The data processing and analysis for the CE-5T1 GNSS experiment

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ScienceDirect Advances in Space Research xxx (2016) xxx–xxx www.elsevier.com/locate/asr

The data processing and analysis for the CE-5T1 GNSS experiment Huicui Liu a,b,⇑, Jianfeng Cao a,b, Xiao Cheng b, Jing Peng c, Geshi Tang a,b a

National Key Laboratory of Science and Technology on Aerospace Flight Dynamics, Beiqing Road 26, Beijing 100094, China b Beijing Aerospace Control Center, Beiqing Road 26, Beijing 100094, China c College of Electronic Science and Engineering, National University of Defense Technology, Changsha 410073, China Received 30 January 2016; received in revised form 25 June 2016; accepted 27 June 2016

Abstract In this paper the performance of a high sensitivity GPS/GLONASS receiver mounted on CE-5T1 Service Module is studied and the data received on the first Earth-lunar transfer orbit is processed and analyzed. At least four GLONASS satellites are visible for 46% of the data span while for 98% of the data span at least four GPS satellites are visible. GLONASS serves as a necessary supplement to GPS in real time positioning whenever less than four GPS satellites are tracked, and helps to optimize the observation geometry by reducing the Position Dilution of Precision (PDOP) values by up to 77%. However, noisier GLONASS pseudorange data should be properly weighted in order not to deteriorate the positioning accuracy. Studies indicate that when the inverse square of the pseudorange measurement error of each satellite is applied as the weight value, single point positioning (SPP) accuracy improves from 57.7 m (RMS) with GPS data alone to 44.6 m (RMS) with the addition of GLONASS data. Transmitter antenna Equivalent Isotropic Radiated Power (EIRP)s of all the four blocks of GPS satellites as well as GLONASS satellites are derived from the received C/N0 data and show significant variance in sidelobe power patterns. In general, the EIRP patterns of GPS Block IIR-M and GLONASS satellite antennas have a comparatively flat power level of around 10 dB W within the off-boresight angle range of 30–80° and roll off at the off-boresight angle of about 80°, offering deep space applications greater benefits than the other three blocks of GPS satellites. In addition, an interesting close encounter happens between CE-5T1 spacecraft and GLONASS satellite R06. Investigations indicate that the PDOP value increases up to 1.4 times and the SPP accuracy deteriorates by more than 142% if satellite R06 is excluded in the positioning computation. Ó 2016 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: GNSS; CE-5T1; Sidelobe; Positioning error; PDOP; Space applications

1. Introduction GNSS (Global Navigation Satellite System)-based PNT (Positioning, Navigation and Timing) service for space mission applications is increasingly of interest to both the commercial space industry and the scientific community due to the significant improvement in navigation perfor⇑ Corresponding author at: National Key Laboratory of Science and Technology on Aerospace Flight Dynamics, Beiqing Road 26, Beijing 100094, China. E-mail addresses: [email protected] (H. Liu), [email protected] (J. Cao), [email protected] (X. Cheng), [email protected] (J. Peng), [email protected] (G. Tang).

mance, large increase in autonomous operations, remarkable decrease in ground interventions, as well as rapid trajectory maneuver recovery support (Parker et al., 2016). The Earth-directed transmitter antennas of GNSS, including those of the GPS, GLONASS, Galileo, and Beidou constellations, provide signal coverage to Low Earth Orbit (LEO) satellite applications (Kaplan and Hegarty, 2006; Bock et al., 2014). Since the 1990s precise GNSS-based positioning of Geostationary Earth Orbit (GEO), High Earth Orbit (HEO), and even deep space exploration satellites has also been considered feasible. These satellites operate above the GNSS constellations and can only be covered by GNSS sidelobe signals from

http://dx.doi.org/10.1016/j.asr.2016.06.035 0273-1177/Ó 2016 COSPAR. Published by Elsevier Ltd. All rights reserved.

Please cite this article in press as: Liu, H., et al. The data processing and analysis for the CE-5T1 GNSS experiment. Adv. Space Res. (2016), http://dx.doi.org/10.1016/j.asr.2016.06.035

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H. Liu et al. / Advances in Space Research xxx (2016) xxx–xxx

