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Research Report on GNSS Interoperability

LU XIAOCHUN
National Time Service Center, Chinese Academy of Sciences

LU JUN
Beijing Institute of Tracking and Telecommunications Technology

BAI YAN, HAN TAO, and WANG XUE
National Time Service Center, Chinese Academy of Science

INTRODUCTION

Interoperability has become a focus of Global Navigation Satellite Systems (GNSS) and a development aspect, and incurs much focus among the world. For the purpose of maximum benefit, a series of interoperability researches and cooperation are put forward.

Research of interoperability includes both technical factors, such as signal design, satellite payload, and user terminal, and nontechnical factors, such as market and industry. One should consider not only combining with other systems, but also vindicating one’s own benefits and maintaining some independence.

Thus, research of interoperability should be approached with consideration of both technical factors and nontechnical factors.

INTEROPERABILITY DEFINITION PARSING

Elements of interoperability include coordinate reference frame, time reference frame, open signal, and the constellation. The resources probably applied in interoperability are global navigation satellite systems and their services, regional navigation satellite systems and their services, and augmentation systems and their services. Finally, the purpose of interoperability is to provide better services at the user level, including higher accuracy, better reliability, better cost-effectiveness, and more user satisfaction (different types of users, different application areas).



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Research Report on GNSS Interoperability LU XIAOCHUN National Time Service Center, Chinese Academy of Sciences LU JUN Beijing Institute of Tracking and Telecommunications Technology BAI YAN, HAN TAO, and WANG XUE National Time Service Center, Chinese Academy of Science INTRODUCTION Interoperability has become a focus of Global Navigation Satellite Systems (GNSS) and a development aspect, and incurs much focus among the world. For the purpose of maximum benefit, a series of interoperability researches and cooperation are put forward. Research of interoperability includes both technical factors, such as signal design, satellite payload, and user terminal, and nontechnical factors, such as market and industry. One should consider not only combining with other systems, but also vindicating one’s own benefits and maintaining some independence. Thus, research of interoperability should be approached with consideration of both technical factors and nontechnical factors. INTEROPERABILITY DEFINITION PARSING Elements of interoperability include coordinate reference frame, time refer- ence frame, open signal, and the constellation. The resources probably applied in interoperability are global navigation satellite systems and their services, regional navigation satellite systems and their services, and augmentation sys - tems and their services. Finally, the purpose of interoperability is to provide better services at the user level, including higher accuracy, better reliability, better cost-effectiveness, and more user satisfaction (different types of users, different application areas). 35

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36 GLOBAL NAVIGATION SATELLITE SYSTEMS DISCUSSION OF INTEROPERABILITY PHASES BETWEEN BEIDOU GLOBAL SYSTEM AND GPS Actuality of BeiDou and GPS Coordinate Reference Frame BeiDou uses CGS2000 as its coordinate reference frame and keeps a centimeter level bias with ITRF. GPS uses WGS84 as its coordinate reference frame and keeps a centimeter level bias with ITRF. Time Reference Frame The time reference frame of BeiDou is called “BDT”; the difference between BDT and UTC (NTSC) is less than 50 ns. The time reference frame of GPS is called “GPST”; the difference between GPST and UTC (USNO) is less than 28 ns. Open Signals The civil signal in the GPS L1 band is C/A code-BPSK(1); future civil sig - nals in the L1 band are MBOC (TMBOC in pilot channel and BOC(1,1) in data channel). The civil signal in the second phase of BeiDou is BPSK(2), and it provides service in the important area (30E~180E, 70S~70N); the civil signal in the third phase is MBOC (TMBOC in pilot channel and BOC(1,1) in data channel) (Tables 1 and 2). TABLE 1 Signal in the Second and Third Phases of BeiDou Center Code frequency rate Phase Signal (MHz) (MHz) Modulation Services BeiDou 2nd. B1(I) 1561.098 2.046 BPSK Open BeiDou 3rd. B1x/y 1575.42 1.023 TMBOC(6,1,4/33)+BOC(1,1) Open TABLE 2 Signal of Current GPS and Future GPS Center Code frequency rate Phase Signal (MHz) (MHz) Modulation Services Current GPS L1C/A 1575.42 1.023 BPSK Open Future GPS L1 C 1575.42 1.023 TMBOC(6,1,4/33)+BOC(1,1) Open

