Compass Signal Structure and First Measurements

Compass Signal Structure and First Measurements T. Grelier, A. Ghion, J. Dantepal, L. Ries, A. DeLatour, J.-L. Issler, CNES, France J.A. Avila-Rodriguez, S. Wallner, G.W. Hein University FAF Munich, Germany

BIOGRAPHY Thomas Grelier is a navigation engineer in the Transmission Techniques and Signal Processing department (TT) at CNES since December 2004. GNSS signal analysis is one of its activities of research. Alain Ghion has worked in the CNES TT Department since September 2005 as associate manager of the laboratory. He is responsible for some experimental activities using CNES GNSS receiving system. Joel Dantepal is currently laboratory manager at CNES in the TT Department since 1996. He is well experienced in the field of GNSS generators, receivers, simulators, pseudolites and digitizers, among other RF hardware. Lionel Ries is a navigation specialist in the TT Department at CNES since June 2000. He is responsible of research activities on GNSS2 signals, including BOC modulations and modernised GPS signals (L2C & L5). Antoine de Latour has been a navigation engineer in the CNES TT Department since 2003. He is involved in the design of Galileo signals, in the use of GNSS for space applications and in GPS/Galileo compatibility assessment. Jean-Luc Issler is head of the TT Department at CNES. He is involved in the development of several GNSS and TTC equipments in Europe, for space and ground users. He is French delegate of the Galileo Signal Task Force. José-Ángel Ávila-Rodríguez is research associate at the Institute of Geodesy and Navigation at the University of the FAF Munich. He is responsible for research activities on MBCS modulations and modernized GNSS signals. Stefan Wallner is research associate at the Institute of Geodesy and Navigation at the University of the FAF Munich. He is responsible for research activities on Spreading Codes and Radio Frequency Compatibility.

Guenter W. Hein is Full Professor and Director of the Institute of Geodesy and Navigation at the University FAF Munich. In 2002 he received the prestigious “Johannes Kepler Award“ from the US Institute of Navigation (ION) for ”sustained and significant contributions to satellite navigation“.

ABSTRACT On April 13 China launched the first middle earth orbiting (MEO) satellite in its Compass GNSS system, 21,550 kilometers above the Earth. China’s radionavigation satellite system which is currently under development is planned to begin operation in 2012. It will consist in a constellation of 27 MEO, 3 Inclined Geosynchronous (IGSO) and 5 geostationary satellites. The current frequency filings for radio bands made by China to the International Telecommunications Union (ITU) indicate that Compass satellites will broadcast signals in four frequency bands in which the carrier frequencies are 1561 MHz (E2’), 1589 MHz (E1’), 1268 MHz (E6) and 1207 MHz (E5b). Compass signals will then overlap some of GPS, Galileo and Glonass signals. On the one hand this suggests potential benefit for users as interoperability of Compass with GPS, Galileo and Glonass would mean increased user performance. On the other hand it arouses concerns as radionavigation L-bands are already congested. It is then necessary that development of Compass is done in cooperation with existing systems to ensure compatibility and interoperability. Compass first MEO satellite began transmitting signal on three frequencies shortly after launch. The satellite was then tracked at both CNES (Toulouse, France) and Monitoring Earth Station of Leeheim (Germany) using high gain parabolic antennas. Compass signals could be observed at frequencies 1207 MHz, 1268 MHz and 1561 MHz. Processing of recorded signals enabled determination of Compass code structure. Results are presented here.

