that of BDS-2, and the B3I signals are at almost the same level. The new BDS-3 B2a/B2b signals have approximately similar signal strength to the GPS L5 and Galileo E5a/E5b signals, while BDS-3 B1C signals exhibit the lowest signal strength of all the analyzed signals, which is consistent with the results of analysis of BDS-3e satellites in Zhang et al. [5]. For the pseudo-range noise and multipath errors, the BDS-3 B1I/B3I signals show smaller errors compared with BDS-2, and B1C/B2a/B2b signals have no significant differences compared to GPS L1/L5 and Galileo E1/E5a/E5b signals. Differently from other GNSS satellites, the BDS-2 code measurements are affected by satellite-induced biases larger than 1 m from horizon to zenith, which will inevitably affect the precise applications which involve the BDS-2 code measurements, such as single-frequency PPP [20] and PPP ambiguity resolution [21,22]. We use observations collected from different receivers/antennas to analyze the existence and magnitude of the satellite-induced code biases for the BDS-3 satellites. No obvious elevation-dependent and receiver-dependent code biases are identified for all of the Remote Sens. 2018, 10, 1736 12 of 14 available BDS-3 signals. Similar results have been found in previous experimental satellites [5,33]. This indicates that BDS-3 satellites have improved satellite internal hardware device design and navigation unit assembly to avoid the problem of satellite-induced code biases present in BDS-2, providing better performance. For the IFCB variations, which have been identified to exist in BDS-2 and GPS Block IIF satellites [24,26], the small bias variations with peak amplitudes of about 1 cm can been found in BDS-3 triple-frequency carrier phase combinations. However, no apparent bias variations were observed for BDS-3e satellites in Zhang et al. [5] and Pan et al. [27]. Differences between the BDS-3 and BDS-3e satellites may be a cause of different results. In addition, it must be kept in mind that the triple-frequency carrier phase combinations show a comprehensive effect of the carrier phase noise, multipath, and the variations of the satellite phase hardware delay, namely IFCB. The results shown in Figure9 might be affected by several factors, including the quality of the receiver and antenna, surroundings of station, and satellite internal hardware devices. The effect of such IFCB variations of BDS-3 satellites on multi-frequency data processing, e.g., triple-frequency PPP, need to be further investigated. It should be noted that previous BDS-3e satellites, for the purpose of experiment validation, will not serve as a part of the BDS-3 satellite network. At present, the test task has been completed and these experimental satellites have been supplemented to the BDS-2 constellation. Although the signal characteristics of BDS-3e satellites have been analyzed in previous publications [5,13], it is still necessary to evaluate the performance of authentic BDS-3 satellites. The results of the satellite-induced biases are in line with the experimental satellites, and the results of IFCB variations are slightly different.

5. Conclusions Following BDS-2 and BDS-3e satellites, China is rapidly launching its BDS-3 satellites. By June 2018, eight BDS-3 MEO satellites have been successfully launched into space. The transmitted signals of multi-GNSS satellites, including recently deployed BDS-3, have been tracked by a subset of the GNSS monitoring stations in the iGMAS network, enabling a meaningful characteristic analysis and performance assessment for various GNSS signals. The conclusions are summarized as follows:

1. For the old B1I and B3I signals, BDS-3 shows a better performance compared with BDS-2. For the signals with overlapping frequencies of BDS-3, GPS, and Galileo, the observational quality of BDS-3 B2a/B2b signals is comparable to that of corresponding GPS and Galileo signals, while the BDS-3 B1C signal exhibits the lowest C/N0 values. 2. Satellite-induced code biases are not obvious for all the available signals of BDS-3 satellites and may be negligible for high-precision applications. 3. Inter-frequency clock bias still exists in BDS-3 signals, but the variations are small and generally conﬁned to an amplitude of about 1 cm. 4. The recently deployed BDS-3 satellites show similar signal tracking characteristics to the previous BDS-3e satellites.

The results mentioned above are expected to provide a useful understanding of performance, not only for the present BDS-3 satellites, but also for the iGMAS tracking network, hence potentially contributing to improvements to receivers and antennas. The performance of the BDS-3 system will be further veriﬁed and its long-term operation will be continuously analyzed via observation of the BDS-3 satellites. To facilitate further analysis, the GNSS receiver ﬁrmware in the iGMAS network should be upgraded to provide better tracking performance.

Author Contributions: R.F. conceived and designed the experiments; X.X. performed the data analysis and wrote the article; T.G. and G.W. reviewed and improved the paper; Q.Z. and J.L. supervised the experiments. Funding: This research was funded by the National Natural Science Foundation of China (Grant Nos. 41631073, 41874038, 41674004, 41574030) and Key Laboratory of Geospace Environment and Geodesy, Ministry of Education, Wuhan University (Grant No. 413100041-17-02-10). Remote Sens. 2018, 10, 1736 13 of 14

Acknowledgments: The contribution of data from iGMAS is greatly appreciated. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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