Title of Invention	A RECEIVER FOR USE WITH A GLOBAL NAVIGATION SATELLITE SYSTEM (GNSS) THAT TRANSMITS ALTERNATE BINARY OFFSET (ALTBOC) SIGNALS AND A METHOD OF DETERMINING GLOBAL POSITION FROM SAID ALTBOC SIGNALS RECEIVED FROM THE GNSS
Abstract	There is disclosed a receiver for use with a global navigation satellite system that transmits Alternate binary offset carrier, or AltBOC signals. The receiver includes a local composite code generator (24) for producing real and imaginary code components (Ipilot, Qpilot) of a local version of an AltBOC composite code obtained by combining locally produced codes (c3, c4) with real and imaginary components (cr, sr) of upper and lower subcarriers (er, er*) ; a correlation subsystem (22, 26) for producing correlation signals (Iprompt, Qprompt) resulting from correlating the locally produced composite code with the composite code in a received AltBOC signal by combining products produced by multiplying baseband inphase and quadrature components (Ibaseband, Qbaseband) of the received signal by the locally produced real and imaginary composite code components ; and a controller (40) for adjusting the local composite code generator to align the local composite code with the corresponding composite code in the received AltBOC signal based on the correlation signals, the controller comprising means for determining a global position based on timing differences between times the AltBOC composite codes are transmitted and the times the codes are received.

Title of Invention

A RECEIVER FOR USE WITH A GLOBAL NAVIGATION SATELLITE SYSTEM (GNSS) THAT TRANSMITS ALTERNATE BINARY OFFSET (ALTBOC) SIGNALS AND A METHOD OF DETERMINING GLOBAL POSITION FROM SAID ALTBOC SIGNALS RECEIVED FROM THE GNSS

Abstract

There is disclosed a receiver for use with a global navigation satellite system that transmits Alternate binary offset carrier, or AltBOC signals. The receiver includes a local composite code generator (24) for producing real and imaginary code components (Ipilot, Qpilot) of a local version of an AltBOC composite code obtained by combining locally produced codes (c3, c4) with real and imaginary components (cr, sr) of upper and lower subcarriers (er, er*) ; a correlation subsystem (22, 26) for producing correlation signals (Iprompt, Qprompt) resulting from correlating the locally produced composite code with the composite code in a received AltBOC signal by combining products produced by multiplying baseband inphase and quadrature components (Ibaseband, Qbaseband) of the received signal by the locally produced real and imaginary composite code components ; and a controller (40) for adjusting the local composite code generator to align the local composite code with the corresponding composite code in the received AltBOC signal based on the correlation signals, the controller comprising means for determining a global position based on timing differences between times the AltBOC composite codes are transmitted and the times the codes are received.

Full Text	A RECEIVER FOR USE WITH A GLOBAL NAVIGATION SATELLITE SYSTEM (GNSS) THAT TRANSMITS ALTERNATE BINARY OFFSET (AltBOC) SIGNALS AND A METHOD OF DETERMINING GLOBAL POSITION FROM SAID AltBOC SIGNALS RECEIVED FROM THE GNSS BACKGROUND OF THE INVENTION Field of the Invention The invention relates generally to GNSS receivers and, in particular, to receivers that operate with Galileo AltBOC satellite signals, and to a method of determining global position from said AltBOC signals received from the GNSS. Background Information Global navigation satellite system (GNSS) receivers, such as GPS receivers, determine their global positions based on the signals received from orbiting GPS and other satellites. The GPS satellites, for example, transmit signals using two carriers, namely, an LI carrier at 1575.42 MHz and an L2 carrier at 1227.60 MHz. Each carrier is modulated by at least a binary pseudorandom (PRN). code, which consists of a seemingly random sequence of ones and zeros that periodically repeat. The ones and zeros in the PRN code are referred to as "code chips," and the transitions in the code from one to zero or zero to one, which occur at "code chip times," are referred to as "bit transitions." Each GPS satellite uses a unique PRN code, and thus, a GPS receiver can associate a received signal with a particular satellite by determining which PRN code is included in the signal. The GPS receiver calculates the difference between the time a satellite transmits its signal and the time that the receiver receives the signal. The receiver then calculates its distance, or "pseudorange," from the satellite based on the associated time difference. Using the pseudoranges from at least four satellites, the receiver determines its global position. To determine the time difference, the GPS receiver synchronizes a locally generated PRN code with the PRN code in the received signal by aligning the code chips in each of the codes. The GPS receiver then determines how much the locally- generated PRN code is shifted, in time, from the known timing of the satellite PRN code at the time of transmission, and calculates the associated pseudorange. The more closely the GPS receiver aligns the locally-generated PRN code with the PRN code in the received signal, the more precisely the GPS receiver can determine the associated time difference and pseudorange and, in turn, its global position. The code synchronization operations include acquisition of the satellite PRN code and tracking the code. To acquire the PRN code, the GPS receiver generally makes a series of correlation measurements that are separated in time by a code chip. After acquisition, the GPS receiver tracks the received code. It generally makes "early- minus-late" correlation measurements, i.e., measurements of the difference between (i) a correlation measurement associated with the PRN code in the received signal and an early version of the locally-generated PRN code, and (ii) a correlation measurement associated with the PRN code in the received signal and a late version of the local PRN code. The GPS receiver then uses the early-minus-late measurements in a delay lock loop (DLL), which produces an error signal that is proportional to the misalignment between the local and the received PRN codes. The error signal is