Advanced global navigation satellite system receiver design (797918), страница 8
Текст из файла (страница 8)
Thenew civil signal on the L2 carrier, akin to the C/A code signal, uses BPSK-R(1)modulation. However, there are subtleties in the generation of the PRN codes forwhich the L2C signal yields unique distinction.The L2C signal combines two PRN codes of different lengths, the moderate length(20msec) CM code and the long (1.5sec) CL code. The L2(C) signal generationscheme is shown in Figure 3-10.
Each code is generated at a rate of 511.5kchip/s.The CM is modulated by data at 50 sps (symbols per second), which is half-rateForward Error Correction (FEC) coded. The CL code has no data modulation. TheCM and CL code are then multiplexed together on a chip by chip basis to provide acombined chip rate of 1.023 Mcps.41Signal characteristicsNavigationalmessage1.023MHz1/225bps½ rateFEC10,230 chipcode generator767,250 chipcode generator50spsL2(C) codeCM codeChip by chipmultiplexerCL codeFigure 3-10, L2(C) code signal generationA code or channel with no data modulation is known as a ‘pilot’, this is an importantdesign feature and is incorporated many times into the modernised GPS and Galileosignal structures. A pilot code or channel allows GNSS receivers to use true PhaseLocked Loop (PLL) discriminators, providing more accurate tracking than the Costasloop discriminators, which are commonly used in existing GNSS receivers.
The pilottones also allow longer integration periods to be easily implemented in the receiverfor weak-signal acquisition and tracking.Combining three signals onto a single carrier frequency requires an additionalmultiplexing or modulation scheme and can impact on signal attributes such as powerand constant envelope properties. The phase II Replacement Modernised (IIR-M)GPS satellites will use a modulation named ‘interplex’ or ‘modified hexaphase’ tocombine the three signals onto both the L1 and L2 carrier waves [GPS ICD 2007].
InSection 7.1, we describe how this modulation can be produced in hardware anddemonstrate an elegant modulation architecture. The impact of interplex modulationon receiver design is discussed in detail in Section 10.3. However, for completenessthe current baseline for the future GPS L1 and L2 signals after interplexing can bewritten as follows [Dafesh et al 2000].S FL1 (t ) =+[[](t ) sin(m)]sin(ω3–112 PI d P (t )a Pi (t ) cos(m) − 2 PQ d M (t ) s (t )a Mi (t ) sin(m) cos(ω L1t )2 PQ d CA (t )aCAi (t ) cos(m) + 2 PI s (t )aCAi (t )a Pi (t )aMit)L142Signal characteristicsS FL 2 (t ) =+[[]3–122 PI d P (t )a Pi (t ) cos(m) − 2 PQ d M (t ) s (t )aMi (t ) sin(m) cos(ω L 2t )]2 PQ d C (t )aCi (t ) cos(m) + 2 PI s (t )aCi (t )a Pi (t )aMi (t ) sin( m) sin(ω L 2t )PI and PQ are the total powers in the in-phase and quadrature channels beforeinterplex modulation, which are set by the choice of modulation index m, s(t) is thesquare wave sub-carrier for BOC modulation with a frequency of 10.23MHz, aCi(t) isthe combination of the L2 CM and CL codes, and aMi(t) is the classified PRNsequence for M code signal.
dM(t), is the bi-phase navigational data signal associatedwith the military M code signal.The new GPS L5 signal potentially offers the most significant benefit to civil GPSusers, primarily through the delivery of a dual frequency civil system with highprecision measurements.The GPS L5 signal is a complex signal consisting of an in-phase carrier modulated bya tiered code spreading sequence (see Appendix A) with navigational data and a dataless pilot channel in phase quadrature modulated only by a tiered code spreadingsequence. The GPS L5 signal can be represented as followsS L 5 (t ) = 2 PL 5 d L 5 (t )a I 5i (t ) cos(ω L 5t ) + 2 PL 5 aQ 5i (t ) sin(ω L 5t )3–13Where, ω L 5 is the L5 carrier frequency, PL5 is the L5 signal power, aI5i and aQ5i are thetiered PRN code sequences for the in-phase and quadrature channels respectively.dL5(t), is the bi-phase navigational data signal associated with the civil L5 signal.43Signal characteristics60GPS L5 signalPower spectral density (dBW/Hz)Carrier Frequency = 1176.45 MHz70PSK10( fP ) 8090100201510505101520fPFrequency offset from carrier (MHz)Figure 3-11, Power spectral density of future L5 GPS signal (unfiltered, 1W signal power)3.3Galileo signal characteristicsUnder the current plans [ESA and GJU 2006] the new European Galileo GNSS willprovide a wide range of radionavigation signals to both civil and regulated users.