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37 RESEARCH REPORT ON GNSS INTEROPERABILITY Currently the BeiDou system has launched eight satellites of phase-2 and elementary form of positioning ability. Domestic industries have designed CMOS chip in L1 band (1561.098 MHz and 1575.42 MHz), and they plan to produce receivers able to receive GPS L1 C/A and BeiDou B1 BPSK(2), which make base of interoperability. Constellation GPS now has 30 satellites in orbit. The BeiDou regional system is designed to have 5 geostationary (GEO), 3 inclined geosynchronous orbit (IGSO) satellites, and 4 medium Earth orbit (MEO) satellites (Figure 1); while the BeiDou global system is designed to have 5 GEO, 3 IGSO, and 24 MEO satellites (Figure 2). Suggestion of Interoperability Between BeiDou and GPS According to current resources, interoperability between BeiDou B1 and GPS L1 should be staged in three phases (Figure 3): Phase 1: From 2012 until GPS TMBOC operates. Phase 2: From GPS TMBOC operation to BeiDou TMBOC functions (2020). Phase 3: After BeiDou TMBOC operates (2020). Phase 1 Interoperability between GPS L1 BPSK(1) in 1575.42 MHz and BeiDou regional system B1 BPSK(2) in 1561.098 MHz (Figure 4). Phase 2 Interoperability between GPS L1 TMBOC in 1575.42 MHz and BeiDou regional system B1 BPSK(2) in 1561.098 MHz (Figure 5). Phase 3 Interoperability between GPS L1-TMBOC in 1575.42 MHz and BeiDou global system B1-TMBOC in 1575.42 MHz (Figure 6).

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38 FIGURE 1 Constellation of GPS and BeiDou regional system. Lu_Fig1.eps bitmap, landscape

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FIGURE 2 Constellation of GPS and BeiDou global system. Lu_Fig2.eps 39 bitmap, landscape

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40 GLOBAL NAVIGATION SATELLITE SYSTEMS GPS TMBOC 2012 operates 2020 GPS L1: BPSK (1) L1: TMBOC BeiDou B1: BPSK (2) B1: TMBOC Phase 1 Phase 2 Phase 3 Lu_Fig3.eps FIGURE 3 Phases of interoperability between BeiDou and GPS. 2 bitmaps, vector type & rules PSD of GPS L1 and BeiDou B1 -55 GPS-bpsk(1) -60 BeiDou bpsk(2)-I -65 -70 -75 PSD -80 -85 -90 -95 -100 1.555 1.56 1.565 1.57 1.575 1.58 1.585 frequency (Hz) 9 x 10 FIGURE 4 Frequency spectrum in interoperability phase 1. Lu_Fig4.eps

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41 RESEARCH REPORT ON GNSS INTEROPERABILITY FIGURE 5 Frequency spectrum in interoperability phase 2. Lu_Fig5.eps bitmap FIGURE 6 Frequency spectrum in interoperability phase 3. Lu_Fig6.eps bitmap

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42 GLOBAL NAVIGATION SATELLITE SYSTEMS FEASIBILITY ANALYSIS OF INTEROPERABILITY BETWEEN BEIDOU B1 BPSK(2) AND GPS BPSK(1) Signal Performance Analysis Correlation peak Correlation peak can be calculated as: Tp ∫S ( t ) ⋅ SRe f ( t − ε ) dt * BB − Pr e Pr oc CCF ( ε ) = 0 (4.1)  Tp   Tp  2 ∫ S BB− Pr e Pr oc ( t ) dt  ⋅  ∫ SRe f ( t ) dt  2 0  0  where S BB− Pr e Pr oc is the base band signal (after pretreatment); reference signal SRe f is the copy of ideally base band signal from receiver; integral time Tp corresponds to the main period of reference signal. The correlation peak is cor- related to ranging accuracy, anti-multipath, and anti-jamming of a signal. The sharper the peak is, the better performance a signal has. From Figure 7, BPSK(2) has a sharper correlation peak than BPSK(1). BPSK(1) 1 BPSK(2) 0.8 0.6 0.4 0.2 0 -0.2 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 FIGURE 7 Correlation peak of BPSK(1) and BPSK(2). Lu_Fig7.eps