used, in turn, to control the PRN code generator, which shifts the local PRN code essentially to minimize the DLL error signal. The GPS receiver also typically aligns the satellite carrier with a local carrier using correlation measurements associated with a punctual version of the local PRN code. To do this the receiver uses a carrier tracking phase lock loop. A GPS receiver receives not only line-of-sight, or direct path, satellite signals but also multipath signals, which are signals that travel along different paths and are reflected to the receiver from the ground, bodies of water, nearby buildings, etc. The multipath signals arrive at the GPS receiver after the direct-path signal and combine with the direct-path signal to produce a distorted received signal. This distortion of the received signal adversely affects code synchronization operations because the correlation measurements, which measure the correlation between the local PRN code and the received signal, are based on the entire received signal - including the multipath components thereof. The distortion may be such that the GPS receiver attempts to synchronize to a multipath signal instead of to the direct-path signal. This is particularly true for multipath signals that have code bit transitions that occur close to the times at which code bit transitions occur in the direct-path signal. One way to more accurately synchronize the received and the locally-generated PRN codes is to use the "narrow correlators" discussed in United States Patents 5,101,416; 5,390,207 and 5,495,499, all of which are assigned to a common assignee and incorporated herein by reference. It has been determined that narrowing the delay spacing between early and late correlation measurements substantially reduces the adverse effects of noise and multipath signal distortion on the early-minus-late measurements. The delay spacing is narrowed such that the noise correlates in the early and late correlation measurements. Also, the narrow correlators are essentially spaced closer to a correlation peak that is associated with the punctual PRN code correlation measurements than the contributions of many of the multipath signals. Accordingly, the early-minus-late correlation measurements made by these correlators are significantly less distorted than they would be if they were made at a greater interval around the peak. The closer the correlators are placed to the correlation peak, the more the adverse effects of the multipath signals on the correlation measurements are minimized. The delay spacing can not, however, be made so narrow that the DLL can not lock to the satellite PRN code and then maintain code lock. Otherwise, the receiver cannot track the PRN code in the received signal without repeatedly taking the time to re-lock to the code. The L1 carrier is modulated by two PRN codes, namely, a 1.023 MHz C/A code and a 10.23 MHz P-code. The L2 carrier is modulated by the P-code. Generally, a GPS receiver constructed in accordance with the above-referenced patents acquires the satellite signal using a locally generated C/A code and a locally generated L1 carrier. After acquisition, the receiver synchronizes the locally generated C/A code and L1 carrier with the C/A code and L1 carrier in the received signal, using the narrow correlators in a DLL and a punctual correlator in the carrier tracking loop The receiver may then use the C/A code tracking information to track the L1 and/or L2 P-codes, which have known timing relationships with the C/A code, and with each other. In a newer generation of GPS satellites, the L2 carrier is also modulated by a C/A code that is, in turn, modulated by a 10.23 MHz square wave. The square wave modulated C/A code, which we refer to hereinafter as the "split C/A code," has maximums in its power spectrum at offsets of ±10 MHz from the L2 carrier, or in the nulls of the power spectrum of the P-code. The split C/A code can thus be selectively jammed, as necessary, without jamming the L2 P-code. The autocorrelation function associated with the split C/A code has an envelope that corresponds to the autocorrelation of the 1.023MHz C/A code and multiple peaks within the envelope that correspond to the autocorrelation of the 10.23 MHz square wave. There are thus 20 peaks within a two chip C/A code envelope, or a square wave autocorrelation peak every 0.1 C/A code chips. The multiple peaks associated with the square wave are each relatively narrow, and thus, offer increased code tracking accuracy, assuming the DLL tracks the correct narrow peak. As discussed in United States Patent 6,184, 822 which is assigned to a common Assignee and incorporated herein by reference, there are advantages to acquiring and tracking the split-C/A code by separately aligning with the received signal the phases of a locally-generated 10.23 MHz square wave, which can be thought of as a 20.46 MHz square-wave code, and a locally-generated 1.023 MHz C/A code. The receiver first aligns the phase of the locally generated square-wave code with the received signal, and tracks one of the multiple peaks of the split-C/A code autocorrelation function. It then shifts the phase of the locally-generated C/A code with respect to the phase of the locally-generated square-wave code, to align the local and the received C/A codes and position the correlators on the center peak of the split-C/A. The receiver then tracks the center peak directly, with a locally generated split-C/A code. The document "Analysis of L5/E5 Acquisition, Tracking and Data Demodulation Thresholds", F. Bastide, O. Julien, C. Macabiau, B. Roturier, in Proceedings of the Institute of Navigation (ION), GPS (24-09-2002), pages 2196- 2207 discloses a receiver adapted to demodulate QPSK-modulated GPS L5 signals (Quaternary Phase Shift Keying), or QPSK-modulated Galileo E5 A and E5B signals. The European Commission and the European Space Agency (ESA) are developing a GNSS known as Galileo. Galileo satellites will transmit signals in the E5aband (1176.45MHz) andE5b band (1207.14MHz) as a composite signal with a center frequency of 1195.795 MHz using a proposed modulation known as Alternate Binary Offset Canier(AltBOC). The generation of the AltBOC signal is described in the Galileo Signal Task Force document"Technical Annex to Galileo SRD Signal