Thecurrent baseline of Galileo signals is believed to be as shown in Table 3-3. Galileowill introduce six data-modulated signals and four data-less signals into the L-bandspectrum, which are to provide the core Galileo services as follows [Dutton et al2002]. Open Service (OS) – This service which provides position, velocity and time(PVT) information, will be available on two frequencies, unencrypted and freeof charge. Commercial Service (CS) – This service will be a controlled access service forprofessional applications with service guarantees.
CS provides two signalswith encrypted data messages at different frequencies to OS broadcasting. Safety of Life (SoL) – This service is targeted at users where navigation safetyis critical (maritime, aviation, trains) and high level global performance isrequired. SoL signals will be broadcast on three carrier frequencies, one ofwhich is separated in frequency from all other services. Public Regulated Service (PRS) – This service is designed to provide a highlevel of data protection in so-called ‘times of extreme tension’. PRS is to be44Signal characteristicsbroadcast on two frequencies and will be heavily encrypted.
Access to thePRS will be controlled through EU member states.Table 3-3, Galileo signals [ESA and GJU 2006]SignalCentreModulationMain lobeTypes ofData ratefrequencyschemebandwidthservice(sps)(MHz)(MHz)E1 A1575.42BOC(15, 2.5)35.805PRSE1 B1575.42BOC(1,1) or4.092OS, SoL, CS4.092Data-less250MBOC(6,1,1/11)E1 C1575.42BOC(1,1) orMBOC(6,1,1/11)E5a I1191.795AltBOC(15, 10)51.15OSE5a Q1191.795AltBOC(15, 10)51.15Data-lessE5b I1191.795AltBOC(15, 10)51.15OS, SoL, CSE5b Q1191.795AltBOC(15, 10)51.15Data-lessE6 A1278.75BOC(10, 5)30.69PRSE6 B1278.75BPSK-R(5)10.23CSE6 C1278.75BPSK-R(5)10.23Data-less502501000The Galileo codes are based on the tiered code concept, the primary and secondarycodes length are shown in Table 3-4.
The table also indicates whether or not thecodes are based on LFSR polynomials or memory stored binary sequences.45Signal characteristicsTable 3-4, Galileo codes [ESA and GJU 2006]SignalPrimary codeSecondaryCode epochGenerator Polynomiallength (chips)code length(ms)(octal)(chips)12E1 AUnknownUnknownUnknownUnknownUnknownE1 B409214Memory storedMemory storedE1 C409225100Memory storedMemory storedE5a I1023020204050350661E5a Q102301001004050350661E5b I10230446402151445E5b Q102301001006402143143E6 AUnknownUnknownUnknownUnknownUnknownE6 B511511UnknownUnknownE6 C1023050100UnknownUnknownFigure 3-12 and Figure 3-13 show the power spectral densities for the Galileo E1 andE6 bands respectively.
Both bands have two data modulated signals and one data-lesschannel, which are to be combined through interplex modulation and can berepresented as follows.S E1 (t ) =[]3–142 PI d E1B (t ) s E1B (t )a E1Bi (t ) cos(m) − 2 PQ s E1C (t )a E1Ci (t ) sin(m) cos(ω L1t ) 2 PQ d E1 A (t ) s E1 A (t )a E1 Ai (t ) cos(m) sin(ω L1t )++ 2 PI s E1 A (t ) s E1B (t ) s E1C (t )a E1 Ai (t )a E1Bi (t )a E1Ci (t ) sin(m)S E 6 (t ) =[]3–152 PI d E 6 B (t )a E 6 Bi (t ) cos(m) − 2 PQ a E 6Ci (t ) sin(m) cos(ω E 6t ) 2 PQ d E 6 A (t ) s E 6 A a E 6 Ai (t ) cos(m) sin(ω E 6t )++ 2 PI s E 6 A (t )a E 6 Ai (t )a E 6 Bi (t )a E 6Ci (t ) sin(m)ω E 6 is the E6 carrier frequency, s1A(t), s1B(t) and s6A(t) are square wave sub-carriers,aE1Ai(t), aE1Bi(t), aE1Ci(t), aE6Ai(t), aE6Bi(t) and aE6Ci(t) are the PRN sequences,including secondary codes for the E1 A, B and C signals and the E6 A, B and C46Signal characteristicssignals, respectively.