INTRODUCTION China´s Compass is the GNSS system planned by China. As with GPS, GLONASS, and Galileo, the system is planned to provide two navigation services: an open service for (commercial) customers and an “authorized” positioning, velocity, and timing communications service. Compass consists of a constellation of 30 nongeostationary satellites, constituted by 27 Medium Earth Orbit (MEO) and three Inclined Geosynchronous (IGSO), together with five Geostationary (GEO) satellites with positions at 58.75º E, 80º E, 110.5º E, 140º E and 160º E. The orbital constellation parameters for each of the four GNSSes are shown in Table 2. Each satellite transmits the same four carrier frequencies for navigational signals. These navigational signals are modulated with a predetermined bit stream, containing coded ephemeris data and time, and having a sufficient bandwidth to produce the necessary navigation precision without recourse to two-way transmission or Doppler integration. China sent three GEO Beidou (Compass first version) navigation test satellites into orbit between 2000 and 2003. Beginning of 2007 two more Beidou satellite launches were successfully achieved. Compass is expected to cover China and parts of neighbouring countries by 2008, before being expanded into a global system. On February 2nd a launch brought the first of the two planned satellites into a geostationary orbit. Not much later, on April 13th 2007, China launched the first MEO satellite in its Compass GNSS system, 21,550 kilometres or about 13,200 miles above the Earth. The spacecraft began transmitting signals on three frequencies within a few days, much quickly than operational satellites in other GNSSes. If confirmed, these new signals will overlay some GPS and Galileo signals’ spectra what clearly evidences why characterizing Compass signals is so important and assessing their impact on other GNSS systems is so necessary. Shortly after launch, the newest Compass (Beidou) satellite was monitored. On April 24th 2007, a series of observations was performed with CNES reception station which integrates a 2.4-m dish. A few weeks later newer recordings were performed at the Monitoring earth station of Leeheim (Germany) which is operated by the Bundesnetzagentur. This station has two steerable parabolic reflector antennas with diameters of 12 and 7 meters. This latter was used to track Compass satellites on May 16th in 2007. The will of China to develop its own global navigation system is clearly reflected in the policy document released by the State Council Information Office on October 12th

2006, which stated that China will ”independently develop application technologies and products in applying satellite navigation, positioning and timing services“ as reported in the November 13th 2006, issue of China Daily [1]. Compass could begin operation in 2012 if all the political statements are brought into reality. As a summary, the overall constellation parameters of the four global GNSSes – GPS, GLONASS, Galileo and the planned Compass – are shown in Table 2 for comparison. COMPASS SIGNALS CDMA is not only used by GPS, and Galileo, but will also be used by the future Compass satellite navigation system. The Russian GLONASS satellite navigation system uses also spread spectrum codes, which provide very accurate pseudorange measurements, but each carrier frequency is shifted from the others, in order to provide similar performances in term of accuracy, signal separation, flexibility and band occupation. According to Chinese filings [3], Compass satellites will broadcast signals in four frequency bands in which the carrier frequencies are 1561 MHz (E2’), 1589 MHz (E1’), 1268 MHz (E6) and 1207 MHz (E5b). Nota Bene: The E1 band is made of the E1’ band, the 24 MHz wide L1 band and the E2’ band. The intended Compass modulations are described in Table 1. It is important to note that the Compass ITU Filings employ slight different names to refer to the different navigation bands. Next table also reflects this. Table 1. Compass frequencies and modulations Frequency (MHz)

Compass Notation

1561.10 (E2’) 1589.74 (E1’) 1268.52 (E6)

B1 B1-2 B3

1207.14 (E5b)

B2

Modulation QPSK(2) QPSK(2) QPSK(10) BPSK(10) + BPSK(2)

Figure 1 shows the frequency plan for GPS, Galileo, and Compass in three separate panels. The top panel shows the GPS signals as they will appear after launch of GPS III satellites with the third civil signal at L5; the middle panel, the Galileo frequency; and the bottom, the three Compass signals monitored by CNES. Moreover, in the top two panels, the Galileo and GPS signals are shown, respectively, in grey. In the bottom panel, the GPS and Galileo signals are shown as a grey background while the Compass spectra are shown in beige. Compass spectra are those measured at CNES on April 23rd 2007. It does not include E1’ signal since this could not be recorded then.

