On Generalized Signal Waveforms for Satellite Navigation (797942), страница 20
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E2-L1-E1) L-band, data and chip rates were alsodefined as well as the Search and Rescue (SAR) up- and downlink frequencies.Furthermore, extensive interference considerations took place in E5a/E5b concerningDistance Measuring Equipment (DME), the Tactical Air Navigation System(TACAN) and the Galileo overlay on GPS L5; in E6 concerning the mutualinterference to/from radars and in E2-L1-E1 frequencies with regard to the Galileooverlay on GPS L1.In addition, by 2002 the EC Signal Task Force and ESA had refined criteria for the codeselection and had also formulated the requirements on each frequency.
Nonetheless, differentcode structures were still under investigation.It is also important to note that the Transport Council of the European Union in its meeting of25th/26th, March 2002, where the development phase of Galileo was finally decided,69Galileo Baseline Evolutionunderlined that compatibility and interoperability to GPS should be one of the key drivers forGalileo. With this signal plan, Galileo presented a good interoperability to GPS but still slightchanges would be required.
Next figure summarizes the characteristics of the resultingbaseline signal plan of 2002.Figure 3.2. Galileo Frequency plan of September 20023.4The Long Way to the AgreementAs we saw in the figure above, the signal plan of 2002 was already very mature in its designand with respect to today’s baseline only little changes can be observed, especially in the E1band, where a slight modification was needed to ensure compatibility between GPS andGalileo. The main changes are summarized next:•••E6: the PRS changed the phasing of the BOC(10,5) signal from sine to cosine.E1: The OS signals changed from BOC(2,2) to BOC(1,1) and the PRS moved fromBOCsin(14,2) to BOCcos(15,2.5) in order to fulfil the Agreement of 2004 as we willmention in the next lines.E5: AltBOC remains until the end as the wideband signal of E5.Let us analyze the changes that took place in a shorter time frame and with special attention tothe most troublesome band: the E1 band.70Galileo Baseline Evolution3.4.1Public Regulated Service in E1Following the guidelines set up by the Transport Council of the European Union, beginning of2004 the negotiations between the European Commission and the United States had clearlyintensified with the objective of reaching compatibility and interoperability.
At that moment,it was clear that Galileo would have to change its PRS signal from BOCsin(14,2) to anothersolution. In the preceding months different solution for the PRS had been thoroughlyassessed:•BPSK(5) at 1593.834 MHz as Public Regulated Service instead of BOCsin(14,2). Aswe can see in the following figure, such a signal would concentrate its PSDasymmetrically, allocating most on the power on the upper part of the E1 band.Figure 3.3. Signal Plan for E1 studied in 2004: PRS with BPSK(5)@1594 MHz•BOC(2.5,2.5) at 1593.834 MHz was another alternative that was object of analysis.Similar to the solution described above, the signal would concentrate all its power onthe upper part of the spectrum.Figure 3.4. Signal Plan for E1 studied in 2004: PRS with BOC(2.5,2.5)@1594 MHz71Galileo Baseline Evolution•BOCcos(15,2.5). This was indeed the solution that was found to be the optimum one.Its Power Spectral Density is shown in the next figure.Figure 3.5.
Signal Plan for E1 studied in 2004: PRS with BOCcos(15,2.5)3.4.2Open Service: BOC(2,2)BOC8(2,2) - BOC(1,1)-BOC(1.5,1.5)-While there was common agreement on both the American and European side that the PRSsignal had to change from BOCsin(14,2) to BOCcos(15,2.5) to preserve compatibility betweenGPS and Galileo, the Open Service signal and Civil signal of Galileo and GPS were stillobject of long discussions. Indeed, in Figure 3.5 above we can clearly recognize that althoughthe PRS corresponds to the actual signal waveform, other signals such as BOC(1.5,1.5) werealso being studied for the OS service. Some results on the performance of such solutions werealso presented by [M. Irsigler et al., 2005].
Additionally, other interesting solutions werebeing explored such as BOC8(2,2), also known as the 8-PSK BOC(2,2), to which we willdedicate more time in chapter 4.5.5.1.3.5Agreement of 2004: BOC(1,1)+BOCcos(15, 2.5)Finally, after many years of fruitful cooperation, the member states of the European Unionand the United States signed on June 26th, 2004, the Agreement on the Promotion, Provision,and Use of Galileo and GPS Satellite-Based Navigation Systems and Related Applications.With it, a new world of possibilities in satellite navigation opened.
The agreement fixedBOC(1,1) as the baseline for both Galileo and GPS future OS signals, but at the same timeopened the door to future possible implementations under the condition that they should havethe current baseline as the core of potential optimizations and that they would be compatiblewith both the GPS M-Code and Galileo PRS. The PRS was raised to the same category as theM-Code. Next figure shows the resulting baseline of 2004:72Galileo Baseline EvolutionFigure 3.6.