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43 RESEARCH REPORT ON GNSS INTEROPERABILITY Gabor Bandwidth Gabor bandwidth is the best index to estimate the connection from receive- bandwidth to tracking accuracy; the greater the Gabor bandwidth is, the better the signal’s tracking accuracy. Under the same code loop bandwidth and same receive carrier to noise ratio, the root mean square (RMS) code tracking accuracy depends on RMS bandwidth. βr (4.2) f 2GS ( f ) df ∫ β rms = 2 β −r 2 where brms is RMS bandwidth (namely Gabor bandwidth), br is bilateral receive bandwidth, and GS(f) is PSD. The RMS bandwidth of different modulations is shown in Table 3. The anti- jamming of BPSK(2) is better than BPSK(1), but weaker than MBOC. TABLE 3 Gabor Bandwidth Modulation BPSK(1) BPSK(2) CBOC+ CBOC- TMBoc BOCsin(2,2) 1.4415×105 2.0525×105 3.0990×105 3.5348×105 3.552×105 3.5558×105 Gabor bandwidth Tracking Error βr / 2 BL (1 − 0.25 BL T ) GS ( f ) sin 2 (π f ∆ ) df ∫ − βr / 2 σ NELP = × 2 2 C  βr / 2  2π ∫ fGS ( f ) sin (π f ∆ ) df  N 0  − βr / 2    βr / 2  GS ( f ) cos 2 (π f ∆ ) df  ∫   − βr / 2 1 + 2 C r   β /2  ∫ GS ( f ) cos (π f ∆ ) df   T N 0  − βr / 2     (4.3) Suppose the interference signal power spectrum is white, one-side power spectrum is N0, signal-receive power is Pc, and forward bandwidth is br. The bilateral power spectrum of interference signal baseband equivalent w(t) is twice the unilateral power spectrum of radio interference; power (Ps) of available sig- nal envelope s(t) is twice of radio signal, influence from the channel is equal to baseband transmit function H(f), GS(f) is signal power spectrum, T is integral time, and ∆ is correlator pace (unit: second) (Table 4).

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44 GLOBAL NAVIGATION SATELLITE SYSTEMS TABLE 4 Simulation Parameters Delt = 5 IntTime = 0.004 s Dllw = 0.5Hz d = 1/(12*1.023*10^6) Space of power spectrum Integral time Loop bandwidth Correlator space frequency Figure 8 shows that BPSK(2) has better tracking error than BPSK(1). Multipath Receive signal with multipath can be equated to: N r ( t ) = a0 e jφ0 x ( t − τ 0 ) + ∑ an e jφn x ( t − τ n ) (4.4) n =1 where a0 is the extent of firsthand signal; f0 is the phase of firsthand signal; x(t) is the complex envelope of sending signal; t0 is the time delay of firsthand signal; N is the number of path of the multipath signal; an is the extent of multi- path signal; fn is the phase of the multipath signal; and tn is the time delay of the multipath signal. 4 BPSK(2) BPSK(1) 3.5 3 2.5 error (m) 2 1.5 1 0.5 0 20 25 30 35 40 45 50 55 60 C/N0 (dB-Hz) FIGURE 8 Tracking error of BPSK(1) and BPSK(2). Lu_Fig8.eps

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45 RESEARCH REPORT ON GNSS INTEROPERABILITY Multipath error can be depicted as: βr / 2 S ( f ) sin ( −2π f τ 1 ) sin (π fd ) df ±2a1 ∫  − βr / 2 ετ ≈ − βr / 2 βr / 2 fS ( f ) sin ( −2π f τ 1 ) sin (π fd ) df fS ( f ) sin (π fd ) df ± 4π a1 ∫ 4π a0 ∫  − βr / 2 − βr / 2 βr / 2 S ( f ) sin ( 2π f τ 1 ) sin (π fd ) df ± a1 ∫   − βr / 2 ≈ (4.5) βr / 2 fS ( f ) sin (π fd ) 1 ± a1 cos ( 2π f τ 1 )  df 2π ∫   − βr / 2 where a1 = a1 / a0 is the extent ratio from the multipath signal to the firsthand  signal, br is the front bandwidth of receiver, and d is the correlator space. The mean multipath error A(t) can be calculated by: τ 1 max ( E ( x )) − min ( E ( x )) dx τ∫ A (τ ) = (4.6) 0 where E(x) is the curve function of multipath error envelope, and t is code time delay. Then the even multipath error is: 1 1  ε (τ ) ϕ = 0 + ε (τ ) ϕ =180  τ ε a (τ 1 ) = τ1 ∫  (4.7)  dτ 2  0  where e(t) is the function of multipath error envelope, ea(t1) is the function of even multipath error, t1 and t are multipath signal time delay (Table 5, Figures 9 and 10). TABLE 5 Simulation Parameters a1 = -5 dB B = 30 MHz d = 1/20 code Extent ratio from multipath signal to firsthand signal Front receive Correlator space bandwidth