Plans", Draft 1,18 July 2001, ref # STF-annexSRD-2001/003, which is incorporated herein in its entirety by reference. Like the GPS satellites, the GNSS satellites each transmit unique PRN codes and a GNSS receiver can thus associate a received signal with a particular satellite. Accordingly, the GNSS receiver determines respective pseudoranges based on the difference between the times the satellites transmit the signals and then times the receiver receives the AltBOC signals. A standard binary offset carrier (BOC) modulates a time domain signal by a sine wave sin(wot), which shifts the frequency of the signal to both an upper sideband and a corresponding lower sideband. The BOC modulation accomplishes the frequency shift using a square wave, or sign(sin(wot)), and is generally denoted as BOC(fs,fc), where fs is the subcarrier (square wave) frequency and fc is the spreading code chipping rate. The factors of 1.023 MHz are usually omitted from the notation for clarity so a BOC(15.345MHz, 10.23 MHz) modulation is denoted BOC(15,10). The BOC modulation, which produces, for example, signals that are similar to the split C/A code discussed above, allows a single spreading, or PRN, code on each of the in-phase and quadrature carriers. The modulation of a time domain signal by a complex exponential ewot shifts the frequency of the signal to the upper sideband oniy. The goal of the AltBOC modulation is to generate in a coherent manner the E5a and E5b bands, which are respectively modulated by complex exponentials, or subcarriers, such that the signals can be received as a wideband "BOC-like signal." The E5a and E5b bands each have associated in-phase and quadrature spreading, or PRN, codes, with the E5a codes shifted to the lower sideband and the E5b codes shifted to the upper sideband. The respective E5a and E5b quadrature carriers are modulated by dataless pilot signals, and the respective in-phase carriers are modulated by both PRN codes and data signals. A GNSS receiver may track either the E5a codes or the E5b codes in a manner that is similar to the tracking of the split C/A code discussed above. There are, however, advantages in both multipath mitigation and tracking accuracy associated with tracking the composite E5a and E5b signals, that is, tracking the wideband AltBOC coherent signal. The respective inphase and quadrature carriers of the composite signal are modulated by complex spreading codes, and thus, the in- phase and quadrature channels each include contributions from both real and imaginary signal components of the E5a and E5b codes. Theoretical analyses of the composite tracking operations have been made using high level mathematics. Accordingly, the associated receivers, which essentially reproduce the high-level mathematical operations, are expected to be both complicated and costly One proposed receiver produces local versions of the AltBOC composite codes using the same look-up tables that the Galileo satellites use to generate the signals for transmission, that is, the tables that correspond to the underlying phase shift keying (PSK) spreading codes. The proposed receiver must thus not only maintain large look- up tables for each of the codes transmitted by the respective Galileo satellites, the receiver must also operate complex circuitry that controls entry to the look-up tables each time a new code chip is received. The tables are even larger and entering them more complicated when different pilot codes are used on the E5a and E5b bands, as is now contemplated. SUMMARY The invention is (a GNSS receiver that tracks the AltBOC (15,10), or composite ) E5a and E5b, codes (using hardware that locally generates the complex composite signal by combining separately generated real and the imaginary components of the complex signal.) To track the dataless composite pilot code signals that are on the quadrature channel of the AltBOC signal, for example, the receiver produces a local version of the composite pilot code as a combination of the locally generated real and imaginary pilot signal components. The receiver thus operates PRN code generators that produce replica E5a and E5b PRN codes and square wave generators that generate the real and imaginary components of the upper and lower subcarriers. The receiver removes the complex composite code from the received signal by multiplying the received signal, which has been downconverted to baseband I and Q signal components, by the locally generated complex composite code. The receiver then uses the results, which are correlated I and Q prompt signal values, to estimate the center frequency carrier phase angle tracking error. The error signal is used to control a numerically controlled oscillator that operates in a conventional manner, to correct the phase angle of the locally generated center frequency carrier. The receiver also uses early and late versions of the locally generated complex composite pilot code in a DLL, and aligns the locally generated composite pilot code with the received composite pilot code by minimizing the corresponding DLL error signal. Once the receiver is tracking the composite pilot code, the receiver determine its pseudorange and global position in a conventional manner Further, as discussed in more detail below, the receiver uses a separate set of correlators to align locally generated versions of the in-phase composite PRN codes with the in-phase channel codes in the received signal, and thereafter, recover the data that is modulated thereon. BRIEF DESCRIPTION OF THE DRAWINGS The invention description below refers to the accompanying drawings, of which: Fig. 1 depicts the frequency spectrum of the AltBOC (15,10) quadrature channel sequence; Fig. 2 depicts the normalized autocorrelation function associated with the signal depicted in Fig. 1; Fig. 3 is a functional block diagram of one channel for GNSS receiver; Fig. 4 is a functional block diagram of a local code generator that is included in the receiver of Fig. 3, Fig. 5 is a functional block diagram of a correlator subsystem that is included in the receiver of Fig. 4; Fig. 6 depicts an autocorrelation function associated with the AltBOC in phase channel; Fig. 7 is a chart of idealized autocorrelation values; Fig. 8 depicts another autocorrelation function associated with the AltBOC in phase