Table 2. Space Constellation Parameters [2] Parameter Constellation GEO Longitudes GSO Equatorial Crossing Eccentricity GSO Inclination MEO Inclination Semi-major axis

Galileo Walker MEO (27/3/1) plus 3 non-active spares -

GPS MEO(24/6) incl 3 active spares -

GLONASS

Compass

MEO(24/3)

GEO(5), MEO(27), IGSO(3)

-

58.75°, 80°, 110.5°, 140° and 160° E

-

-

-

118°

0 56° 29601.297 km

0 55° 26559.7 km

0 64.8° 25440 km

0° 55° 55° 27840 km

GPS III

L5 1.176 GHz

L1 1.575 GHz

L2 1.227 GHz

Galileo

E5a

E2‘

E5b E6 1.278 GHz

E5 1.192 GHz

B2

Beidou 2A B3

L1 1.575 GHz

E1 ‘

B1

24/04/2007

E5a

E2‘

E5b E5 1.192 GHz

E6 1.278 GHz

E1 ‘ L1 1.575 GHz

Figure 1. GPS, Galileo and Compass/Beidou frequency plan The interesting intended Compass signal in the E5b band would allow Galileo/Compass single frequency E5b receivers.

by each signal. Thus the spectra will be plotted in the next figures in grey to underline this fact. Compass B1 Band [2]

As we have seen in the previous lines, the current frequency filings [3] for radio bands made by China to the International Telecommunications Union (ITU) indicate that Compass would overlay both the Galileo Public Regulated Service (PRS) and military GPS M-code at E1/L1, as well as in L2 and E6. We describe in detail in the next lines the proposed spectra of Compass. In addition, it is important to mention that China has not officially announced yet what services will be transmitted

Although still all the technical aspects of the Compass signals in the B1 band are not defined yet, the planned signal waveforms have already been submitted to the ITU and were summarized in Table 1. Next figure [2] shows the spectral characteristics of the intended Compass signals in B1.

Figure 2. Spectra of intended Compass Signals in B1 Furthermore, in order to have a better feeling about the complicated situation that all the navigation systems have to deal with in the E1/L1/B1 band, Figure 3 shows the details of the studied option in the B1 band together with the rest of signals present in the band and around. As we can clearly recognize, the band suffers from an important congestion. From the previous figures it is clear to see that the situation in E1/L1/B1 is especially challenging for all the existing and planned navigation systems. However, and as the Agreement between the US and EU in 2004 has clearly shown, important synergies could result. In fact, all partners could benefit from it if the development of Compass is set on in coordination with the rest of existing systems.

Figure 3. Spectra of GPS, Galileo, GLONASS and Compass Intended Signals in E1/L1 [2]

Figure 4a-c show maps of the PDOP GPS, Compass and the interoperable scenario of combined GPS and Galileo. To stress the benefit of compatibility and interoperability Figure 4a-d shows additionally the important further reduction of the PDOP figures and consequently the benefit to users worldwide that a potential interoperable design of Compass with GPS and Galileo would bring to the GNSS community.

Max. PDOP Compass (min. Elev. 5°)

Max. PDOP GPS (min. Elev. 5°)

90

90

60 Latitude [deg]

Latitude [deg]

60 30 0 -30

1

0 -30 -60

-60 -90

30

-90 -150

1.5 Min: 1.93

-100

2

-50 0 50 Longitude [deg] 2.5

3

Mean: 2.91

3.5

100

4

4.5

150

5

Max: 5.14

Figure 4a. GPS Position Dilution of Precision (PDOP) with a constellation of 27 satellites

1

-150

1.5 Min: 1.24

-100

2

-50 0 50 Longitude [deg] 2.5

3

Mean: 2.26

3.5

100

4

4.5

150

5

Max: 3.02

Figure 4b. Compass Position Dilution of Precision (PDOP) with a constellation of 35 satellites

Max. PDOP GPS+Galileo+Compass (min. Elev. 5°) 90

60

60

30

30

Latitude [deg]

