On Generalized Signal Waveforms for Satellite Navigation (797942), страница 16
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Toconclude some technical characteristics of the Compass B1 signals are given next:43Global Navigation Satellite SystemsTable 2.13. Compass B1 signal characteristics [Compass ITU Filing]GNSS SystemCompassCompassCompassService NameB1 GSOB1 N-GSOB1GSO and N-GSOIPhaseQCentre FrequencyCarriers at 1561.098 MHz (B1)and 1589.742 MHz (B1-2)Frequency BandB1Access TechniqueCDMASpreading modulationQPSK(2)Sub-carrier frequency2.046 MHzCode frequency2.046 MHzSignal ComponentDataDataDataPrimary PRN Code length---Code Family---Secondary PRN Code length---Data rate500 bps50 bps500 bps-163 dBWMinimum Received Power [dBW]Elevation2.6.2.2---Compass B2 BandSimilar to the B1 and B1-2 bands, not all the technical aspects of the Compass signals in B2have been defined yet.
Nonetheless a proposed signal waveform has already been submittedto the ITU [Compass ITU Filing]. Next figure shows the spectral details of the studied option.Figure 2.31. Spectra of Compass Signals in the B2 band44Global Navigation Satellite SystemsAs also done for the rest of GNSS bands, we show in the next figure all the systems together.Figure 2.32. Spectra of Galileo and Compass signals in the E5 - B2 bandsTo conclude, some technical characteristics on the Compass B2 signals are presented more indetail in the next table:Table 2.14. Compass B2 signal technical characteristics [Compass ITU Filing]GNSS SystemCompassCompassCompassService NameB2 GSOB2 N-GSOB2GSO and N-GSOIPhaseQCentre Frequency1207.14 MHzFrequency BandB2Access TechniqueCDMASpreading modulationQPSKSub-carrier frequency2.046 MHzCode frequency10.23 MHzSignal ComponentDataDataDataPrimary PRN Code length---Code Family---Secondary PRN Code length---Data rate500 bps50 bps500 bps-163 dBWMinimum Received Power [dBW]Elevation---45Global Navigation Satellite Systems2.6.2.3Compass B3 BandFinally, the spectral characteristics of the Compass B3 signals are also shown here.
Similar tothe B1, B1-2 and B2 bands, not all the technical aspects of the Compass signals are definedyet. Next figure shows the Power Spectral densities of the proposed Compass signals in B3:Figure 2.33. Spectra of Compass Signals in the E6 - B3 bandIn order to have a better insight on how the Galileo E6 – Compass B3 band looks like, thefollowing figure presents all the planned signals together.Figure 2.34.
Spectra of Galileo and Compass Signals in the E6 - B3 bandTo conclude, some technical characteristics on the Compass B3 signals are provided next[Compass ITU Filing].46Global Navigation Satellite SystemsTable 2.15. Compass B3 signal technical characteristics [Compass ITU Filing]GNSS SystemCompassCompassCompassService NameB3 GSOB3 N-GSOB3GSO and N-GSOIPhaseQCentre Frequency1268.52 MHzFrequency BandB3Access TechniqueCDMASpreading modulationQPSK(10)Sub-carrier frequency10.23 MHzCode frequency10.23 MHzSignal ComponentDataDataDataPrimary PRN Code length---Code Family---Secondary PRN Code length---Data rate500 bps50 bps500 bps-163 dBWMinimum Received Power [dBW]Elevation2.7---Summary on Global Navigation SatelliteSystemsSatellite navigation has become a technology of great acceptance.
Every superpower wants tohave a satellite navigation system and preferably a global one. As suggested in[G.W. Hein et al., 2007a], the real need of having multiple systems is questionable at somepoint from an economic perspective.A very different issue, but one of great importance, is whether we can have so many systemscoexisting together without degrading the performance of one another. The interferencecaused on one system by the rest is technically difficult to measure, and especially if eachsystem would develop its own compatibility and interoperability concept without taking therest into account.
As shown in Appendix M, there are methodologies to assess interferencebut to what extent they accurately describe the real interference environment is a differentissue.In the 2004 Agreement on the Promotion, Provision, and Use of Galileo and GPS SatelliteBased Navigation Systems and Related Applications an interference and compatibilitymethodology was developed following ITU standards. The bilateral Agreement, however, isonly between Americans and Europeans.47Global Navigation Satellite SystemsA natural solution would be to develop general methodologies valid for all GNSS systems ona multilateral basis but then it would be difficult to find out who should or could beresponsible for coordinating these actions. It seems that the United Nations Office of OuterSpace Affairs or the ITU could play such a role, extending its efforts in sponsoring formationof the International Committee on GNSS (ICG). But transforming existing bilateralagreements into compatible multilateral fora is not an easy task.GPS and Galileo are compatible, and interoperable to a high degree, but they are not equal.Although important common actions have occurred in recent years, there is still a long way togo.
And the difficulties are compounded when we compare the evolution of both systems withthat of GLONASS or the planned Compass.No matter what the system designers do, the fact is that the user market will explode in thenext years. GNSS receivers will work better and almost everywhere, and the fusion with othercommunication devices is already on the threshold. International standards and certificationare urgently needed and although it is true that ICAO, ITU, RTCA and other for a alreadyprovide models for certification and regulation, the market forces are stronger and demandfaster reactions.2.8Regional Satellite Navigation SystemsIn addition to the global satellite-based navigation systems already under way, three regionalsatellite navigation systems are also being developed by Japan, India and China: namelyQZSS, IRNSS and Beidou respectively.