‘‘the other side of the Earth” during most of their orbits. The aggregate signal availability of mainlobes and sidelobes determines the level of performance for such space users. However, the sidelobe signals are not measured as thoroughly as the mainlobe signals because they are too weak to reach terrestrial users (Ziedan, 2006). Several theoretical investigations have confirmed the availability of sidelobe signals for space missions above GNSS constellations (e.g. Moreau (2001), Lorga et al. (2010), Zhao et al. (2011), Liu et al. (2014, 2015), Manzano et al. (2014)). NASA has proposed changes to the GPS Space Service Volume for Block III satellites to protect the use of sidelobes in HEO/GEO regimes (Parker et al., 2016). Receivers have been designed with high signal acquisition and tracking sensitivity and enhanced receiver antenna gain in the directions where sidelobe signals are expected (Moreau, 2001; Brown and Mathews, 2007; Stadter et al., 2008; Zin et al., 2015). The Goddard Space Flight Center developed a GPS receiver known as the ’Navigator’ for lunar missions, with tracking threshold down to 22 and 25 dB-Hz, which confirms the possible use of GPS to support lunar exploration satellite autonomous navigation (Bamford et al., 2008). Unfortunately no lunar exploration mission has been conducted to date in order to test this receiver’s performance. The viability of using GPS sidelobe signals in GEO and HEO missions has been demonstrated in several flight tests (Balbach et al., 1998; Kronman, 2000; Moreau et al., 2002). In addition, the European Space Agency’s (ESA) retired GIOVE-A navigation mission was the first civilian satellite to perform GPS position fixes from an orbit more than 1000 km above the GPS satellites in 2013 (http://gpsworld. com/giove-a-uses-gps-side-lobe-signals-for-far-out-spacenavigation). However, no spaceborne experiment using GNSSs other than GPS has been carried out. And most of these previous works focus on final orbit determination performance and lack the analysis of GNSS signal characteristics for spaceborne experiments. At the end of 2014 China launched the CE-5T1 spacecraft as part of the 3rd stage of her lunar exploration plan. A GPS/GLONASS experiment was initiated to support precise positioning of the Return Vehicle before its separation with the Service Module. As its designer, the Beijing Institute of Satellite Information Engineering (BISIE) has described the receiver’s main technical features and introduces the Position, Velocity and Time (PVT) algorithm for the combined processing of GPS and GLONASS measurements in Wang et al. (2015). A brief characteristics analysis of the GNSS signal received during lunar-Earth transfer orbit before the separation operation is also presented in that paper. Furthermore, a detailed orbit determination analysis shows that the orbit prediction errors are reduced to 12% and 29% using GNSS data and using GNSS data combined with ground-based radiometric data respectively, compared with using ground-based radiometric data only (Fan et al., 2015).

This paper analyzes the CE-5T1 GNSS experiment with a focus on the signal characteristics on the Earth-lunar transfer orbit. A general description of the spaceborne GNSS experiment is given in Section 2. A detailed analysis is then carried out. Firstly, the GNSS satellite visibility is investigated, including the number of visible GNSS satellites and PDOP values. Single point positioning solutions obtained with GPS-only and GPS + GLONASS pseudorange data are presented. Secondly, the received signal power is derived from the CN0 data. Furthermore, the transmitter antenna Equivalent Isotropic Radiated Power (EIRP) of all visible GNSS satellites can be calculated based on the received signal power and precise geometry relationships between the spacecraft and the GNSS satellites. Thirdly, this work describes the changes in PDOP values and SPP accuracy due to the CE-5T1 spacecraft’s close encounter with the GLONASS satellite R06. 2. General description of the CE-5T1 GNSS experiment The CE-5T1 spacecraft, launched on 23rd October 2014, consists of two modules – the Service Module (SM) and the Return Vehicle (RV). The RV conducted a circumlunar flight with duration of eight days and landed in Siziwang Banner of China’s Inner Mongolia Autonomous Region. The SM is still in operation in April 2016. GNSS receiver equipments were mounted on both the SM and RV. The RV GNSS receiver came into service only after the separation of the RV and SM when the orbit altitude was lower than that of the GNSS satellites to support the positioning of the RV. Therefore this paper only concerns the SM GNSS receiver, whose basic characteristics are listed in Table 1 (Wang et al., 2015; Fan et al., 2015). Note that there are two quadrifilar helix antennas pointing in opposite directions to ensure sufficient received signals no matter whether the spacecraft is above or below the GNSS constellations. The gain patterns of these two antennas are the same and shown in Wang et al. (2015). The maximum gain is about 6dBi and the 3 dB beam width of both antennas is about ±55°. This receiver can process a maximum of 24 satellite signals at the same time due to restrictions in the payload power consumption and can only track one frequency signal of each GNSS. The GNSS receiver mounted on the SM was powered on three times for about 15 h in total. The second set of data lasts for more than 8 h, and starts from about 3 h before SM-RV separation. This data set, intended to support the precise RV position prediction at separation epoch, has already been analyzed in the two aforementioned CE-5T1 references (Wang et al., 2015; Fan et al., 2015). The third set of data was collected on 18th November 2014 during the extended mission of the SM on the Earth-lunar transfer orbit. The SM-Earth distance reached 140,000 km with the receiver still tracking GNSS signals. However, this data set only contains signals from the two GPS satellites G15 and G29. This paper focuses on the