Latitude [deg]

Max. PDOP GPS+Galileo (min. Elev. 5°) 90

0 -30

1

-30 -60

-60 -90

0

-150

1.5

-100

2

Min: 1.33

-50 0 50 Longitude [deg] 2.5

3

Mean: 1.63

3.5

100

4

4.5

150

5

Max: 2.12

Figure 4c. Position Dilution of Precision (PDOP) of interoperable GPS and Galileo. For Galileo a constellation of 27 satellites was assumed

-90

1

-150

1.5 Min: 0.77

-100

2

-50 0 50 Longitude [deg] 2.5

3

Mean: 1.15

3.5

100

4

4.5

150

5

Max: 1.52

Figure 4d. Position Dilution of Precision (PDOP) of interoperable GPS, Galileo and Compass

Compass B2 Band [2] Similar to the B1 Band, still all the technical aspects of the Compass signals in B2 are not defined yet. B2 is approximately equivalent to the Galileo E5b band and close to the GPS L5 band. Next figure shows the spectral details of the studied option together with the rest of signals present in the band.

Figure 6. Spectra of Galileo and Compass Signals in E6 - B3 [2] COMPASS MEASURED SPECTRA

Figure 5. Spectra of GPS, Galileo, GLONASS and Compass Intended Signals in E5 – B2 [2] Compass B3 Band [2] The Compass B3 band corresponds approximately to the Galileo E6. Next figure shows the Power Spectral densities of the Compass signals in the B3 band together with the Galileo E6 signals that are around.

Compass signals were recorded at CNES on April 23rd 2007. Figure 2, 3 and 4 show Compass signals’ spectra in E2’/L1 (Galileo/GPS anteriority), E6 (Galileo anteriority), and E5b (Galileo anteriority) bands. No signal was recorded at frequency E1’. Measured spectra result from 1000-time averaging. On the measured spectra, theoretical spectra of expected modulations are superimposed. We also superimposed the theoretical spectrum resulting from the sum of noise and signal. This latter plot is really helpful to compare real spectrum with theory. We note that measured spectra are really close to theory. The slight mismatch is very likely due to receiving chain equalization errors. Note that the power spectral density (PSD) decibel values in the vertical axis are relative. Absolute power levels have not been calibrated; so, only the spectral shapes are significant.

Compass signals were also recorded at Leeheim on May 14th 2007. Figure 5, 6 and 7 show spectra in E2’/L1, E6, and E5b bands. Again no signal was recorded at frequency E1’. Measured spectra result from 50-time averaging. The observed spectrum dissymmetries are due to imperfect equalization of Leeheim receiving chain. The higher gain of Leeheim 7-m dish antenna makes the observation of secondary lobes possible. This in return enables to assess the filtering which is performed in the satellite payload. E2’ bandwidth is around 20 MHz, that is about 5 times the spectral width of the transmitted main lobe. This high filtering ratio close to 5 might not be compatible with the RadioAstronomy band protection in 1610.6-1613.8 MHz. The E6 and E5b bandwidth are

around 35 MHz only, that is about 1.75 time the spectral width of the transmitted lobe. A second observation was performed at Leeheim on May 30th 2007. Spectra are represented Figures 8 to 16. They show pure carriers emitted at 1207.14, 1268.52 and 1561.10 MHz. We also observe a residual carrier at 1575.42 MHz, which is 36 dB lower than the carrier at 1561.10 MHz. This unmodulated carrier mode was very likely for test purpose. Note that the spikes and bumps which are present on these spectra come from interferers close to Leeheim station.

