On Generalized Signal Waveforms for Satellite Navigation (797942), страница 11
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Furthermore, we can clearly recognize that GPS L1C concentrates more power athigher frequencies – due to BOC(6,1) – in the pilot channel than in the data channel. This willbe described in detail in chapter 4.7.4Finally, it is important to note that for all the figures next the commonly used expressions forbandwidths in MHz must be understood as multiplied by the factor 1.023.
Thus BPSK(10)refers in reality to a BPSK signal with a chip rate of 10.23 MHz. This remains valid for all thebandwidths in this thesis, unless different stated.2.3.2.2GPS L2 BandGPS is transmitting in the L2 band (1227.60 MHz) a modernized civil signal known as L2Ctogether with the P(Y) Code and the M-Code. The P(Y) Code and M-Code were alreadydescribed shortly in the previous chapter and the properties and parameters are thus similar to14Global Navigation Satellite Systemsthose in the L1 band.
In addition, for Block IIR-M, IIF, and subsequent blocks of SVs, twoadditional PRN ranging codes will be transmitted. They are the L2 Civil Moderate (L2 CM)code and the L2 Civil Long (L2 CL) code. These two signals are time multiplexed so that theresulting chipping rate is double as high as that of each individual signal. We further describethem in the next lines more in detail:••L2 CM Code is transmitted in the IIR-M, IIF, and subsequent blocks. The PRNL2 CM Code for SV number i is a ranging code, CMi(t), which is 20 milliseconds inlength at a chipping rate of 511.5 Kbps. The epochs of the L2 CM Code aresynchronized with the X1 epochs of the P-code. The CMi(t) sequence is a linear patternwhich is short cycled every count period of 10,230 chips by resetting with a particularinitial state.
Furthermore, for Block IIR-M, the navigation data is also Modulo-2 addedto the L2 CM Code. It is interesting to note that the navigation data can be used in oneof two different data rates selectable by ground command:o D(t) with a data rate of 50 bpso D(t) with a symbol rate of 50 symbols per second (sps) which is obtained byencoding D(t) with a data rate of 25 bps coded in a rate 1/2 convolutional code.Finally, the resultant bit-train is combined with the L2 CL Code using timedivision multiplexing.L2 CL Code is transmitted in the IIR-M, IIF, and subsequent blocks.
The PRN L2 CLCode for SV number i is a ranging code, CLi(t), which is 1.5 seconds in length at achipping rate of 511.5 Kbps. The epochs of the L2 CL Code are synchronized with theX1 epochs of the P Code. The CLi(t) sequence is a linear pattern which is generatedusing the same code generator polynomial as of CMi(t). However, the CLi(t) sequenceis short cycled by resetting with an initial state every count period of 767,250 chips.Finally, it is important to note that the GPS L2 band will have a transition period from theC/A Code to L2C and mixed configurations could occur. Next figure shows the baseband L2signal generation scheme. As we can recognize, although the chipping rate of the L2 CM andL2 CL signals is of 511.5 Kbps individually, after the time multiplexing the composite signalresults in a stream of 1.023 MHz.Figure 2.4.
Modulation scheme for the GPS L2 Signals15Global Navigation Satellite SystemsThe technical characteristics of the GPS L2 signals are summarized next:Table 2.4. GPS L2 signal technical characteristicsGNSS SystemGPSGPSGPSGPSService NameL2 CML2 CLP(Y) CodeM-CodeCentre Frequency1227.60 MHz1227.60 MHz1227.60 MHz1227.60 MHzFrequency BandL2L2L2L2Access TechniqueCDMACDMACDMACDMABPSK(10)BOCsin(10,5)SpreadingmodulationBPSK(1) result of multiplexing 2streams at 511.5 kHzSub-carrierfrequency---10.23 MHzCode frequency511.5 kHz511.5 kHz10.23 MHz5.115 MHzDataPilotDataN.A.10,230(20 ms)767,250(1.5 seconds)6.19 x 1012N.A.Combinationand shortcycling of MsequencesN.A.SignalComponentPrimary PRNCode lengthCode FamilySecondary PRNCode lengthM-sequence from a maximalpolynomial of degree 27---N.A.IIF50 bps / 50 spsIIR-M-50 bps /50 spsN.A.MinimumReceived Power[dBW]II/IIA/IIR-164.5 dBWIIR-M-161.5 dBWIIF-161.5 dBWII/IIA/IIR-164.5 dBWIIR-M-161.4 dBWIIF-160.0 dBWN.A.Elevation5°5°5°Data rateAlso 25 bps50 sps with FECThe spectra of the different signals described in the preceding lines are shown in the nextfigure:16Global Navigation Satellite SystemsFigure 2.5.
Spectra of the GPS Signals in L22.3.2.3GPS L5 BandThe GPS L5 (1176.45 MHz) signal will be transmitted for the first time on board IIFsatellites. The GPS carriers of the L5 band are modulated by two bit trains in phasequadrature: the L5 data channel and the L5 pilot channel.
Moreover, two PRN ranging codesare transmitted on L5: the in-phase code (denoted as the I5-code) and the quadraphase code(denoted as the Q5-code). The PRN L5-codes for SV number i are independent, but timesynchronized ranging codes X Ii (t ) and X Qi (t ) , of 1 millisecond in length at a chipping rate of10.23 Mbps [GPS ICD-705, 2005]. For each code, the 1-millisecond sequences are themodulo-2 sum of two sub-sequences referred to as XA and XBi with lengths of 8,190 chips and8,191 chips respectively, which restart to generate the 10,230 chip code. The XBi sequence isselectively delayed, thereby allowing the basic code generation technique to produce thedifferent satellite codes.The generation scheme can be shown graphically as follows:Figure 2.6.