Please cite this article in press as: Liu, H., et al. The data processing and analysis for the CE-5T1 GNSS experiment. Adv. Space Res. (2016), http://dx.doi.org/10.1016/j.asr.2016.06.035

H. Liu et al. / Advances in Space Research xxx (2016) xxx–xxx Table 1 Some primary characteristics of the GNSS equipment on the CE-5T1 SM. Item

Values

Frequency Antenna

GPS L1/GLONASS L1 2 (quadrifilar helix, nadir direction/zenith direction) 24 (16 GPS sat. & 8 GLONASS sat.)
175 dB W
180 dB W (including receiver antenna gain)

Parallel satellite channels Acquisition sensitivity Tracking sensitivity

data collected from 2014.10.23 18:56:32.275 to 21:53:35.000 UTC when the CE-5T1 spacecraft flew towards the Moon for the first time. According to the precise ephemeris provided by the Beijing Aerospace Control Center (BACC), the orbit altitude of the CE-5T1 spacecraft is shown in Fig. 1. It can be seen that the CE-5T1 spacecraft rose from about 11,392 km to 53,003 km over this period, and flew across the GNSS constellations from UTC time 19:23:05 to 19:28:05. The GNSS data collected with a sampling interval of 3 s includes carrier phase, pseudo-range, Doppler, and carrier-to-noise ratio (CN0). 3. GNSS data processing and analysis 3.1. GNSS satellite visibility and positioning performance Satellite visibility for this space application concerns two issues: (1) the light-of-sight is not obstructed by the Earth limb; and (2) the received signal power is sufficiently high for the receiver to carry out acquisition and tracking (Moreau, 2001). Thus, the word ‘‘visible” mentioned in this paper means not only ‘‘in view” but also ‘‘tracked”. As mentioned in Section 2, the number of visible GNSS satellites at the CE-5T1 spacecraft is not only constrained by the signal visibility itself but also by the receiver processing

3

restrictions. As shown in Fig. 2, the number of visible GNSS satellites does not exceed the receiver processing ability. However, the demand in parallel satellite channels increases with acquisition and tracking sensitivity. It can be seen in Fig. 2 that there is no significant change in the number of visible GPS satellites after the CE-5T1 spacecraft flies across the GNSS constellation, whereas there is a clear decrease in the number of visible GLONASS satellites. There is even no visible GLONASS satellite during a period of about 210 s (the purple circle in Fig. 2). It is also clear that there are far more visible GPS satellites than GLONASS satellites. Statistic results show that there are at least four visible GPS satellites during about 98% of the total time, while this drops to about 46% for the GLONASS constellations. At least five satellites should be visible if using both GPS and GLONASS signals for positioning in order to solve for the system bias. However, this bias keeps stable within a short period, and can be estimated based on the processing of previous data. In view of this, the only one visible GLONASS satellite serves as an efficient supplement to GPS in real time positioning when there are only three visible GPS satellites (as shown in the purple rectangle in Fig. 2). Thus, more than 99% epochs of this data set can satisfy positioning requirement. The PDOP (Position Dilution of Precision) values are shown in Fig. 3(a) where the black curve represents PDOPs calculated using both GPS and GLONASS satellites, and the blue bold curve represents PDOPs calculated using only GPS satellites. Despite GPS’s superiority over GLONASS with respect to the number of visible satellites, the PDOP values are improved by 77% at the most and 20% on average when using GPS + GLONASS satellite compared to those when using only GPS satellites. It can be seen in Fig. 3(a) that both curves bulge up to over 40 when the visible satellite number falls to four. At other times, the PDOP value is less than 15 for the combined GPS + GLONASS satellite processing and less than 20 when only GPS satellites are used. GPS+GLO GPS GLO

16 50,000

CE-5T1

40,000

12

Visible satellite number

Orbit altitude (km)

14

30,000

upper limit of GNSS constellation orbit height

20,000

10

8

6

4

lower limit of GNSS constellation orbit height 2 19

20

21

22

Time from 10-23-2014 00:00:00 (UTC), unit: hour

Fig. 1. The orbit altitude of the CE-5T1 spacecraft in relation to the GPS & GLONASS constellations.

0

19

20

21

22

Time from 10-23-2014 00:00:00 (UTC), unit: hour

Fig. 2. The number of visible GNSS satellites.

Please cite this article in press as: Liu, H., et al. The data processing and analysis for the CE-5T1 GNSS experiment. Adv. Space Res. (2016), http://dx.doi.org/10.1016/j.asr.2016.06.035

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(a) 80

GPS+GLO GPS

PDOP

60 40 20 0

19

20

21

22

Time from 10-23-2014 00:00:00 (UTC), unit: hour (b) 10

GPS+GLO GPS

PDOP

8 6 4 2 0 19

19.2

19.4

19.6

19.8

20

20.2

20.4

Time from 10-23-2014 00:00:00 (UTC), unit: hour Fig. 3. The PDOP values for GPS + GLONASS constellation, and GPS-only constellation.