5 Measured PSD Theoretical signal PSD Theoretical signal+noise PSD 0

PSD (dB)

-5

-10

-15

-20

-25 1555

1560

v

1565

1570

Frequency (MHz)

Figure 7. Compass spectrum in E2’ (CNES 23/04/07)

Figure 8. Compass spectrum in E2’ (Leeheim 14/05/07)

5 Measured PSD Theoretical signal PSD Theoretical signal+noise PSD 0

PSD (dB)

-5

-10

-15

-20

-25 1250

1255

1260

1265

1270

1275

1280

1285

Frequency (MHz)

Figure 9. Compass spectrum in E6 (CNES 23/04/07)

Figure 10. Compass spectrum in E6 (Leeheim 14/05/07)

5 Measured PSD Theoretical signal PSD Theoretical signal+noise PSD 0

PSD (dB)

-5

-10

-15

-20

-25 1190

1195

1200

1205

1210

1215

1220

1225

Frequency (MHz)

Fig 12. Compass spectrum in E5b (Leeheim 14/05/07) Figure 11. Compass spectrum in E5b (CNES 23/04/07)

Figure 13. Compass spectrum in E2’ (Leeheim 30/05/07)

Figure 14. Compass spectrum in L1 (Leeheim 30/05/07)

Figure 15. Compass spectrum in E6 (Leeheim 30/05/07)

Figure 16. Compass spectrum in E5b (Leeheim 30/05/07)

Baseband signal modulation The first processing step consists in removing the Doppler frequency offset and the residual carrier phase so as to obtain baseband signals. Compass signal are acquired using codeless acquisition techniques. After removal of Doppler and carrier phase residual the signal is decomposed in its two components: in-phase (I) and quadra-phase (Q). Accumulations over chip duration are performed to reduce noise and estimate chip values. Chip duration is the inverse of the chip rate and depends on the processed signal (either 2.046 or 10.23 MHz). I-Q diagram are then plotted to determine signal modulation. The three Compass signals appear to be balanced QPSK: QPSK(2) in E2’, QPSK(10) in E6 and BSPK(10)+BSPK(2) in E5b. Figure 12 shows I-Q diagram in E2’. The 4 spots of a QPSK constellation clearly appear. The same kind of diagram is obtained for E6 and E5b.

Secondary code We notice that quadra-phase correlation peaks are either positive or negative and that the binary sequence repeats itself every 20 ms. This means that a 20-ms periodic secondary code is present. The 20-chip sequence is sometimes inverted which suggests that a 50-Hz data stream modulates the chips. 2000

1000

Correlation

PRN CODE DEMODULATION

0

-1000

-2000

0

5

10

15

20

25

30

35

40

45

Time (ms)

Figure 18. Correlation of the E2’-I bit sequence with a slice of itself Secondary code sequence Data = 1

Secondary code sequence Data = -1

2000

Correlation

1000

0

-1000

Figure 17. I-Q diagram of E2’ signal Code period determination The second step consists in determining the period of PRN codes. Thanks to the high gain of CNES dish, bits can be estimated easily. The bit sequence is correlated with a small slice of itself (1000 bits). The correlation plots for E2’ in-phase and quadra-phase components are represented on Figure 18 and Figure 19. The time separation between peaks is the code period. For E2’-Q correlation peaks appear every 1 ms therefore PRN code period is 1 ms (2046 chips). No second correlation peak appears on E2’-I plot. We can then deduce that the code period is greater than the bit sequence duration. The longest sequence that was processed is 400 ms for E2’ and 160-ms for E6 and E5b.

-2000

0

5

10

15

20

25

30

35

40

45

Time (ms)

Figure 19. Correlation of the E2’-Q bit sequence with a slice of itself The same process was repeated for E6 and E5b signals and lead to estimations of PRN codes. Results are synthesized in Table 2. Note that E2’ and E5b primary code are identical. Note also that the secondary code is identical for the three frequencies. The code is the following (the sign of the sequence being ambiguous the initial chip was set positive): 1 1 1 1 1 -1 1 1 -1 -1 1 -1 1 -1 1 1 -1 -1 -1 1

Table 3. Compass signal characteristics Frequency

E2’