channel; Fig. 9 is a functional block diagram of a local code generator; Fig. 10 is a functional block diagram of a correlator subsystem; and Fig. 11 is a functional block diagram that combines the correlator subsystems of Figs. 5 and 10. DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT The Galileo AltBOC modulation scheme generates an AltBOC (15,10) signal that is a "BOC(15,10)-like" signal with the E5a and E5b bands having their own respective spreading, or PRN, codes on their in-phase and quadrature carriers. The AltBOC (15,10) signal has a center carrier frequency of 1191.795 HMz and a subcarrier frequency of 15.345 MHz, with the E5a band (1176.45 MHz) as the lower sideband and the E5b band (1207.14 MHz) as the upper sideband. The AltBOC (15,10) signal is generated on the satellite as a constant envelope signal that includes on the in-phase channel a composite of the E5a and E5b spreading, or PRN, codes and data, on the quadrature channel and a composite of the E5a and E5b dataless PRN, or pilot, codes. Fig. 1 depicts the frequency spectrum of an AltBOC PRN quadrature channel sequence. The idealized normalized autocorrelation function for the AltBOC (15, 10) signal is shown in Fig. 2. The envelope 100 of the autocorrelation function 111 is the autocorrelation function of a 10.23 MHz chipping rate signal and the multiple peaks of the autocorrelation function 111 are associated with the 15.3454 MHz; subcarrier, which can be thought of as a complex square wave code. The operations of a GNSS receiver 10 in tracking the AltBOC (15, 10) Galileo satellite signals are discussed below. In Section 1, the operations for tracking the quadrature dataless pilot codes as a composite code are discussed. In Section 2 the operations for recovering the E5a and E5b data from the in-phase composite data codes are discussed. The discussion below assumes that the receiver has acquired the center frequency carrier using a conventional carrier tracking loop (not shown). Section 1. Tracking the Composite Pilot Code The AltBOC signal is given by: where c1 is the in-phase E5b code, c2 is the in-phase E5a code, c3 is the quadrature E5b code and c4 is the quadrature E5a code, and the E5b and E5a spreading codes are modulated, respectively, on the upper carrier er(t) and the lower carrier er(t), which is the complex conjugate of er(l). The upper carrier er(t) is: where cr(t) = sign(cos(2π fst)), sr(t) = sign(sin7(2π fst)) and fs is the subcarrier frequency. The lower carrier er(t) is. The composite pilot code, which is on the quadrature channel of the AltBOC (15,10) signal, includes the E5a and E5b quadrature codes c4(t) and c3(1:). An expression Xq(t) for the quadrature channel signal is derived from equation 1 by setting the in-phase channel codes to zero and substituting the expressions for the carriers: The receiver produces the local version of the complex composite pilot code by combining locally generated real and imaginary signal components, as discussed in more detail with reference to Fig. 4. The receiver then correlates the locally generated composite pilot code with the corresponding composite pilot code in the received signal, as discussed in more detail with reference to Fig. 5 below. The receiver then determines associated pseudoranges and its global position in a conventional manner. Referring now to Fig. 3, a GNSS receiver 10 receives over an antenna 12 a signal that includes the AltBOC composite codes transmitted by all of the satellites that are in view. The received signal is applied to a downconverter 14 that, in a conventional manner, converts the received signal to an intermediate frequency ("IF") signal that has a frequency which is compatible with an analog-to-digital converter 18. The IF signal is next applied to an IF bandpass filter 16 that has a bandpass at the desired center carrier frequency. The bandwidth of the filter 16 should be sufficiently wide to allow the primary harmonic of the AltBOC composite pilot code to pass, or approximately 1192 MHz. The wide bandwidth results in relatively sharp bit transitions in the received code, and thus, fairly well defined correlation peaks. The analog-to-digital converter 18 samples the filtered IF signal at a rate that satisfies the Nyquist theorem and produces corresponding digital inphase (I) and quadrature (Q) signal samples in a known manner. The 1 and Q digital signal samples are supplied to a Doppler removal processor 20 that operates in a known manner, to / produce baseband Ibaseband and Qbaseband samples by rotating the signals in accordance with an estimate of the center frequency carrier phase angle. The estimate of the carrier phase angle is based in part on the signals produced by a carrier numerically controlled oscillator ("carrier NCO") 30, which is adjusted in accordance with carrier phase error tracking signals produced by a correlation subsystem 22. The operations of the correlator subsystem are discussed below with reference to Fig. 5. The Ibaseband and Qbaseband samples are next supplied to the correlator subsystem 22, which makes correlation measurements by multiplying the samples by early, prompt and late or early minus late versions of locally generated composite pilot codes produced by a composite code generator 24. The operations of the composite code generator and the correlator subsystem are discussed below with reference to Figs. 4 and 5, respectfully. The I and Q correlation measurements associated with the early, prompt and late or early minus late versions of the local-composite pilot code are supplied to an integrate and dump circuit 26, which separately accumulates the respective I and Q measurements over predetermined intervals. At the end of each interval, the integrate and dump circuit 26 supplies the results of the respective I and Q accumulations, that is, the I and Q correlation signals, to a controller 40. The controller then controls the carrier NCO 30 and the composite code generator 24, to align the locally generated composite pilot code with the corresponding composite code in the received signal. The GNSS receiver 10 tracks the AltBOC (15, 10) signals using a locally generated composite pilot code that is generated from locally produced real and imaginary composite signal components. The operations performed by the composite