But before we describe them in detail together withall the augmentation systems that already exist or are planned to be set up in the comingyears, it is the right moment to make some reflexions on the need of regional andaugmentation systems if, as we saw in the lines above, in a not so far future four globalsystems might already be reality.As we have seen above, four global navigation systems might be operational in two decadesproviding thus an excellent coverage of most of the locations on earth. Today with GPS aloneas the only real operational system, an average of approximately 10 satellites can be seen atany point of the earth.
When GPS, GLONASS, Galileo, and Compass are in operation andassuming they would be fully interoperable, four times more satellites could be available fornavigation, positioning, and timing [G.W. Hein et al., 2007b].Locations with poor coverage today might not need any more a regional augmentation. As aresult, the added value of using regional systems when all the planned global systems wouldalready deliver good accuracy is questionable.48Global Navigation Satellite SystemsIn the framework of Galileo, different studies have been carried out in the past years to assessthe effect of increasing the number of satellites of an existing constellation as shown in[G.W.
Hein et al., 2006a]. Here the effect of doubling the number of satellites was studied. Interms of positioning accuracy, the improvement resulting from the better geometry is clear tosee in [G.W. Hein et al., 2006a]. Indeed, the step from GPS alone to Galileo + GPS representswith no doubt a clear gain for the final user. Nevertheless, once a pretty dense constellation ofsatellites is achieved, other measures such as the increase of power in the satellite would be ofmore profit to the final user. Let us think of a hypothetical scenario of 110 satellites asdescribed in [G.W. Hein et al., 2007b].Indeed, one might expect that the relative gain brought by 30 satellites when there alreadyexist 30 is higher than that of 30 additional satellites when there are already 60.
This isequivalent to saying that the marginal gain diminishes as the size of the constellation grows.Such a conclusion should not come as a surprise, because it reflects a well known economiclaw that applies to most goods and services in the world.Figure 2.35. Qualitative analysis of the expected marginal gain as a function of thenumber of GNSS satellitesThe four global satellite systems now in existence or under development will have around 110satellites altogether. As shown by [G.W. Hein et al., 2007b], it seems that the saturation levelin terms of geometry lies around this number.The problem gains even more multidimensionality if we recall the development of thesemiconductor industry in recent years. It is not so unrealistic to think that not too far in thefuture pseudolites could be a cheap product that anyone could place in locations with poorGNSS coverage.
These single-chip pseudolites (SCPL) could thus meet users’ positioningrequirements in areas where the satellites signals could not be received.Only time will tell if there will be a real need of regional and augmentation systems in thefuture. Nevertheless, since they are still an important component of today’s GNSS, we willpay them the attention they deserve in the next lines.49Global Navigation Satellite Systems2.8.1Quasi Zenith Satellite System(QZSS)2.8.1.1QZSS System OverviewQZSS is the Japanese regional system that will serve as enhancement for GPS in Japan.
Theconstellation consists of three satellites inclined in elliptic orbits with different orbital planesin order to pass over the same ground track. QZSS was designed so as to guarantee that at anytime at least one of its three satellites is close to the zenith over Japan.Initially, QZSS was conceived as a government – private sector program aiming for newsatellite business, in which the private sector would be responsible for mobilecommunications and mobile broadcasting while the government would be responsible for thenavigation part.
However, due to the lack of participation by the Japanese communicationindustry, the QZSS satellites will not carry any communication payloads but rather willconcentrate on the navigation element funded by the government alone.QZSS and GPS will be fully interoperable and the first satellite launch date is planned for theyear 2008.
Figure 2.36 below shows in detail the ground track of the three QZSS satellites.The ground tracks of the Indian Regional Navigation Satellite System (IRNSS) satellites, themodernization of GAGAN, are also shown. Next chapter will be dedicated to this regionalnavigation satellite system.The overall constellation parameters of both systems together are shown next in Table 2.16.Table 2.16. Space Constellation Parameters of QZSS and IRNSSParameterQZSSIRNSSConstellationGSO(3)GEO(3)+GSO(4)GEOLongitudes-34°, 83°, 132° EGSO EquatorialCrossing-55°(2), 112°(2)Eccentricity0.0990Inclination45°29°Semi-major axis42,164.0 km42,164.0 kmThe ground tracks of the two systems are graphically shown as follows:50Global Navigation Satellite SystemsFigure 2.36.
Ground Tracks of QZSS and IRNSS. This figure was generated using theSatellite Tool Kit (STK) [Satellite Tool Kit STK, 2006]Moreover, the Maximal Number of QZSS Satellites visible at a Minimum Elevation Angle of10° is shown in the next figure.Figure 2.37. Number of visible QZSS Satellites at Minimum Elevation Angle of 10°2.8.1.2QZSS Signal PlanQZSS and GPS have the highest level of interoperability among all the Satellite NavigationSystems as we will see in the following tables. In fact, the spectral properties are equivalent tothose that we saw in chapter 2.3.2 and therefore will not be shown here. The characteristics ofthe different signals in particular are summarized in detail in the following tables. For the caseof the L1 band the technical characteristics of the QZSS signals are presented next:51Global Navigation Satellite SystemsTable 2.17.
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