On Generalized Signal Waveforms for Satellite Navigation (797942), страница 10
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This results in the energy used in transmitting the information ofthe baseband signal being spread over a wider bandwidth. Typically, the SS power level dropsbelow the RF noise floor, which makes the SS signal invisible for unauthorized users. Theratio (in dB) between the spread baseband and the original signal is called processing gain andtypical SS processing gains range from 10 dB to 60 dB [G.W. Hein et al., 2006c].8Global Navigation Satellite SystemsFor the particular case of the DSSS technique, the Pseudo Random Noise Code (PRN) isinserted at the data level.
How this is realized makes the difference between CDMA, FDMAand TDMA. In the case of GPS and Galileo, the pseudo-random sequence is mixed ormultiplied with the information signal (CDMA) while GLONASS employs differentfrequencies for each signal (FDMA).To facilitate the reading in the next chapters and given that the different existing and plannedGNSSes refer sometimes to different bands with the same notation and other times to thesame band with different names, the following table of correspondences is presented forclarification.
Furthermore, it is interesting to note that Galileo used for a long time L1 to referto the actual E1 band. In fact, only after [Galileo SIS ICD, 2008] the notation has changedfrom L1 to E1 to make it consistent with the rest of the bands.Table 2.2. Correspondence between frequency bands of different GNSSesSystemGPSGLONASSGalileoCompassFrequency BandL1L1E1B1Centre Frequency1575.42 MHz1602 MHz1575.42 MHz1561.098 MHz (B1)1589.742 MHz (B1-2)SystemGPSGLONASSGalileoCompassFrequency BandL2L2--Centre Frequency1227.60 MHz1246 MHz--SystemGPSGLONASSGalileoCompassFrequency BandL5-E5a-Centre Frequency1176.45 MHz-1176.45 MHz-SystemGPSGLONASSGalileoCompassFrequency Band--E6B3Centre Frequency--1278.75 MHz1268.52 MHzSystemGPSGLONASSGalileoCompassFrequency Band-L3E5bB2Centre Frequency-1201 MHz1207.14 MHz1207.14 MHzFinally, Figure 2.1 next shows the signal structure of all the existing and planned GNSSes.Negotiations among the involved countries are still needed to ensure compatibility (and tofulfil ITU regulations) and interoperability of the signals.
It is important to note that the GPSL1C pilot and data signals are shown in quadrature in the figure although according to[GPS ICD-800, 2006] the final phasing is still open.9Global Navigation Satellite SystemsFigure 2.1. GPS, GLONASS, Galileo, and planned Compass signals (Courtesy of Stefan Wallner)10Global Navigation Satellite Systems2.3The Global Positioning System (GPS)2.3.1GPS System OverviewGPS is made up of a network of initially 24 active satellites placed into orbit by the U.S.Department of Defense. Although originally developed for military applications, the U.S.government made GPS available to civilians, transforming it into the dual-use system it istoday. Accordingly, certain signal capabilities are reserved for U.S. and allied militaryapplications while the civilian signals are open and free for worldwide use.The GPS baseline constellation consists of 24 satellites (21 + 3 active spares) in six orbitalpositioned circular MEO planes at a nominal average orbit semi-major axis of 26559.7 km,and at an inclination of the orbital planes of 55 degrees with reference to the equatorial plane.The first developmental satellites were launched beginning in 1978, and the first operationalsatellites went on orbit in 1989.
The system reached initial operational capability (IOC) in1993 and FOC, in 1995. The present GPS constellation exceeds the baseline constellationwith 32 orbiting satellites after the last successful launch was on March 15th, 2008. Thehistory of all GPS launches can be seen in Figure 2.2.Figure 2.2. Launch History of GPS11Global Navigation Satellite Systems2.3.2GPS Signal Plan2.3.2.1GPS L1 BandThe GPS L1 band (1575.42 MHz) has turned to be the most important band for navigationpurposes.
Indeed most of the applications in the world nowadays are based on the signalstransmitted at this frequency. As stated in [GPS ICD 200], three signals are transmitted at themoment by GPS in L1: C/A Code, P(Y) Code and M-Code. In the future, an additional newcivil signal, known as L1C, will also be transmitted. We describe all of them in the next lines:•The Coarse/Acquisition (C/A) code signal was primarily thought for acquisition of theP (or Y) code and has become nowadays the most important signal for mass marketapplications. The PRN C/A Code for SV ID number i is a Gold code, Gi(t), of 1millisecond in length at a chipping rate of 1.023 Mbps.
The Gi(t) sequence is a linearpattern generated by the Modulo-2 addition of two subsequences, G1 and G2i, each ofthem being a 1023 chip long linear pattern. The epochs of the Gold code aresynchronized with the X1 epochs of the P-code.•The P Code is the precision signal and is coded by the precision code.