code generator 24 to produce the locally generated composite pilot code components are now discussed in detail with reference to Fig. 4. The composite code generator 24 includes c3 and c4 PRN code generators 242 and 243 'that produce, respectively, local versions of the E5a and E5b PRN codes for a given GNSS satellite. The code generator 24 further includes two square wave generators 244 and 245 that produce values of cr and sr that correspond to the real and imaginary components of the upper and lower carriers er(t) and er(t). As discussed in more detail below with reference to Fig. 5, the controller uses the correlation signals to control the relative timing of the locally produced c3 and c4 code chips and the transitions of the cr and sr square waves, which can be thought of as respective code patterns of 0, 1,0...., and so forth. The composite code generator 24 adds together the c3 and c4 code chips in an adder 240 and multiplies the sum in multiplier 246 by the value of cr, which is sign(cos(πfst), to produce the real component of the locally generated composite pilot code. The real component of the composite pilot code is hereinafter referred to as "Ipilot" The composite code generator produces the imaginary component of the composite pilot code by inverting the c3 code chips in an inverter 248, adding the inverted c3 code chip to the corresponding c4 code chip in an adder 2.50 and, in multiplier 252, multiplying the result by sr, which is sign(sin(πfssf). The imaginary component of the locally generated composite pilot code is hereinafter referred to as "Qpilot" The local replica of the composite pilot code is then the sum: As discussed in more detail below, the correlator subsystem 22 multiplies the received composite pilot code by the locally generated composite pilot code. Based on the results, the controller 40 adjusts the PRN code and square wave generators 242-245 to align the local code to the received code. Referring now to Fig. 5, the operations of the correlator subsystem 22 are explained in terms of the operations involving the prompt version of the locally generated composite pilot code. The receiver includes similar circuits for early and late or early-minus-late versions of the locally generated composite pilot code that operate as part of a delay lock loop, or DLL, which operates in a known manner to produce an associated DLL error signal. The correlation subsystem 22 multiplies together two complex signals, namely, the locally generated composite pilot code and the received composite signal. The correlator subsystem thus performs the operation: As depicted in Fig. 5, the correlation subsystem manipulates the baseband signals Ibaseband and Qbaseband provided by the Doppler removal processor 20 and the locally-generated real and imaginary signal components Ipilot and Qpilot provided by the code generator 24, to produce the real and imaginary components of the correlation signal. The correlation subsystem 22 multiples the Ibaseband signal by the Ipilot signal in multiplier 502, and the Qbaseband signal by the Qpilot signal in multiplier 510. An adder 506 then adds the two products together and provides the result to an integrate and dump circuit 516. The integrate and dump circuit 516 accumulates the sums produced by the adder 506 and at appropriate times produces a corresponding real component, or Iprompt, signal. To produce the imaginary components, the correlation subsystem multiplies the Qbaseband signal by the Ipilot signal in a multiplier 508 and multiplies the Ibaseband signal by the Qpilot signal in a multiplier 504. The product produced by the multiplier 504 is inverted by inverter 512 and added to the product produced by the multiplier 508 in an adder 514. The adder 514 then supplies the sums to the integrate and dump circuit 518, which accumulates the sums and at appropriate times produces a corresponding Qprompt signal. The controller 40 (Fig. 3) manipulates the Iprompt and Qprompt signals, to determine the center carrier tracking phase error as the arctangent of QPrompt/IPrompt. The phase error signal is then used, in a known manner, to control the carrier NCO 30, which in turn controls the Doppler removal processor 20. As discussed above, the controller 40 also receives early and late or early- minus-late I and Q correlation signals. Based on these signals the controller 40 adjusts the generators 242-245 to align the local composite code in the received code and thus minimize the associated DLL error signal Section 2. Recovering the Data from the Composite In-Phase Signals The AltBOC (15, 10) signals include both data and spreading codes on the E5a in-phase and E5b in-phase channels The E5a in-phase channel will carry data that is transmitted at a particular data rate and the E5b in-phase channel will carry different data that is transmitted at a different data rate. The data transitions on the E5a and E5b in-phase channels will, however, occur at corresponding times. The GNSS receiver 10 acquires and tracks the AltBOC (15,10) signal using the composite pilot code, as discussed above. After the removal of the carrier, the receiver recovers the data from the composite in-phase signal using a separate set of correlators, as discussed in more detail below with reference to Fig. 10. The AItBOC(15, 10) complex in-phase baseband signal, that is, the signal with the quadrature pilot codes set to zero is: where the subcarrier is included in the expression as a sinusoid instead of the corresponding rectangular functions cr(t) ± sr(t) and assuming, for the moment, that c1(t) and c2(t) are free of data. Note that the term c1(t) •ej2xf + c2(t)•e-j2xf is similar to the expression for the quadrature spreading, or PRN, codes discussed in Section 1 above. If the baseband in-phase signal is correlated with a local replica of the composite in-phase spreading code, the result is. where Rk denotes the auto-correlation function for signal k. The cross-terms will be filtered over the predetection interval, particularly since the E5a and E5b in-phase spreading codes are designed to have low cross-correlation values. Removing the cross-terms and expanding the complex exponentials the expression becomes: In the case of datafree signals, the autocorrelation functions R1 and R2 will be equal and the expression simplifies