Without dynamic constraints, the single point positioning (SPP) performance depends on undifferenced pseudorange measurement error and PDOP. An orbital transfer was implemented at around UTC time 20:30:00, with significant impact on positioning. Thus, the data from UTC time 19:00:00 to 20:30:00 is used in this analysis. As the mission control and operation center, BACC provides precise orbit determination results (POD) of the CE-5T1 SM with a conservatively estimated orbital accuracy of better than 50 m, by employing dynamical and measurement models as listed in Table 2. It should be stressed here that according to the POD results the spaceborne receiver clock is not stable and is reset when its offset to GPS Time exceeds 0.5 ms (as shown in Fig. 4). The post-fit residuals of the undifferenced pseudo-range measurements for GPS and GLONASS using POD results as a reference are shown in Fig. 5. The RMSs of the resid-

Table 2 Primary models of POD. Item

Description

GPS measurement model

Undifferenced Pseudo-ranging with 3 s sampling IGS final products of GPS ephemerides and 30 s clocks (ftp://cddisa.gsfc.nasa.gov) GGM03C (16  16) (Tapley et al., 2007) Luni-solar-planetary gravity (DE421) (Folkner et al., 2008) Solar radiation model with fixed area-mass ratio

Gravitational forces Non-gravitational forces Reference frame

Estimation Estimated parameters

ITRF 2005 IERS 2010 reference frame transformation (Petit and Luzum, 2010) Batch least squares Position, velocity and receiver clock error

uals are 5.9 m, 13.3 m and 8.3 m for the GPS-only, GLONASS-only and equal-weighted GPS + GLONASS data respectively. It can be seen in Fig. 5 that the RMS of GLONASS R06 residuals is only 4.1 m, much lower than that of other GLONASS satellites and even lower than the general level of GPS data; and Section 3.3 provides the explanation on this issue. As shown in Fig. 3(b), the PDOP value in this period is less than 8.5 for GPSonly and less than 6.6 for GPS + GLONASS, thus the corresponding maximum SPP error (1-sigma, 68% confidence) is estimated to be about 50.2 m and 54.8 m (equal-weighted GPS + GLONASS) respectively. Thus the coarse GLONASS pseudorange measurements should be processed with a lower weight value than GPS ones in order not to influence the SPP precision. More comprehensive discussions on data weighting can be found in Jin et al. (2005) and Jin et al. (2010). According to Gussan–Markov theory, the optimal weight value of each visible satellite is r12 if the pseudorange measurement error follows Gaussian distribution, where r is the RMS of pseudorange measurement error (Kaplan and Hegarty, 2006). Then the differences between weighted SPP and POD are shown in Fig. 6, where the red, blue and green dots represent the results in the X, Y and Z components (World Geodetic System 1984) respectively. It should be noted in Fig. 6 that the big gap is due to an on board attitude maneuver. The influence of this operation is already included in the POD calculation thus there is no gap in Fig. 3–5. It is clear in Fig. 6(a) and (b) that the dispersion of the two sets of SPP results is more significant after the UTC time 19:48:00 than before. The increase in PDOP value responds to this change according to Fig. 3 (b). Furthermore, the RMSs of these differences in X, Y and Z components are about 36.9 m, 35.2 m and 27.0 m

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ange data and GLONASS L1 data in SPP calculation (Jin et al., 2016). However, given the noise level of the data, the DCB correction (usually several nanoseconds) is ignored in this study.

0.1

0

Clock offset (ms)

5

-0.1

3.2. The received signal power and GNSS satellite transmitter antenna EIRP

-0.2

From the definition of CN0, the received signal power P s can be computed using the following equation (Kaplan and Hegarty, 2006):

-0.3

P s ¼ CN 0
10log10 ðKT Þ

-0.4

-0.5 19

19.2

19.4

19.6

19.8

20

20.2

20.4

Time from 10-23-2014 00:00:00 (UTC), unit: hour

Fig. 4. The clock offset of the CE-5T1 spaceborne GNSS receiver.

when using only GPS data; and 32.3 m, 25.3 m and 17.5 m when using weighted GPS + GLONASS data. These correspond to 3-D position errors of 57.7 m and 44.6 m respectively. The properly weighted dual-GNSS processing improves the SPP accuracy by 23% compared with the GPS-only processing. It should be noted that the tropospheric and ionospheric effects are very small and thus ignored in the SPP calculation since the CE-5T1 was flying above 10,000 km during the data span studied in this paper (Fan et al., 2015; Jin et al., 2015). In addition, POD processing uses IGS orbit and clock products which refer to L1/L2 ionosphere-free combinations, then the Difference Code Bias (DCB) should be corrected for the processing of GPS L1 C/A pseudor-

