On Generalized Signal Waveforms for Satellite Navigation (797942), страница 15
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For the caseof the standard accuracy signals (C/A), the PR ranging code is a sequence of the maximumlength of a shift register (M-sequence) with a period of 1 millisecond and a bit rate of511 kbps. The navigation message is sent at 50 bps and the auxiliary meander at 100 Hz.Figure 2.23. Spectra of GLONASS Signals in L236Global Navigation Satellite SystemsWe show in the figure next all the signals of GPS and GLONASS in the L2 band togetherFigure 2.24.
Spectra of GPS and GLONASS Signals in L2To finalize some details on the technical characteristics of the GLONASS L2 signals arepresented in the next table:Table 2.11. GLONASS L2 signal technical characteristicsGNSS SystemGLONASSGLONASSService NameC/A CodeP CodeCentre Frequency(1242.9375…1248.625) MHz ± 0.511 MHzFrequency BandL2L2Access TechniqueFDMAFDMASpreading modulationBPSK(0.511)BPSK(5.11)Sub-carrier frequency--Code frequency0.511 MHz5.11 MHzSignal ComponentDataDataPrimary PRN Code length511N/ACode FamilyM-sequencesN/A.Meander sequence100 HzN/AData rate50 bpsN/AMinimum Received Power[dBW]-167 dBWN/AElevation5°N/AIt is important to note again that unlike for the case of GPS and Galileo in the previouschapters, the frequencies do not have to be multiplied by the factor 1.023.37Global Navigation Satellite Systems2.5.2.2GLONASS L3 BandAs shown during the Munich Satellite Navigation Summit of 2008, GLONASS is planning totransmit navigation signals also on the L3 band, although the definite signal plan has not beendecided yet.
Indeed, four possible scenarios are being studied at the moment:••••Option 1: GLONASS K satellites would use a bandwidth of approximately 15 MHzwith 16 channels. Both the in-phase and quadrature signals would be BPSK(4) with achip rate of 4.092 MHz. It is interesting to note that as GPS and Galileo, the factor 4must be understood as multiplied by 1.023 although we talk about FDMA signals.Option 2: GLONASS-L3 would have a 24 MHz bandwidth and would transmitBPSK(8) for the in-phase channel and BPSK(2) for the quadrature signals.Option 3: This option is identical to option 1 but shifted by 3 MHz to higherfrequencies to achieve better isolation with Galileo E6.Option 4: This option is identical to option 2 but shifted by 3 MHz to higherfrequencies, also to improve the spectral isolation with other signals in the band.Figure 2.25.
Spectra of GLONASS Signals in L3 (Option 1)And now with GPS and Galileo together:Figure 2.26. Spectra of GPS, Galileo and GLONASS Signals in L3 (Option 1)38Global Navigation Satellite SystemsWe analyze now the second option for the GLONASS signals in L3. The following figureshows the different spectra.Figure 2.27. Spectra of GLONASS Signals in L3 (Option 2)and again, with GPS and Galileo together:Figure 2.28. Spectra of GPS, Galileo and GLONASS Signals in L3 (Option 2)To finalize, the technical characteristics of the GLONASS L3 signals are summarized in thenext table:39Global Navigation Satellite SystemsTable 2.12.
GLONASS L3 signal technical characteristicsGNSS SystemGLONASSGLONASSService NameL3 IL3 Q1201 MHzCentre Frequency2.5.3Frequency BandL3L3Access TechniqueFDMAFDMASpreading modulationOption 1BPSK(4)Option 2BPSK(8)Option 1BPSK(4)Option 2BPSK(2)Sub-carrier frequency--Code frequencyOption 14.092 MHzOption 28.184 MHzOption 14.092 MHzOption 22.046 MHzData Rate100 or 125 bpsN.A.GLONASS ModernizationSimilar to GPS, GLONASS is on the way to modernizing its infrastructure. Apart from thesignals in the L1 band, the Russian system has already established a second civil signal at L2upon launch of the first GLONASS-M satellite in 2003.
The new GLONASS-M satelliteshave better signal characteristics as well as a longer design life (7-8 years instead of thecurrent 3 years) and a third civil signal at the L3 band is planned to start in 2010 aboardGLONASS-K satellites. The different options have been discussed some lines above. Inaddition, GLONASS also intends to transmit CDMA signals in L1 and L5.This last generation of satellites is planned to be based on low mass satellites with aguaranteed lifespan of 10 to 12 years. The new GLONASS satellites are planned to reducetheir weight by 50 %.
This will allow to significantly lower launch costs by using the SoyuzU launch vehicles. Regarding the number of satellites, it seems that GLONASS envisionsupdating its constellation up to 36 satellites.GLONASS is not yet fully compatible and interoperable with the rest of GNSS systems.Indeed, major differences between GPS and Galileo using the Code Division Multiple Access(CDMA) and GLONASS using the Frequency Division Multiple Access (FDMA) approachstill exist. A deeper insight into this topic will be provided in the next chapters. However, it isimportant to note at this point that in spite of the fundamental differences that the FDMA andCDMA approaches represent, solutions for common receivers can be found today.Nonetheless, common receiver architectures are considerably more complex and expensive.40Global Navigation Satellite SystemsThe experience of Galileo has shown how important the roles of interoperability andcompatibility with GPS were from the very beginning.