Modulation

Component

2.046 Mcps 2.046 Mcps

2046

>400 ms 1 ms

Quadrature

Secondary code Data Code Primary code Secondary code Data Code Primary code Secondary code Data

1 kHz 50 Hz 10.23 Mcps 10.23 Mcps 1 kHz 50 Hz 10.23 Mcps 2.046 Mcps 1 kHz 50 Hz

20 10230 20 2046 20 -

20 ms >160 ms 1 ms 20 ms >160 ms 1 ms 20 ms -

Quadrature In-phase

E5b

QPSK

Period

Code Primary code

QPSK

QPSK

Length

In-phase

In-phase E6

Rate

Quadrature

CONCLUSION Collection of signal from recently launched first Compass MEO satellite has enabled us to determine Compass signal structure in E2’, E6 and E5b bands. All three frequencies are QPSK modulated, very likely with an authorized service in-phase and a public/commercial service in quadrature. Open/commercial service codes were determined: primary codes are 1-ms long and are modulated by a 20-ms secondary code. Observing both E2’ and E1’ signals simultaneously would be very instructive as it would allow us to determine if signals are generated independently or if a specific modulation is used to generate both signals. Finally it is important to note that China has chosen to superimpose the intended Compass frequency plan on Galileo one rather than on GPS or GLONASS one, or rather to use the non-used RNSS C-band candidate for the second generation of GALILEO. The cohabitation of future Galileo and Compass signals will concern several signals. Indeed Compass E1’ and E2’ BSPK(2) fall on main lobes of Galileo PRS BOC(15,2.5), Compass E6 BPSK(10) falls on lower lobe of Galileo PRS BOC(10,5) and Compass E5b BPSK(10) falls on higher lobe of Galileo AltBOC(15,10). Therefore compatibility and interoperability between both systems will have to be seriously analysed. CNES EQUIPMENT Compass signals were recorded using CNES tracking and recording system which was developed in collaboration with the European Space Agency (ESA). This system is composed of a tracking station, a broadband digitizer and a high capacity recorder (called datalogger). It enables the

observation of GNSS satellites in the 1.1-1.7 GHz band and the post-processing of recorded signals. The tracking station includes a 2.4-m steerable parabolic dish developed by Datatools, based in Dakar (Senegal) and can track automatically a specified satellite. The digitizing equipment, was developed by SMP, a SME based in Toulouse (France) and allows the sampling of 2 to 4 GNSS bands simultaneously. The datalogger used to store the samples was developed by M3Systems, another SME based close to Toulouse. It offers a recording rate of 250 MB/s and a total capacity of several hundreds of GBytes. ACKNOWLEDGMENTS The authors would like to thank Klaus Mecher, Peter Steiner and Harald Hellwig of Bundesnetzagentur for their considerable work at Leeheim earth station. They thank Jean-Luc Gerner, Gonzalo Seco Granados of ESA/ESTEC for a fruitful collaboration on the developments of the GNSS datalogger and bitgrabber equipments mentioned in the paper. The authors would like also to thank Frédéric Couturier, of ANFR with which CNES also collaborates on GNSS spectrum studies. REFERENCES [1] See China Daily, Beijing, 13 November 2006 [2] J.A., Avila-Rodriguez, On Optimized Signal Waveforms for GNSS, Ph.D. Thesis, 2007, University FAF Munich, Neubiberg, Germany [3] International Telecommunication Union, Annex 3 to Document 8D/274 on the Chinese Satellite Navigation System Compass: 8D-300 CHN Compass 1164-1215 MHz, 8D-301 CHN Compass 1260-1300

MHz, 8D-302 CHN Compass description and 8D-303 CHN Compass 1559-1610 MHz, 16 January 2006. [4] Initial Observation and analysis of Compass MEO satellite signals - T.Grelier, J.Dantepal, A.De Latour, A.Ghion, L. Ries (CNES), Inside GNSS issue May/June 2007