Modulation scheme for the GPS L5 signals17Global Navigation Satellite SystemsFor more details on L5, refer to [E. D. Kaplan and C. Hegarty, 2006]. The different signalspresent the following spectrum:Figure 2.7. Spectra of GPS Signals in L5To conclude, the technical characteristics of the GPS signals in L5 can be summarized asfollows:Table 2.5. GPS L5 signal technical characteristicsGNSS SystemGPSGPSService NameL5 IL5 QCentre Frequency1176.45 MHz1176.45 MHzFrequency BandL5L5Access TechniqueCDMACDMASpreading modulationBPSK(10)BPSK(10)Sub-carrier frequency--Code frequency10.23 MHz10.23 MHzSignal ComponentDataPilotPrimary PRN Code length1023010230Code FamilyCombination and short-cycling ofM-sequencesSecondary PRN Code length1020Data rate50 bps / 100 sps-Minimum Received Power [dBW]-157.9 dBW-157.9 dBWElevation5°5°18Global Navigation Satellite Systems2.3.3GPS ModernizationBefore December 2005 the basic GPS capability consisted of the Standard Positioning Service(SPS) provided by the C/A Code on the L1 frequency and the Precise Positioning Service(PPS) provided by the P(Y) Code on L1 and L2.
Although those services are of relativelygood quality, the United States envisaged modernizing the signals in order to improve thequality and protection of both civil and military users. To that objective, the following GPSModernization plan was proposed. This can be timely divided in the following three blocks:•Block IIR-M (Replenishment-Modernized) satellites. This generation of spacecrafthas introduced a second civil signal with improved services (L2C) and is planning toreach the 24-satellite FOC around 2012. Additionally, for military purposes themodernized M-Code — BOC(10,5) — will be placed on L1 and L2 signals. BlockIIR-M satellites also have antijam flex power capabilities for military needs.
Moredetails on the planned signal plan can be found in Figure 2.1. The first operationalIIR-M satellite was launched on December 16th, 2005 and the last onMarch 15th, 2008.•Block IIF (Follow-on) satellites. With the design of GPS Block IIF, the Americannavigation system has undergone an important transformation that has finallyredefined GPS from a military service with the guarantee of civil use to a true dualservice [P.A. Dafesh et al., 1999a]. Indeed, what started as a modest upgrade tointroduce a new civil frequency quickly evolved into a complement of new signals forenhanced military and civil use.
The culmination of this transformation will be GPSBlock III. The third civil signal (L5), namely BPSK(10), will begin with the IIFsatellites and the FOC with 24 satellites is expected to be reached around 2015.•Block III is still in the design phase. GPS Block III includes prospectiveimprovements to both the ground and space segments. These will most likely includeincreased anti-jam power, increased security, increased accuracy, navigation surety,backward compatibility, assured availability, system survivability, and controlledintegrity among other improvements.
The fourth civil signal (L1C) will also beintroduced with this block. In the 2004 GNSS cooperation Agreement between theUnited States and the European Union, the two parties agreed to have BOC(1,1) as thebaseline waveform on both the GPS L1C and Galileo E1 Open Service (OS) signals.Nevertheless, a group of experts from Working Group A set up under the 2004Agreement proposed to optimize this signal using MBOC(6,1,1/11) as shown in[G.W. Hein et al., 2006a] and [G.W. Hein et al., 2006b]. Although an earlier scheduleis under consideration, the first Block III satellite launch will probably occursomewhere around 2014 with FOC being reached by about 2020.19Global Navigation Satellite Systems2.4Galileo2.4.1Galileo System OverviewGalileo is the European global navigation satellite system (ENSS) and provides a highlyaccurate, guaranteed global positioning service under civilian control.
According to[Galileo SIS ICD, 2008], the system will be interoperable with GPS and — at least to someextent, excluding the real-time high-precision part — with GLONASS, the two global satellitenavigation systems available today.The fully deployed Galileo system will consist of 30 satellites (27 operational + 3 non-activespares), positioned in three circular MEO planes at a nominal average orbit semi-major axis of29,601.297 kilometres, and at an inclination of the orbital planes of 56 degrees with referenceto the equatorial plane. Once FOC is achieved, the Galileo navigation signals will providegood coverage even at latitudes up to 75 degrees north and 75 degrees south.
Galileo providesenhanced distress localization and call features for the provision of a Search And Rescue(SAR) service interoperable with the COSPAS-SARSAT system.Figure 2.8. Galileo Space Segment [Figure from ESA Website]The Galileo orbit altitude results in a repeat cycle of ten sidereal days during which eachsatellite completes seventeen revolutions. Indeed this repetition period of seventeenrevolutions was chosen because simulation analyses showed that with seventeen terms torepresent the series of the gravitational field satisfactory results were obtained.
In order toavoid the gravitational resonance associated with a 12 hour orbital period as it is the case ofGPS, the Galileo satellites will have an altitude approximately 3000 km higher than that ofGPS. The relatively short repeat period is convenient for mission planning purposes. Theconstellation lifetime is of 20 years, while the Galileo satellites have a design lifetime of 12years [R.
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