Fortunately, it seems that GLONASSis taking important steps towards increasing compatibility and interoperability in the future asthe common GPS/GLONASS statements in the framework of Working Group 1 (WG 1) seemto show. Indeed, during the GPS/GLONASS meeting in December 2006 in Yaroslavl(Russia), both sides emphasized the benefit to the user community that a common approachconcerning the use of FDMA and CDMA would bring in terms of interoperability.
Thememorandum textually states ”Both sides noted that concerning the question of the use ofFDMA and CDMA, significant progress was made in understanding the benefit to the usercommunity of using a common approach.” [US-Russia Statement, 2006].Russia announced then that they would come to a decision on the change from FDMA toCDMA by the end of 2007 [US-Russia Statement, 2006]. Finally, on February 15th, 2008, agovernment decree on new GLONASS requirements announced that GLONASS will alsotransmit open CDMA BOC(2,2) signals at 1575.42 MHz (GPS and Galileo E1/L1 band) and aBOC(4,4) signal centred at 1176.45 MHz (GPS L5 or Galileo E5a bands).
It is however alsopossible that GLONASS could offer MBOC in E1 and BPSK(10) in L5 instead in its finalCDMA Signal baseline plan in order to enhance interoperability with the rest of navigationsystems in the band, namely with GPS and Galileo. As we have seen, these frequenciesessentially correspond to the centre points of GPS and Galileo. According to this, GLONASSwill be able to implement CDMA signals on L1 and L5 already in the GLONASS-K satellitesplanned for Phase 3 of the GLONASS modernization.
This will imply a tremendous effort inthe design of the satellite’s payload regarding the power consumption, because if GLONASSmoves to CDMA, this does not mean that FDMA will be completely abandoned. Indeed, forlegacy and security reasons, FDMA and CDMA payloads will fly together increasingconsiderably their weight and required power. If all the modernization plans are thus realized,GLONASS will transmit FDMA signals in the current L1 and L2 GLONASS FDMA bands,together with the new FDMA signals in L3. In addition, CDMA open signals will also beprovided in E1/L1 and L5. It is important to note that as shown in Table 2.2, the currentGLONASS FDMA L1 band and the GPS and Galileo L1 band do not coincide.
However, thenew GLONASS CDMA signals will be allocated in the same E1/L1 band and the sameE5a/L5 band as Galileo and GPS. This implies that GLONASS will transmit 5 bands in total,what is a considerable effort.41Global Navigation Satellite Systems2.6Compass2.6.1Compass System OverviewCompass is the GNSS system planned by China [G.W. Hein et al., 2007a] and will consist ofa constellation of 30 Non-GEOstationary (3 IGSOs and 27 MEOs) satellites and 5GEOstationary satellites with positions at 58.75º E, 80º E, 110.5º E, 140º E and 160º E.
Eachsatellite transmits the same four carrier frequencies for navigational signals, where B1 andB1-2 are counted as separate bands. These navigational signals are modulated with apredetermined bit stream, containing coded ephemeris data and time.China has sent three Compass navigation test satellites into orbit between 2000 and 2003. Thelaunch of the two Beidou (Compass first version) satellites early 2007 is expected to coverChina and parts of neighbouring countries by 2008, before expanding into a global system.The Compass satellites will be developed, manufactured, and launched by the ChinaAerospace Science and Technology Corporation (CASTC).The will of China to develop its own global navigation system is clearly reflected in thepolicy document released by the State Council Information Office of October 12th, 2006.
Hereit was stated that China will independently develop application technologies and products inapplying satellite navigation, positioning and timing services. Compass could begin operationin 2012 if the political statements are brought into reality.Although the intentions and services that Compass will provide are still unclear, it seems thatCompass will offer two levels of services: free open and commercial services for users inChina, and licensed service for the military.
As in GPS, Galileo and GLONASS, the licensedservice will be more accurate than the free service and might be used for communication too.In October 2003 the Galileo Joint Undertaking (GJU) signed a cooperation agreement withChina. The National Remote Sensing Centre of China (NRSCC) was designated as theEuropean Union’s Chinese partner on the Galileo Project.2.6.2Compass Signal PlanThe current frequency filings [Compass ITU Filing] for radio bands made by China to theInternational Telecommunications Union (ITU) indicate that it would overlay both the GalileoPublic Regulated Service (PRS) and the military GPS M-code at E1/L1, as well as in the L2band. Given the importance of these protected signals and bands, we describe next in detailthe intended signal plan of Compass.
Since the use that China plans to do with Compass isstill unclear, the spectra will be plotted in the next figures in grey to underline this fact.42Global Navigation Satellite Systems2.6.2.1Compass B1 BandAlthough not all the technical aspects of the Compass signals in B1 are defined yet, anenvisaged signal waveform has already been submitted to the ITU [Compass ITU Filing].Next figure shows the spectral details of the studied option in the B1 and B1-2 bands.Figure 2.29.
Spectra of Compass Signals in B1 and B1-2 bandsAs also done in previous chapters, in order to have a better overview of all the GNSS signalsaround the Compass B1 and B1-2 bands, next figure depicts the spectral environment:Figure 2.30. Spectra of GPS, Galileo, GLONASS (Option 2) and Compass Signals in L1It is important to note that the GPS L1C pilot and data signals are shown in quadrature in thefigure although according to [GPS ICD-800, 2006] the final phasing is still to be decided.
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