to: The corresponding correlation function is depicted in Fig 6. Note that this is similar to the correlation function for the composite quadrature pilot code, which is shown in Fig. 2. If the assumption that the E5a and E5b in-phase codes c1(t) and c2(t) are datafree is removed, the maximum normalized ideal value of the individual correlation functions R1 and R2, and their sum and differences are shown in the table of Fig. 7. Accordingly, the original data sequence can be recovered if the receiver recovers both (R1+R2) and (R2-R1) from the composite in-phase signal, The receiver may recover the (R1+R2) data directly from the in-phase I prompt signal. However, the recovery of the (R2-R1) data is not as straight forward. As depicted in Fig. 6, the R1+R2 component of the in-phase composite signal, namely, (R1 (ι) + R2 (r)) • cos(2πfsτ) has an autocorrelation function that is similar to the autocorrelation function of the composite quadrature channel signal. However, as depicted in Fig. 8, the R1-R2 signal component of the in-phase composite signal j (R2 (τ) - R1 (τ)) sin(2πfτ) does not have a similar autocorrelation function because the sin term goes to zero when r goes to zero. To compensate for the sin term going to zero when x goes to zero, a correlation operation could be offset so that the operation tracks the correlation peak that corresponds to j (R2(τ) -R1 (τ)) sin(2πfτ) and the (R2-R1) data sequence may then be read from the Qprompt correlator, but with reduced power. Alternatively, the circuitry that generates the local version of the composite signal may instead generate a signal that produces for the (R2 -R1) term the autocorrelation function (R2 (τ) - Rx (τ)) cos(2πfsτ) The local in-phase composite signal thus becomes: To demodulate the data using this method, the receiver locally produces two combinations of the c1(t) and c2(t) spreading codes, namely, combinations that correspond, respectively, to the R1 +R2 data and the R2-R1 data To recover the (R1+R2) data the receiver produces the local signal: Referring now to Fig. 9, the real and imaginary components of the R2-R1 and R1 +R2 combinations are locally produced by a code generator 54, which may be part of the local composite code generator 24 (Fig. 3). To produce the real component of the R1 +R2 combination, the code generator adds together the c1 and c2 codes in an adder 540 and multiplies the result by the square wave code cr in a multiplier 546. To produce the imaginary component of the R1 +R2 combination, the code generator adds together the c1 code and an inverted c2 code in an adder 550 and multiples the result by the square wave code sr in a multiplier 562. The generator also multiplies the sum c1 + c2 produced by the adder 540 by the square wave code sr in a multiplier 560, to produce the imaginary component of the R2-R1 combination. The generator further multiplies the sum c1-c2 produced by the adder 550 by the square wave code cr in a multiplier 522, to produce the real component of the R2 -R1 code The receiver then uses the locally produced real and imaginary components of the R1 +R2 and R2-R1 combinations to recover the data from the composite in-phase code. Referring now to Fig. 10, the system multiplies the in-phase baseband signal Ibaseband by the real component of R1 +R2 in a multiplier 602, and the quadrature baseband signal Qbaseband by the imaginary component of R1 +R2 in a multiplier 606. The products produced by the multipliers 602 and 606 are then added together in an adder 608 and the sum provided to an integrate and dump circuit 615. To produce the correlation signal relating to R1-R2, the correlation subsystem multiplies the quadrature baseband signal Qbaseband by the imaginary component of R2-R1 in a multiplier 610. Further, the system multiplies the real component of the baseband signal Ibaseband by the real component of R2-R1, in a multiplier 604 code. The two sums are added together in an adder 612 and provided to an integrate and dump circuit 616. The integrate and dump circuits 615 and 616 accumulate the correlation values produced by the adders 608 and 612, respectively, and at appropriate times produce the R1 + R2prompt and R1- R2prompt signals. The results are then used to recover the data in accordance with the chart of Fig. 7. The circuitry of Figs. 5 and 10 can be combined, as depicted in Fig. 11, to produce a system that tracks the quadrature AltBOC composite code and recovers E5a and E5b data from the in phase AltBOC composite code. WE CLAIM : 1. A receiver for use with a global navigation satellite system that transmits Alternate binary offset carrier, or AltBOC signals, the receiver comprising : a local composite code generator (24) for producing real and imaginary code components (Ipilot, Qpilot) of a local version of an AltBOC composite code obtained by combining locally produced codes (c3, c4) with real and imaginary components (cr, sr) of upper and lower subcarriers (er, er); a correlation subsystem (22, 26) for producing correlation signals (Iprompt, Qprompt) resulting from correlating the locally produced composite code with the composite code in a received AltBOC signal by combining products produced by multiplying baseband inphase and quadrature components (Ibaseband, Qbaseband) of the received signal by the locally produced real and imaginary composite code components; and a controller (40) for adjusting the local composite code generator to align the local composite code with the corresponding composite code in the received AltBOC signal based on the correlation signals, the controller comprising means for determining a global position based on timing differences between times the AltBOC composite codes are transmitted and the times the codes are received. 