Galileo baseline after the Agreement of 2004We can also take a closer look at the L1 band to see more in detail the spectral environment ofthe band. It must be noted that RAS refers to the Radio-Astronomy Service.Figure 3.7. Galileo and GPS baseline in E1/L1 after the Agreement of 20043.6The Way to Today’s Baseline3.6.1Crazy BPSK, CBOC(5) and OthersShortly after the Agreement of 2004 was signed, experts from both sides of the Atlanticstarted to work together to find possible alternatives to the common BOC(1,1) modulationthat would clearly outperform the Open Service and Civil signals of the baseline and fulfil atthe same time the requirements of the Agreement.
Among the many solutions that wereinvestigated at that time, we underline the following:••MBOC(5) as the result of multiplexing BOC(1,1) and BOC(5,1)Crazy BPSK. This signal is a particular BCS sequence with 1.5 MHz chip rate and can73Galileo Baseline Evolution•be described as BCS([15x0,1,4x0],1.5). The notation [15x0,1,4x0] denotes that thecode chip is divided in 20 parts of equal length being the first 15 subchips logicalzeros (or physically -1), then comes a +1 and finally 4 zeros (-1) are placed at the end.For more details on the mathematical properties of these signals refer to chapter 4.3.Given its great similarity with a BPSK signal, but with an additional quick flip, thesignal was baptized as crazy BPSK.BCS signals with chip rates of 1.023 MHz or multiplexed versions with BOC(1,1).3.6.2Composite BCS (CBCS)As we underlined in the chapter above, although the Agreement fixed BOC(1,1) as thebaseline for both Galileo E1 Open Service and GPS future L1C signals, it also stated that theParties would work together toward achieving optimization of that modulation for theirrespective systems, within the constraints of the Agreement.
In September 2005[G.W. Hein et al., 2005], a sophisticated signal known as CBCS was presented by members ofthe Signal Task Force of the EC. This signal promised improvement of more than 40 % inmultipath performance with respect to BOC(1,1) under certain conditions. We will talk aboutit more in detail in chapter 4.6.CBCS was highly compatible with BOC(1,1) receivers and fulfilled to a high degree therequirements of the Agreement of 2004. Moreover, it offered an important improvement interms of performance.3.6.3Alternating Composite BCS (CBCS*)CBCS had some inconvenient properties that we will analyze more in detail in chapter 4.6.Among them, the existence of a tracking bias that could potentially appear due to the crosscorrelation between the CBCS signal and BOC(1,1) legacy receivers.
This problem could besolved in different manners being the most interesting that of alternating the BCS sequence.The resulting signal thus received the name CBCS* where the * refers to the phase-alternationof the BCS component.3.6.4MBOC(4,1)Shortly before MBOC(6,1) was selected, one more signal was intensely studied as potentialalternative to BOC(1,1). This signal was MBOC(4,1). MBOC(4,1) was the result ofmultiplexing BOC(1,1) and BOC(4,1) but due to its spectral properties it showed a lowerdegree of growth potential than MBOC(6,1) and was thus abandoned.74Galileo Baseline Evolution3.7MBOC(6,1)Finally, a joint design activity involving experts from the United States and Europe produceda recommended optimized spreading modulation for the L1C signal and the Galileo E1 OSsignal [MBOC Recommendation, 2006] and [GPS ICD-800, 2006]. The spreadingmodulation design places a small amount of additional power at higher frequencies in order toimprove signal tracking performance.
The signal was found to be satisfactory to both partiesand significantly improved BOC(1,1). More details on MBOC will be provided in chapter 4.7.Next picture shows the working group members that participated in the informal meeting ofMunich on March 9th, 2005 where MBOC was selected as candidate for the E1 Open Serviceand the L1 Civil Signal in L1.Figure 3.8. Members of the US/EU working group celebrate their agreement on arecommended common MBOC structure for GPS and Galileo L1 civil signals: from leftto right: Chris Hegarty, Tony Pratt, Jean-Luc Issler, John Owen,José-Ángel Ávila-Rodríguez, John W. Betz, Sean Lenahan, Stefan Wallner,and Günter W. Hein75Galileo Baseline EvolutionThe final frequency plan of Galileo is shown in the next figure in detail:Figure 3.9. Final Galileo Frequency Plan76GNSS Signal Structure4. GNSS Signal StructureOnce a general overview on all the current and planned navigation systems has beenprovided, we will deal in the next chapters with the task of finding general expressions todefine any navigation signal.4.1GNSS Modulation SchemesGalileo and GPS signals are generated using the well known Direct Sequence SpreadSpectrum (DSSS) technique.
DSSS is a particular case of Spread Spectrum (SS) technique.The SS principle seems simple and evident, but its implementation is indeed complex. Inorder to accomplish this objective, different SS techniques are available, but they all have onething in common: they perform the spreading and dispreading operation by means of a pseudorandom noise (PRN) code attached to the communication channel. The manner of insertingthis code into the transmitting chain before the antenna is actually what defines the particularSS technique, as next figure shows:Figure 4.1. SS techniques classification depending on the point in the system at whichthe PRN code is inserted in the communication channelAccording to this, a DSSS signal, s(t) can be represented as follows [C.J.
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