ð1Þ

where the Boltzmann constant K = 1.3806505  1023 J/K and the system temperature T = 298.5 K. As shown in Fig. 7, all the signal power values fluctuate within the range (
160 dB W,
180 dB W), which corresponds to the tracking sensitivity in Table 1, except that of GLONASS satellite R06. R06’s signal power is higher than that of other satellites, and above the normal level for ground users (about
160 dB W). Section 3.3 discusses this matter further. Fig. 7 also shows that the received GPS signal power seldom exceeds
165 dB W when the CE-5T1 SM flies above the GNSS constellations while the GLONASS signal power can still approach
160 dB W even when the orbit altitude is about 50,000 km. The received GNSS signal power at the CE-5T1 SM can be written from the view of link budget as: P s ¼ EIRP ðht Þ þ Lp þ Gr ðhr Þ

ð2Þ

where the free space loss LP can
k be calculated using the well where k is the wavelength known equation Lp ¼ 20 log 4pd of the GNSS signal and d is the propagation distance, hr GPS

Residuals (m)

40 20 0 -20 -40 19

19.2

19.4

19.6

19.8

20

20.2

20.4

Time from 10-23-2014 00:00:00 (UTC), unit: hour GLONASS Residuals (m)

200

R06

100 0 -100 -200 19

19.2

19.4

19.6

19.8

20

20.2

20.4

Time from 10-23-2014 00:00:00 (UTC), unit: hour Fig. 5. The undifferenced pseudo-range measurement residuals for period of analysis.

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Received signal power (dBW)

Received signal power (dBW)

Fig. 6. The difference of GNSS SPP and dynamic orbit determination results for the period of analysis.

GPS -160 -165 -170 -175 -180

19

20

21

22

Time from 10-23-2014 00:00:00 (UTC), unit: hour GLONASS -140

R06

-145 -150 -155 -160 -165 -170 -175 -180

19

20

21

22

Time from 10-23-2014 00:00:00 (UTC), unit: hour Fig. 7. The received GNSS signal powers for GPS and GLONASS satellites.

and ht is the departure angle (off-boresight angle) and the incidence angle (off-boresight angle) respectively, and Gr is the receiver antenna gain. No losses due to tropospheric and ionospheric effects are added because the CE-5T1 spacecraft flies well above the atmosphere during the period of time studied in this paper. EIRP characterizes the performance of the transmitter antenna and its value for the sidelobe is especially impor-

tant for space GNSS applications. NASA conducted the project GPS Antenna Characterization Experiment (ACE), to investigate the EIRP variance across different types of GPS satellite antennas and support the design of the future Block III satellites (Martzen et al., 2015). Typical EIRP models for Block IIA, IIR and IIF (Czopek, 1993; Moreau, 2001; Lorga et al., 2010) are shown in Fig. 8(a), (b) and (d), and the corresponding mainlobe width (3 dB

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H. Liu et al. / Advances in Space Research xxx (2016) xxx–xxx

beam width) values are listed in Table 3. It is clear that Block IIR satellites have narrower mainlobe and lower mainlobe gain compared to other two generations of GPS satellites. Although the maximum sidelobe gains of the three models are almost the same, the Block IIR satellites have flatter gain pattern and higher average power level in the off-boresight angle range of 30–50°. No relevant literature has been found about the transmitter EIRP model for GPS Block IIR-M and GLONASS satellites. The transmitter antennas’ EIRP of all visible GNSS satellites can be calculated using P s , precise GPS ephemerides and the aforementioned POD results of the CE-5T1 spacecraft, as shown in Figs. 8 and 9. Although the three sets of data for GPS Block IIA, IIR and IIF do not fit the models well because of unknown precision of EIRP model and the receiver’s C/N0 estimation, variances

among different satellites as well as other error sources, they nevertheless correspond with each other in terms of trends and general power levels. More specifically, the double-peaked sidelobe pattern of GPS Block IIF satellite antennas and the comparatively flat sidelobe pattern of Block IIR satellite antennas are clearly sketched as shown in Fig. 8(d) and (b). According to Fig. 8(c), Block IIR-M satellites also have flat sidelobe patterns but slower rolloff when the off-boresight angle is larger than 60° compared with Block IIR satellites, which confirms the results of GPS ACE (Parker et al., 2016). The CE-5T1 GNSS experiment gives a good chance to investigate the EIRPs of GLONASS satellite antennas which are seldom discussed in previous works. The good consistency among the data of the 13 visible GLONASS satellites can be easily seen in Fig. 9. In addition, the

G04 Model

30 25 20 15 10 5 0 -5 0

20

40

60

80

100

Transmitter antenna off-boresight angle (deg.) (c) Block IIR-M

30

G05 G07 G08 G15 G29 G31

25 20 15 10 5 0 -5 -10

0

20

40

60

80

100

Transmitter antenna off-boresight angle (deg.)

