On Generalized Signal Waveforms for Satellite Navigation (797942), страница 18
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BeforeWAAS, the U.S. National Airspace System (NAS) did not have the capability of providinghorizontal and vertical navigation for aviation precision approach operations for all users at alllocations. WAAS is constituted by four geostationary satellites.59Global Navigation Satellite Systems2.9.1.2European Geostationary Navigation Overlay Service (EGNOS)The European Geostationary Navigation Overlay Service (EGNOS) is a satellite-basedaugmentation system (SBAS) under development by the European Space Agency (ESA), theEuropean Commission (EC), and EUROCONTROL. EGNOS supplements GPS by reportingon the reliability and accuracy of the signals.
It consists of three geostationary satellites(AOR-E, IOR-W and ARTEMIS) and a network of ground stations. The system started itsinitial operations in July 2005, and is intended to be certified for use in safety of lifeapplications in 2008.2.9.1.3MTSAT Space-based Augmentation System (MSAS)The Japanese equivalent to WAAS and EGNOS incorporates the Multifunctional TransportSatellite (MTSAT) into MSAS (MTSAT Space-based Augmentation System). In addition totransmitting correction and integrity data for GPS, the MTSAT satellites are used formeteorological observations and communication services following a multi-mission concept.After failing with the initial launch of the first MTSAT satellite in 1999 the substitute satelliteMTSAT-1R was set into orbit in February 2005. An additional satellite, MTSAT-2, was putinto mission in February 2006.2.9.1.4Russian Differential Correction and Monitoring (SDCM) SystemThe Russian System for Differential Correction and Monitoring (SDCM) is the Russianversion of SBAS.
It is being developed under the leading role of the Russian Institute ofSpace Device Engineering (RISDE) and will augment GLONASS, GPS and Galileo signals inE1/L1, providing real-time corrections for all the systems over the Russian territory from twogeostationary satellites. SDCM will use eight tracking stations within Russia, plus others onforeign ground yet to be decided, and will provide WAAS- and EGNOS-like capabilities overthe Russian airspace, monitoring the integrity and quality of the GLONASS, GPS and Galileonavigation services. The design of the system started in 2002 in accordance with the contractsigned with the Federal Space Agency of Russia. As announced during the Second Meeting ofthe International Committee on GNSS (ICG) celebrated in Bangalore, India, in 2007, thesystem is planned to be validated in 2010 and operational by 2011.2.9.1.5GPS and GEO Augmented Navigation system (GAGAN)The GPS and GEO Augmented Navigation system (GAGAN) is India’s SBAS for the SouthAsian region.
Established by the Indian Space and Research Organization (ISRO) and theAirports Authority of India to aid civil aviation in the country, GAGAN will expand intoIRNSS.60Global Navigation Satellite SystemsThe first geostationary navigation payload in the C-band and the L1 and L5 frequencies(L-band) will be carried on an Indian geostationary satellite, GSAT-4, placed at 82°E. Twomore satellites, GSAT8 and GSAT9 will follow it to complete the augmentation system, withFOC expected by 2009.2.9.1.6Nigerian Communications Satellite System (NIGCOMSAT)With its Communications Satellite (NIGCOMSAT-1), Nigeria is the first African country inplanning to enter the stage of GNSS Augmentation Systems by transmitting two L-bandaugmentation signals in L1 and L5.
The manufacturing of the satellite was assigned toChina’s state-owned space hardware manufacturer and is thus China’s first satellite exportsale [GAGAN, 2006d]. The satellite was launched by a Long March 3B carrier rocket at theXichang Satellite Launch Centre on May 14th, 2007. Two ground stations are going to be builtin Nigeria and in China. NIGCOMSAT-1 will be placed in a geostationary orbit at 42°E,although it is a suboptimal location for covering Nigeria.2.9.1.7Other Satellite Augmentation Navigation Systems2.9.1.7.1 Canadian Wide Area Augmentation System (CWAAS)The United States Wide Area Augmentation System (WAAS) that was described in chapter2.9.1.1 can only be partially used in southern Canada.
In order the full territory of Canada tohave too augmentation of the GNSS signals, the Canadian WAAS (CWAAS) is underdevelopment. First studies started as soon as mid 1990s.2.9.1.7.2 South American Satellite Augmentation System (CSTB)The Caribbean and South American Region have taken initial steps to establish a GNSSAugmentation Test Bed (CSTB) throughout the region to support and facilitate research,development, acquisition, and implementation efforts associated with an operational transitionto satellite navigation.2.9.1.7.3 Chinese Satellite Navigation Augmentation System (SNAS)Finally, also China plans its own satellite augmentation system.