dE!B(t), dE!A(t), dE6B(t) and dE6A(t) are bi-phase navigational datasignals for the respective signals.60Power spectral density (dBW/Hz)Carrier Frequency = 1575.42 MHz70BOCs ( fP )MBOCs ( fP ) 80BOC15c( fP )90100201510505101520fPFrequency offset from carrier (MHz)Galileo E1 B+C, BOC(1,1)-sineGalileo E1 B+C, MBOC(6,1,1/11)Galileo E1 A, BOC(15,2.5)-cosineFigure 3-12, Power spectral density of the Galileo E1 signals (1W signal power)60Galileo E6 B+CGalileo E6 APower spectral density (dBW/Hz)Carrier Frequency = 1278.75 MHz70PSK5( fP )80BOCc( fP )90100201510505101520fPFrequency offset from carrier (MHz)Figure 3-13, Power spectral density of the Galileo E6 signals (1W signal power)47Signal characteristicsThe Galileo E5a and E5b bands will each transmit a data-modulated and a data-lesschannel.
These four signals are combined onto a single centre frequency, this isaccomplished though the use of Alternate BOC (AltBOC) modulation. This highbandwidth complex signal offers unprecedented code tracking accuracy and multipathmitigation properties. This generation of this signal is detailed in Section 7.1, receiverdesign for AltBOC signal is covered in Section 6.3. The Galileo E5a and E5b bandscan be represented as follows.s E 5 (t ) = 2 PE 5 × [a E 5 aI (t ) × d E 5a (t ) + a E 5bI (t ) × d E 5b (t )] × s (t ) × cos(ω E 5t )+ 2 P × [a (t ) × d (t ) − a (t ) × d (t )] × ~s (t ) × sin(ω t )E5E 5 aI[× [aE 5aE 5bIE 5b](t )]× s(t ) × sin(ω3–16E5+ 2 PE 5 × a E 5 aQ (t ) + a E 5bQ (t ) × ~s (t ) × cos(ω E 5t )− 2 PE 5E 5 aQ(t ) − a E 5bQE5t)ω E 5 is the E5 carrier frequency, s(t), is a 15.345MHz sine square wave sub-carrier,~s (t ) is a 15.345MHz cosine square wave sub-carrier, aE5aI(t), aE5aQ(t), aE5bI(t),aE5bQ(t), are the tiered code spreading sequences for the E5 band.
dE5a(t) and dE5b(t)are bi-phase navigational data signals for the respective signals. The power spectraldensity of the Galileo E5 band is shown in Figure 3-14 [Rebeyrol et al 2005].70Carrier frequency = 1191.795 MHzPower spectral density (dBW/Hz)7580AltBOCs ( fP ) 859095100402002040fPFrequency offset from carrier (MHz)Figure 3-14, Power spectral density of the Galileo E5 band (1W signal power)48Signal characteristicsThe predicted received power levels of the Galileo signals are shown in Table 3-5.Table 3-5, Received power levels of Galileo signals [ESA and GJU 2006]E1BSatellite TXpower,[PT]TX Antenna gain,[GTX](max pointingerror 14.3°)Required SV EIRP=[PT] + [GTX]Atmosphericlosses,[LA]Propagation loss,[LPR](R = 25236km)Receiving antennagain,[GRX]Minimum receivedpowerE1CE6BE6CE5a(I)15.66 dB15.85 dBE5aE5b(Q)(I)15.24 dB13.4 dB13.4 dB13.4 dB29.06 dBW29.25 dBW28.64 dBW0.5 dB0.5 dB0.5 dB185.56 dB183.75 dB183.140 dB0 dB0 dB-157 dBW-155 dBW-155 dBWE5b(Q)= [PT ] + [GTX ] − [LA ]− [LPO ] − [LPR ] + [GRX ]494PSK and BOC signalsIn this chapter, the advantages and disadvantages of BOC signals are discussed andcomparison is made with PSK systems.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