Transmitter antenna EIRP (dBW)

(b) Block IIR

Transmitter antenna EIRP (dBW)

Transmitter antenna EIRP (dBW)

Transmitter antenna EIRP (dBW)

(a) Block IIA 35

-10

7

30 G02 G11 G13 G14 G16 G19 G20 G21 G23 G28 Model

25 20 15 10 5 0 -5 -10

30

0

20

40

60

80

100

Transmitter antenna off-boresight angle (deg.) (d) Block IIF G06 G09 G25 G27 G30 Model

25 20 15 10 5 0 -5 -10

0

20

40

60

80

100

Transmitter antenna off-boresight angle (deg.)

Fig. 8. The GPS satellite transmitter antennas’ EIRP derived from the received signal power for different generations of satellites. Table 3 The mainlobe width of GPS and GLONASS transmitter antennas and signal availability time. GNSS/Block GPS

GLONASS

IIA IIF IIR IIR-M

Mainlobe width

Mainlobe signal available time percentage

Sidelobe signal available time percentage

17.9 16.9 16.2 17.9 17.9

/ 18.0 8.4 12.0 5.8

/ 82.0 91.6 88.0 94.2

Please cite this article in press as: Liu, H., et al. The data processing and analysis for the CE-5T1 GNSS experiment. Adv. Space Res. (2016), http://dx.doi.org/10.1016/j.asr.2016.06.035

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GLONASS 30

R01 R02 R03 R04 R05 R06 R11 R12 R14 R16 R18 R23 R24

Transmitter antenna EIRP (dBW)

25

20

15

10

5

0

-5

-10 0

10

20

30

40

50

60

70

80

90

100

110

Transmitter antenna off-boresight angle (deg.)

19.2

19.3

19.4

0

Time from 10-23-2014 00:00:00 (UTC), unit: hour Doppler changing rate (Hz/s)

(c) 100

10000

50

0 19.1

5000

19.2

19.3

19.4

0

Time from 10-23-2014 00:00:00 (UTC), unit: hour

100 5000 50

0 19.1

19.2

19.3

19.4

0

Time from 10-23-2014 00:00:00 (UTC), unit: hour (d) 3

10000 PDOPR06

2 5000 1 PDOP 0 19.1

19.2

19.3

0

19.4

0

Time from 10-23-2014 00:00:00 (UTC), unit: hour

Range between R06 and CE-5T1(km)

-180 19.1

5000

10000

Range between R06 and CE-5T1(km)

-160

(b) 150

Depature angle (deg)

10000

PDOP

Received signal power (dBW)

(a) -140

Range between R06 and CE-5T1(km) Range between R06 and CE-5T1(km)

Fig. 9. The GLONASS satellite transmitter antennas’ EIRP derived from the received signal power.

Fig. 10. Variation of received signal power, departure angle, Doppler change rate and PDOP during the CE-5T1 encounter with GLONASS satellite R06.

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sidelobes power levels are around 10 dB W within the offboresight angle range of 30–80° and roll off at the offboresight angle of about 80°, similar to those of GPS Block IIR-M. Thus the conclusion can be made that the EIRP patterns of GPS Block IIR-M satellites and GLONASS satellites are more suited for long distance space users than other GPS ones. It can be seen in these two figures that the majority of received signals are from the sidelobes; and as for GLONASS satellite R06, signals from the off-boresight angle up to more than 100° can be still be received. According to Table 3 the received signals of all six visible Block IIF satellites are from the transmitter antenna sidelobes for more than 82% of the total time period. This percentage rises to 91.6%, 88% and 94.2% for GPS Block IIR, IIRM and GLONASS satellites respectively. Only one Block IIA satellite is visible during this time period thus no statistic calculation is done for this generation of satellites. Note that the mainlobe widths of both GPS Block IIR-M and GLONASS satellite antennas are assumed to be the same as that of GPS Block IIA satellite antennas and the above statistic results only make senses in the CE-5T1 SM flight scenario.