The Chinese version receivesthe name of Satellite Navigation System (SNAS) and is under current development.The overall constellation parameters of those GNSS Augmentation Systems whosespecifications are well defined are shown next in Table 2.20.61Global Navigation Satellite SystemsTable 2.20. GNSS Augmentation Systems Constellation ParametersParameterWAASEGNOSSDCMMSASGAGANNIGCOMSATConstellationGEO(4)GEO(3)GEO(2)GEO(2)GEO(3)GEO(1)GEOLongitudes53° W98° W120° W178° W15.5° W25.0° E21.5° ETBD140° E145° E34° E83° E132° E42° ESemi-majoraxis (km)42,164.042,164.042,164.042,164.042,164.042,164.0Figure 2.44 next shows the coverage region of the satellite-based augmentation systemsvisible to users at elevations higher than 10 degrees.Figure 2.44.
Ground tracks of EGNOS, WAAS, MSAS, GAGAN and NIGCOMSAT[Satellite Tool Kit STK, 2006]2.9.2 Other Augmentation systems (GBAS, LAAS)The previous chapters dealt with the different regional and augmentation systems on asatellite basis. In this chapter we will shortly describe other approaches to generate rangingsignals from the ground, being the Ground Based Augmentation (GBAS) and the Local AreaAugmentation System (LAAS) the two most important exponents.62Global Navigation Satellite Systems2.9.2.1Ground Based Regional Augmentation System (GRAS)The Ground-based Regional Augmentation System (GRAS) is the Australian system toprovide GNSS augmentation from the ground. The system is complementary to any of theSatellite Based Augmentation Systems (SBAS) that we saw above and also complements theGround Based Augmentation Systems (GBAS) in general.The user receives information directly from ground based transmitters allowing thuscontinuous reception of the service over a large geographical area of approximately 200nautical miles.
GRAS supports GNSS operations in all the phases of flight including en-route,terminal and instrument approach. GRAS is made up of multiple ground stations withoverlapping coverage.Regarding the operational requirements, The GRAS SARPs (Standards and RecommendedPractices) have been submitted to ICAO for acceptance and amendment in the ICAOAnnexure-10 volume I.2.9.2.2Local Area Augmentation System (LAAS)The Local Area Augmentation System (LAAS) is a ground-based augmentation to GPS thatfocuses its service on the airport area (approximately a 20-30 mile radius) for precisionapproach, departure procedures and terminal area operations.LAAS broadcasts its correction message via a very high frequency (VHF) radio data linkfrom a ground-based transmitter. LAAS is expected to yield the extremely high accuracy,availability, and integrity necessary for Category I, II, and III precision approaches, and willprovide the ability for flexible, curved approach paths.
LAAS demonstrated accuracy is lessthan 1 meter in both the horizontal and vertical axis.Figure 2.45. Typical Architecture of LAAS [Federal Aviation Administration FAA]63Global Navigation Satellite Systems2.10 PseudolitesA pseudolite (or pseudo-satellite) is a ground-based transmitter that broadcasts GNSS likesignals [T.G. Morley, 1997]. The concept of a terrestrial GNSS transmitter is actually quitemature. In fact, as early as 1977, the Inverted Range, a GPS user equipment test facilitylocated at the Yuma Proving Ground, used four ground transmitters (GTs) to augment thelimited number of space-borne GPS satellites during initial GPS trials. Since then, interest inaugmenting GPS with pseudolites (PLs) has grown steadily.
One of the first papers to suggestthe use of PLs for civilian aviation and maritime users was presented by[D. Klein and B.W. Parkinson, 1984].We can also find some modifications to this concept in what is known as synchrolites orsynchronized pseudolites, which derive their timing from individual GNSS satellites.Synchrolites are especially interesting for differential applications [H. S. Cobb, 1997].One of the clearest examples of the pseudolite concept even for experimentation is theEuropean GATE project.
As we could see some lines above, the European satellite navigationsystem Galileo is under development, with a planned operational availability around 2011.The development of Galileo based applications and products becomes thus a matter of majorimportance. In the region of Bavaria, Germany, the GATE project has been set up. GATE isthe German Galileo Test and Development Environment, a ground based, realistic test bedthat was developed to cover the needs of receiver manufacturers and application developers[GATE Testbed].
It must be noted that the GATE transmissions are continuous and notpulsed.Figure 2.46. German Galileo Test and Development Environment [GATE Testbed]64Galileo Baseline Evolution3. Galileo Baseline Evolution3.1IntroductionOnce we have taken a close look at the different existing and planned navigation systems, it istime to start analyzing one of the main elements of any navigation system: its signals. Ourobjective in the coming chapters and indeed in the whole thesis is to underline the problematicof developing optimum signal waveforms for navigation. With that in mind, the Galileo signaldesign and GPS modernization will be special focus of this work. Nonetheless, given theimportance that the Galileo programme has played in the last years, we dedicate this chapterto talk a little bit on the historical process that has lead to the Galileo Signal and FrequencyPlan that we have today.Galileo is an impressive technological achievement and one of the most important projectsthat are meant to unite the different countries of Europe.
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