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64 GLOBAL NAVIGATION SATELLITE SYSTEMS Error curves of TMBOC with different receiver front bandwidths are shown in Figure 28 (Table 13) 3 B=16MHz B=20MHz 2.5 B=24MHz B=30MHz B=40MHz 2 1.5 1 0.5 0 15 20 25 30 35 40 45 50 55 60 FIGURE 28 TMBOC error curves with different receiver front bandwidths. Lu_Fig28.eps TABLE 13 Simulation Parameters Delt = 5 B = 0.2 IntTime = 0.004 Frequency space of power spectrum Loop bandwidth Integral time

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65 RESEARCH REPORT ON GNSS INTEROPERABILITY Error curves of TMBOC with different receiver loop bandwidths are shown in Figure 29 (Table 14). 3 Bn=0.1 Hz Bn=0.2 Hz 2.5 Bn=0.5 Hz Bn=0.75 Hz Bn=1 Hz 2 1.5 1 0.5 0 15 20 25 30 35 40 45 50 55 60 FIGURE 29 TMBOC error curves with different receiver loop bandwidths. Lu_Fig29.eps TABLE 14 Simulation Parameters Delt = 5 IntTime = 0.004 Dllw = [0.1 0.2 0.5 0.75 1] d = 1/24 Frequency space of Integral time Loop bandwidth Correlator space power spectrum

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66 GLOBAL NAVIGATION SATELLITE SYSTEMS Error curves of TMBOC with different integral time are shown in Figure 30 (Table 15). 4 IntTime=0.001 s IntTime=0.002 s 3.5 IntTime=0.005 s IntTime=0.0075 s 3 IntTime=0.01 s 2.5 2 1.5 1 0.5 0 15 20 25 30 35 40 45 50 55 60 FIGURE 30 TMBOC error curves with different integral time. Lu_Fig30.eps TABLE 15 Simulation Parameters Delt = 5 Dllw = 0.5 Dllw = [0.1 0.2 0.5 0.75 1] d = 1/24 Frequency space of Loop bandwidth Loop bandwidth Correlator space power spectrum

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67 RESEARCH REPORT ON GNSS INTEROPERABILITY Error curves of signals with different modulations are shown as Figure 31 (Table 16). FIGURE 31 Error curves of signals with different modulations. Lu_Fig31.eps bitmap TABLE 16 Simulation Parameters Delt = 5 Dllw = 0.5 IntTime = 0.004 s d = 1/24 Frequency space of Loop bandwidth Integral time Correlator space power spectrum

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68 GLOBAL NAVIGATION SATELLITE SYSTEMS Multipath Generally, we can get different multipath performance for each kind of navi - gation signal based on its multipath error and multipath running average error. Assume that receiver front bandwidth is 30 MHz, with correlator space of 1/20 chip and the ratio of multipath to direct path of -6dB. Figure 32 shows the envelop curves of multipath average errors for BPSK(10), BOC(1,1), BOC(2,2), TMBOC Pilot, and CBOC Pilot. If we put the modulated signal with best performance in the first place, and that with worst performance in the end, then we can easily get the following results: BPSK(10), BOC(2,2), TMBOC(6,1,4/33), CBOC-(6,1,1/11), BOC(1,1), BPSK(1); where the difference between TMBOC(6,1,4/33) and CBOC-(6,1,1/11) is very small. FIGURE 32 Multipath running average error. Lu_Fig32.eps bitmap

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69 RESEARCH REPORT ON GNSS INTEROPERABILITY Brief Summary Both BeiDou B1 signal in phase 3 and GPS modernized signals adopt the design of TMBOC(6,1,4/33) + BOC(1,1). The particular technologies of time division in sub-carrier wave, second coding for ranging codes, channel separa- tion, and message coding make the TMBOC signal have a better acquisition per- formance, better tracking performance, better demodulation performance, better anti-jamming performance, and better anti-multipath performance. Service Performance In phase 3 of interoperability between BeiDou and GPS, not only did the signals of BeiDou system change, but its satellite constellation also changed (Figures 33–38). FIGURE 33 PDOP in China. Lu_Fig33.eps bitmap