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lite R06 also provides positive contributions to PDOP because it is relatively close to the spacecraft compared with the other GNSS satellites.PDOPx ðtÞ is the PDOP value calculated using all the visible satellites except satellite x (x = G01, . . ., G32, R01, . . ., R24) at the time t, and PDOP0 ðtÞ is calculated with all visible satellites. From Fig. 10(d) it can be seen that PDOPR06 ðtÞ is obviously larger than PDOP0 ðtÞ and the difference is more evident before the encounter than after the encounter. This contribution can be quantified using the following equation: P PDOPx ðtÞ ð3Þ C PDOP ðxÞ ¼ Pt t PDOP0 ðtÞ where t is the whole visible period of satellite R06 and all the visible GNSS satellites. C PDOP is plotted in Fig. 12, (a) The geometry before the close encounter

3.3. The CE-5T1’s close encounter with GLONASS satellite R06 The processing results above show some distinctive performances of GLONASS satellite R06. By investigating the range between satellite R06 and the CE-5T1 spacecraft (Fig. 10), it can be seen that the spacecraft flies close to this GLONASS satellite with a minimum distance of only about 2000 km. During the total visible period, the departure angle (off-boresight angle) of the received R06 signal increases from about 24° to more than 100°, which corresponds to transitioning from the sidelobe to the backlobe. In addition, the close encounter leads to a rise not only in the signal power but also in the rate of change of Doppler. As illustrated in Fig. 10(c), the maximum rate of change of Doppler between R06 and CE-5T1 is 96 Hz/s which almost triples the values of other GNSS satellites. This high dynamic challenge is not difficult to address because a raw estimate of the Doppler and its change rate obtained beforehand using orbit parameters of the CE-5T1 spacecraft and GNSS satellites would narrow the Doppler search range. The 3-D relative positions between the CE-5T1 spacecraft and all visible GNSS satellites before and after the encounter are plotted in Fig. 11. The epoch is UTC time 2014.10.23 19:15:54.275 and 19:24:32.275. Fig. 11(a) shows all other visible satellites except satellite R06 are on the Earth-directed side of the CE-5T1 spacecraft whereas satellite R06 is on the other side before the encounter. This unique geometry optimizes the observation configuration and therefore contributes to the decrease in PDOP. After the encounter all the visible GNSS satellites are on the Earth-directed side as shown in Fig. 11(b), however satel-

(b) The geometry after the close encounter

Fig. 11. The geometry of CE-5T1 and all visible GNSS satellites before and after the close encounter with GLONASS R06.

Please cite this article in press as: Liu, H., et al. The data processing and analysis for the CE-5T1 GNSS experiment. Adv. Space Res. (2016), http://dx.doi.org/10.1016/j.asr.2016.06.035

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H. Liu et al. / Advances in Space Research xxx (2016) xxx–xxx 1.4

1.35

1.3

CPDOP

1.25

1.2

1.15

1.1

1.05

1

G02

G04

G07

G11

G13

G17

G28

R03

R06

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x Fig. 12. The contribution of each visible GNSS satellite to PDOP.

(a) +R06 40 20

Differences of SPP and POD (m)

0 -20 -40 -60 19.1

19.2

19.3

19.4

19.5

Time from 10-23-2014 00:00:00 (UTC), unit: hour (b) -R06 40 20 0 -20 -40 -60 -80 19.1

19.2

19.3

19.4

19.5

Time from 10-23-2014 00:00:00 (UTC), unit: hour Fig. 13. The SPP accuracy differences caused by the absence of satellite R06.

where we can see that the PDOP value increases up to 1.4 times if satellite R06 is excluded in the positioning computation. This influence is much larger than for the other GNSS satellites. This close encounter also brings a significant decrease in the RMS of satellite R06 pseudorange measurement error, which is already shown in Fig. 5. Thus a positive contribution to SPP accuracy improvement is expected. Fig. 13(a) shows the differences of SPP and POD when the pseudor-

ange data of all visible GNSS satellites are employed in the SPP calculation while Fig. 13(b) shows the differences when satellite R06 data is absent. To maintain consistency with the SPP calculation in Section 3.1, the weight value of each satellite is also the inverse square of pseudorange measurement error RMS, and the red, blue and green dots represent the results in the X, Y and Z components (World Geodetic System 1984) respectively. Statistic results show the 3-D RMSs of the differences in these two cases are