2. The receiver as claimed in claim 1, wherein the local composite code generator (24) comprises : square wave code generators (244, 245) for producing real and imaginary components (cr, sr) of the upper and lower carriers (er, er), a first PRN code generator (243) for producing a first code (c3) that is modulated on the upper carrier (er), a second PRN code generator (242) tor producing a second code (c4) that is modulated on the lower carrier (er), and adders (240, 250) and multipliers (246, 248, 252) for combining the first and second codes (c3, c4) with the real and imaginary components (cr, sr) of the upper and lower carriers (er, er) to produce the real and imaginary components (Ipilot, Qpilot) of the local composite code. 3. The receiver as claimed in claim 1 or 2, wherein the local composite code generated (Ipilot, Qpilot) corresponds to a dataless composite pilot code. 4. The receiver as claimed in anyone of claims 1 to 3, wherein the local composite code generator (24) comprises means for producing combinations (ki+R2, QRI+R2, IR1-R2, QR1-R2) that correspond respectively to a sum and a difference of a first autocorrelation function (R1) associated with a third code (c1) and a second autocorrelation function (R2) associated with a fourth code (c2), the correlation subsystem (22) comprises means for producing combination correlation signals ((Rl+R2)prompt, (Rl-R2)prompt) that correspond to the respective combinations, and the controller (40) comprises means for recovering data from the combination correlation signals. 5. The receiver as claimed in anyone of claims 2 to 4, wherein the adders (240, 248, 250) comprise at least one inverter (248) for selectively inverting the codes (c3, c4). 6. The receiver as claimed in anyone of claims 3 to 5, wherein the local code generator comprises : a third PRN code generator (542) for producing a third code (c1) that is modulated on the upper carrier (er); a fourth PRN code generator (543) for producing a fourth code (c2) that is modulated on the lower carrier (er); adders (540, 548, 550) for combining the third and fourth codes to produce associated sums; and multipliers (546, 552, 560. 562) for multipling each sum separately by the first and second square waves (cr, sr) to produce real and imaginary components (IR1+R2, QR1+R2, IR1-R2, QR1-R2) of the respective associated combination correlation signals ((Rl+R2)prompt, (Rl-R2)prompt)). 7. A method of determining global position from Alternate binary offset carrier, or AltBOC signals received from a global navigation satellite system, the method comprising the steps of: producing real and imaginary code components (Ipilot, Qpilot) of a local version of an AltBOC composite code obtained by combining locally produced codes (c3, c4) with real and imaginary components (cr, sr) of upper and lower subcarriers (er, er) ; producing in-phase and quadrature components of the received AltBOC signal; correlating the locally produced composite code with the composite code in the received AltBOC signal by combining products produced by multiplying the baseband inphase and quadrature components (Ibaseband, Qbaseband) of the received AltBOC signal by the locally produced real and imaginary composite code components to produce associated correlation signals (Iprompt, Qprompt) ; adjusting the local composite code generator based on the correlation signals to align the local composite code with the corresponding composite code in the received AltBOC signal; and determining a global position based on timing differences between times the received AltBOC composite codes are transmitted and the times the codes are received. 8. The method as claimed in claim 7, wherein the step of producing the local version of the AltBOC composite code comprises steps of: producing square waves (cr, sr) that correspond to real and imaginary components of upper and lower carriers (er, er), producing a first code (c3) that is modulated on the upper carrier, producing a second code (c4) that is modulated on the lower carrier, and selectively combining the first and second codes and multiplying the results by the real and imaginary components of the upper and lower carriers to produce the real and imaginary components (Ipilot, Qpilot) of the local composite code. 9. The method as claimed in claim 8, wherein the step of selectively combining the first and second codes comprises the steps of producing first and second sums that are associated respectively with the real and imaginary components (Ipilot, Qpilot), the first sum corresponding to the addition of the second code (c4) to an inverted first code (-c3) and the second sum corresponding to the addition of the two codes. 10. The method as claimed in anyone of claims 7 to 9, wherein the local composite code generated (Ipilot, Qpilot) corresponds to a dataless composite pilot code. 11. The method as claimed in anyone of claims 7 to 10, comprising steps of: producing a third code (c1) that is modulated on the upper carrier, producing a fourth code (c2) that is modulated on the lower carrier, producing two combinations of the third and fourth codes with the first and second square waves to obtain real and imaginary components (IRI+R2, QR1+R2, IR1-R2, QR1-R2) of combinations R1 + R2 and R2 - Rl, where Rl is the autocorrelation function associated with the third code (cl), and R2 is the autocorrelation function associated with the fourth code (c2), correlating the combinations so as to obtain combination correlation signals ((Rl+R2)prompt, (Rl-R2)prompt) for each combination, and recovering data from the combination correlation signals. There is disclosed a receiver for use with a global navigation satellite system that transmits Alternate binary offset carrier, or AltBOC signals. The receiver includes a local composite code generator (24) for producing real and imaginary code components (Ipilot, Qpilot) of a local version of an AltBOC composite code obtained by combining locally produced codes (c3, c4) with real and imaginary components (cr, sr) of upper and lower subcarriers (er, er*) ; a correlation subsystem (22, 26) for producing correlation signals (Iprompt, Qprompt) resulting from correlating the locally produced composite code with the composite code in a received AltBOC signal by combining products produced by multiplying baseband inphase and quadrature components (Ibaseband, Qbaseband) of the received signal by the locally produced real and imaginary composite code components ; and a controller (40) for adjusting the local composite code generator to align the local composite code with the corresponding composite code in the received AltBOC signal based on the correlation signals, the controller comprising means for determining a global position based on timing differences between times the AltBOC composite codes are transmitted and the times the codes are received.