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70 GLOBAL NAVIGATION SATELLITE SYSTEMS FIGURE 34 PDOP in the United States. Lu_Fig34.eps bitmap FIGURE 35 TDOP in China. Lu_Fig35.eps bitmap

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71 RESEARCH REPORT ON GNSS INTEROPERABILITY FIGURE 36 TDOP in the United States. Lu_Fig36.eps bitmap FIGURE 37 GDOP in China. Lu_Fig37.eps bitmap

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72 GLOBAL NAVIGATION SATELLITE SYSTEMS FIGURE 38 GDOP in the United States. Lu_Fig38.eps bitmap Accuracy In China, for an interoperability constellation of BeiDou + GPS, the average PDOP reduction is 30.14 percent compared with that of GPS, while the average TDOP reduction is 28.76 percent. When it is compared with BeiDou, the average PDOP reduction is 17.75 percent, and the average TDOP reduction is 20.14 percent. Thus, compared with GPS, the interoperability system obtains a 30.14 percent improvement in positioning accuracy and a 28.76 percent improvement in timing accuracy. When it is compared with BeiDou, the interoper- ability system obtains a 17.75 percent improvement in positioning accuracy and a 20.14 percent improvement in timing accuracy. In the United States, for an interoperability constellation of BeiDou + GPS, the average PDOP reduction is 35.10 percent compared with that of GPS and the average TDOP reduction is 36.66 percent. When it is compared with BeiDou, the average PDOP reduction is 35.86 percent and the average TDOP reduction is 37.97 percent. Thus, compared with GPS, the interoperability system obtains a 35.10 percent improvement in positioning accuracy and a 36.66 percent improve - ment in timing accuracy. When it is compared with BeiDou, the interoperability system obtains a 35.86 percent improvement in positioning accuracy and a 37.97 percent improvement in timing accuracy. Availability In China, for an interoperability constellation of BeiDou + GPS, the average PDOP reduction is 30.14 percent compared with that of GPS and the average TDOP reduction is 28.76 percent. When it is compared with BeiDou,

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73 RESEARCH REPORT ON GNSS INTEROPERABILITY the average PDOP reduction is 17.75 percent and the average TDOP reduction is 20.14 percent. Thus, compared with GPS, the interoperability system obtains a 30.14 percent improvement in positioning availability and a 28.76 percent improvement in timing availability. When it is compared with BeiDou, the interop- erability system obtains a 17.75 percent improvement in positioning availability and a 20.14 percent improvement in timing availability. In the United States, for an interoperability constellation of BeiDou and GPS, the average PDOP reduction is 35.10 percent and the average TDOP reduction is 36.66 percent. When it is compared with BeiDou, the average PDOP reduction is 35.86 percent and the average TDOP reduction is 37.97 percent. Thus, com - pared with GPS, the interoperability system obtains a 35.10 percent improvement in positioning availability and 36.66 percent improvement in timing availability. When it is compared with BeiDou, the interoperability system obtains a 35.86 per- cent improvement in positioning availability and a 37.97 percent improvement in timing availability. Integrity The ranging accuracy of interoperable signal in phase 3 is better than that of phase 2; while the GDOP in phase 3 is better than that of phase 2. Thus, the integrity detection ability will be improved. Continuity Increase of the number of BeiDou satellites also adds the number of interop - erability constellation satellites in phase 3, which will lead to a better continuity than that of phase 2. Brief Summary In phase 3, there will be a smaller ranging error, advanced URE, more visible satellites, and lower positioning and timing threshold. As a result, service avail - ability, integrity, and continuity will be improved. So we can draw the conclusion that, if BeiDou and GPS could achieve interoperability, then there will be a great improvement in the service performance of both systems. SUMMARY From the analysis above, we can see that: 1. BeiDou and GPS could achieve interoperability. 2. Interoperability between BeiDou and GPS will be of great benefit to the improvement in service performance.

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74 GLOBAL NAVIGATION SATELLITE SYSTEMS 3. With the development of each system, interoperability between the two systems will deepen. 4. Interoperability could be achieved between BeiDou B2 signal and GPS L5 signal as well, so dual-frequency interoperability can also be realized.