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12.8 m and 18.2 m respectively, which means that the SPP accuracy deteriorates by more than 142% because of the absence of satellite R06. 4. Concluding remarks GNSS-based satellite navigation is receiving increasing interest because of a range of foreseeable economic and operational benefits. To support the precise positioning prediction of the RV-SM separation point, China’s CE5T1 spacecraft carried two GNSS receivers and collected data at orbital altitude from 10,000 km to 60,000 km, and even higher. This paper processes the GNSS data received on the first Earth-lunar transfer orbit by the SM receiver and presents the analysis results. The analysis results show that the 24-channel receiver tracked at least four GPS satellites during 98% of the total time period, which verifies the feasibility of using only GPS signals in positioning when the orbit altitude is below 60,000 km. This ratio can be increased to over 99% when employing GPS + GLONASS measurements if the system bias can be fixed as a constant based on previous processing. Besides the positioning feasibility, dual-GNSS also improves the PDOP performance by 77% at the most and 20% on average. However, the measurement error in GLONASS pseudorange data is 13.3 m while that for GPS is 5.9 m. Thus the GLONASS data should be properly weighted in SPP calculation in order not to deteriorate the positioning accuracy. In this paper, the inverse square of pseudorange measurement error RMS is chosen as the weight value for each visible GNSS satellites. And in this case the dual-GNSS brings an improvement of 23% in SPP accuracy compared with the GPS-only results. The GNSS sidelobe signals play a critical role in space missions because only these signals can be tracked from the other side of the Earth. Different types of GNSS satellites have different antenna EIRP designs, especially in the case of the sidelobe signals. The EIRP of all visible GNSS satellites can be calculated using received signal power, precise GNSS ephemerides and the CE-5T1 spacecraft POD results. Due to unknown model precision and other errors, the EIRP values of GPS Block IIA, IIF and IIR do not fit the models precisely but still sketch the main features of the EIRP pattern. Above all, the EIRPs of GLONASS satellites are illustrated in this paper with a nice consistency among different satellites. According to the analysis in Section 3.2, the sidelobe power level of GPS Block IIR-M and GLONASS satellite antennas keeps around 10 dB W from the off-boresight angle of 30°, and begins to decrease at the off-boresight angle of about 80° degrees. The EIRP patterns of these two types of satellites are more popular to space users compared with those of other three generations of GPS satellites because of the high sidelobe power lever and slow roll-off. The close encounter between the CE-5T1 spacecraft and the GLONASS R06 satellite is discussed in this paper. The encounter leads to a rise in signal power and a change in

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the Doppler rate. More importantly, the PDOP value increases up to 1.4 times and the SPP accuracy deteriorates by more than 142% if satellite R06 is excluded in the positioning computation. As one of the few spaceborne GNSS experiments conducted above the altitudes of the GNSS constellations, the data collected in China’s CE-5T1 mission is very useful for studies into how space GNSS application can be addressed. Nevertheless there is still considerable research to be undertaken to improve the GNSS techniques so as to support future lunar missions. Acknowledgment This work was supported by the National Natural Science Foundation of China (41304026). The authors would like to thank Prof. Xiaogong Hu and Ms. Min Fan of Shanghai Astronomical Observatory for their supports. In addition, the authors would like to thank Prof. Chris Rizos of University of New South Wales for his great help in improving English writing. All the reviewers’ valuable comments and helpful suggestions are highly appreciated. References Balbach, O., Eissfeller, B., Hein, G., et al., 1998. Tracking GPS above GPS satellite altitude: first results of the GPS experiment on the HEO mission Equator-S. In: Proceedings of IEEE PLANS, Palm Spring, CA, pp. 243–249. Bamford, W., Heckler, G., Holt, G., et al., 2008. A GPS receiver for lunar missions. In: Proceedings of ION NTM 08 Conference, San Diego, CA, pp. 268–276. Bock, H., Jaggi, A., Beutler, G., et al., 2014. GOCE: precise orbit determination for the entire mission. J. Geod. 88 (11), 47–60. Brown, A., Mathews, B., 2007. Constrained beamforming for space GPS navigation. In: Proceedings of ION GNSS 2007, Ft. Worth, TX, pp. 2357–2363. Czopek, F., 1993. Description and performance of the GPS Block I and II L-band antenna and link budget. In: Proceedings of ION GPS 93, Salt Lake City, UT, pp. 37–43. Fan, M., Hu, X., Dong, G., et al., 2015. Orbit improvement for Chang’E5T lunar returning probe with GNSS technique. Adv. Space Res. 56, 73–82. Folkner, W., Williams, J., Boggs, D., 2008. The planetary and lunar ephemeris DE 421. JPL Memorandum IOM 343R-08-003, url: ftp:// ssd.jpl.nasa.gov/pub/eph/planets/ioms/de421.iom.v1.pdf. Jin, S., Wang, J., Park, P., 2005. An improvement of GPS height estimates: stochastic modeling. Earth Planets Space 57 (4), 253–259. Jin, S., Luo, O., Ren, C., 2010. Effects of physical correlations on longdistance GPS positioning and zenith tropospheric delay estimates. Adv. Space Res. 46 (2), 190–195. Jin, S., Jin, R., Li, D., 2016. Assessment of BeiDou differential code bias variations from multi-GNSS network observations. Ann. Geophys. 34 (2), 259–269. Jin, S., Occhipinti, G., Jin, R., 2015. GNSS ionospheric seismology: recent observation evidences and characteristics. Earth-Sci. Rev. 147, 54–64. Kaplan, E., Hegarty, C., 2006. Understanding GPS: Principles and Applications, second ed. Artech House INC. Kronman, J., 2000. Experience using GPS for orbit determination of a Geosynchronous satellite. In: Proceedings of ION GPS 2000, Salt Lake City, UT, pp. 1622–1626.

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