Moreover theY-Code is used in place of the P-code whenever the Anti-Spoofing (AS) mode ofoperation is activated as described in the ICDs 203, 224 and 225. The PRN P-code forSV number i is a ranging code, Pi(t), 7 days long at a chipping rate of 10.23 Mbps.The 7 day sequence is the Modulo-2 sum of two sub-sequences referred to as X1 andX2i with 15,345,000 chips and 15,345,037 chips, respectively. The X2i sequence is anX2 sequence selectively delayed by 1 to 37 chips allowing the basic code generationtechnique to produce a set of 37 mutually exclusive P-code sequences 7 days long.The modernized military signal (M-Code) is designed exclusively for military use andis intended to eventually replace the P(Y) code [E.
D. Kaplan and C. Hegarty, 2006].The M-Code provides better jamming resistance than the P(Y) signal, primarilythrough enabling transmission at much higher power without interference with C/Acode or P(Y) code receivers [B.C. Barker et al., 2000]. Moreover, the M-Codeprovides more robust signal acquisition than is achieved today, while offering bettersecurity in terms of exclusivity, authentication, and confidentiality, along withstreamlined key distribution. In other aspects, the M-Code signal provides much betterperformance than the P(Y) Code and more flexibility.The L1 Civil signal (L1C), defined in the [GPS ICD-800, 2006], consists of two maincomponents; one denoted L1CP to represent the pilot signal, consisting of a timemultiplexing of BOC(1,1) and BOC(6,1), thus without any data message, and L1CD,with a pure BOC(1,1), for the data channel. This is spread by a ranging code andmodulated by a data message.
The pilot channel L1CP is also modulated by an SVunique overlay secondary code, L1CO.••12Global Navigation Satellite SystemsFor more details on the code generation refer to the [GPS ICD 200] and[GPS ICD-800, 2006]. Finally, the technical characteristics of the existing and planned GPSsignals in the L1 band are summarized in the following table.Table 2.3. GPS L1 signal technical characteristicsGNSS SystemGPSGPSGPSGPSService NameC/AL1CP(Y) CodeM-CodeCentre Frequency1575.42 MHz1575.42 MHz1575.42 MHz1575.42 MHzFrequency BandL1L1L1L1Access TechniqueCDMACDMACDMACDMASignal ComponentDataDataN.A.ModulationBPSK(1)BPSK(10)BOCsin(10,5)Sub-carrierfrequency [MHz]--10.23Code frequency1.023 MHz1.023 MHz10.23 MHz5.115 MHzPrimary PRNCode length1023102306.19·1012N.A.Weil CodesCombinationand shortcycling of MsequencesN.A.DataPilotTMBOC(6,1,1/11)1.023 &6.1381.023Code FamilyGold CodesSecondary PRNCode length--1800-N.A.Data rate50 bps /50 sps50 bps /100 sps-50 bps /50 spsN.A.Minimum ReceivedPower [dBW]-158.5-157-161.5N.A.Elevation5°5°5°5°Of all the signals above, the C/A Code is the best known as most of the receivers that havebeen built until today are based on it.
The C/A Code was open from the very beginning to allusers, although until May 1st, 2000 an artificial degradation was introduced by means of theSelect Availability (SA) mechanism which added an intentional distortion to degrade thepositioning quality of the signal to non-desired users. As we have already mentioned, theC/A Code was thought to be an aid for the P(Y) Code (to realize a Coarse Acquisition). TheM-Code is the last military signal that has been introduced in GPS.For a long time different signal structures for the M-Code were under consideration[J.W. Betz, 2001] being the Manchester code signals (BPSK) and the binary offset carrier13Global Navigation Satellite Systems(BOC) signals the two favoured candidates. Both solutions result from the modulation of anon-return to zero (NRZ) pseudo random noise spreading code by a square-wave sub-carrier.While the Manchester code has a spreading code of rate equal to that of the square-wave, theBOC signal does not necessarily have to be so, being the only constraint that the rate of thespreading code must be less than the sub-carrier frequency.The interesting aspect about these signals is that, like the conventional sub-carrier modulation,the waveform presents a zero at the carrier frequency due to the square-wave sub-carrier.
Infact, their split-power spectra clearly facilitate the compatibility of the GPS military M-Codesignal with the existing C/A Code and P(Y) Code.Figure 2.3. Spectra of GPS Signals in L1It is important to note that although the GPS L1C pilot and data signals are shown inquadrature in the figure above, according to [GPS ICD-800, 2006] the final phasing is still tobe decided.
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