Full Text

A RECEIVER FOR USE WITH A GLOBAL NAVIGATION SATELLITE SYSTEM
(GNSS) THAT TRANSMITS ALTERNATE BINARY OFFSET (AltBOC) SIGNALS
AND A METHOD OF DETERMINING GLOBAL POSITION FROM SAID AltBOC
SIGNALS RECEIVED FROM THE GNSS
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates generally to GNSS receivers and, in particular, to receivers that operate
with Galileo AltBOC satellite signals, and to a method of determining global position from said
AltBOC signals received from the GNSS.
Background Information
Global navigation satellite system (GNSS) receivers, such as GPS receivers, determine
their global positions based on the signals received from orbiting GPS and other satellites. The
GPS satellites, for example, transmit signals using two carriers, namely, an LI carrier at
1575.42 MHz and an L2 carrier at 1227.60 MHz. Each carrier is modulated by at least a binary
pseudorandom (PRN). code, which consists of a seemingly random sequence of ones and zeros
that periodically repeat. The ones and zeros in the PRN code are referred to as "code chips,"
and the transitions in the code from one to zero or zero to one, which occur at "code chip
times," are referred to as "bit transitions." Each GPS satellite uses a unique PRN code, and
thus, a GPS receiver can associate a received signal with a particular satellite by determining
which PRN code is included in the signal.
The GPS receiver calculates the difference between the time a satellite transmits its signal
and the time that the receiver receives the signal. The receiver then calculates its distance, or
"pseudorange," from the satellite based on the associated time difference. Using the
pseudoranges from at least four satellites, the receiver determines its global position.
To determine the time difference, the GPS receiver synchronizes a locally
generated PRN code with the PRN code in the received signal by aligning the code
chips in each of the codes. The GPS receiver then determines how much the locally-
generated PRN code is shifted, in time, from the known timing of the satellite PRN

code at the time of transmission, and calculates the associated pseudorange. The more
closely the GPS receiver aligns the locally-generated PRN code with the PRN code in
the received signal, the more precisely the GPS receiver can determine the associated
time difference and pseudorange and, in turn, its global position.
The code synchronization operations include acquisition of the satellite PRN
code and tracking the code. To acquire the PRN code, the GPS receiver generally
makes a series of correlation measurements that are separated in time by a code chip.
After acquisition, the GPS receiver tracks the received code. It generally makes "early-
minus-late" correlation measurements, i.e., measurements of the difference between (i)
a correlation measurement associated with the PRN code in the received signal and an
early version of the locally-generated PRN code, and (ii) a correlation measurement
associated with the PRN code in the received signal and a late version of the local PRN
code. The GPS receiver then uses the early-minus-late measurements in a delay lock
loop (DLL), which produces an error signal that is proportional to the misalignment
between the local and the received PRN codes. The error signal is used, in turn, to
control the PRN code generator, which shifts the local PRN code essentially to
minimize the DLL error signal.
The GPS receiver also typically aligns the satellite carrier with a local carrier
using correlation measurements associated with a punctual version of the local PRN
code. To do this the receiver uses a carrier tracking phase lock loop.
A GPS receiver receives not only line-of-sight, or direct path, satellite signals
but also multipath signals, which are signals that travel along different paths and are
reflected to the receiver from the ground, bodies of water, nearby buildings, etc. The
multipath signals arrive at the GPS receiver after the direct-path signal and combine
with the direct-path signal to produce a distorted received signal. This distortion of the
received signal adversely affects code synchronization operations because the
correlation measurements, which measure the correlation between the local PRN code
and the received signal, are based on the entire received signal - including the multipath
components thereof. The distortion may be such that the GPS receiver attempts to
synchronize to a multipath signal instead of to the direct-path signal. This is
particularly true for multipath signals that have code bit transitions that occur close to
the times at which code bit transitions occur in the direct-path signal.

One way to more accurately synchronize the received and the locally-generated
PRN codes is to use the "narrow correlators" discussed in United States Patents
5,101,416; 5,390,207 and 5,495,499, all of which are assigned to a common assignee
and incorporated herein by reference. It has been determined that narrowing the delay
spacing between early and late correlation measurements substantially reduces the
adverse effects of noise and multipath signal distortion on the early-minus-late
measurements.
The delay spacing is narrowed such that the noise correlates in the early and late
correlation measurements. Also, the narrow correlators are essentially spaced closer to
a correlation peak that is associated with the punctual PRN code correlation
measurements than the contributions of many of the multipath signals. Accordingly,
the early-minus-late correlation measurements made by these correlators are
significantly less distorted than they would be if they were made at a greater interval
around the peak. The closer the correlators are placed to the correlation peak, the more
the adverse effects of the multipath signals on the correlation measurements are
minimized. The delay spacing can not, however, be made so narrow that the DLL can
not lock to the satellite PRN code and then maintain code lock. Otherwise, the receiver
cannot track the PRN code in the received signal without repeatedly taking the time to
re-lock to the code.
The L1 carrier is modulated by two PRN codes, namely, a 1.023 MHz C/A code
and a 10.23 MHz P-code. The L2 carrier is modulated by the P-code. Generally, a
GPS receiver constructed in accordance with the above-referenced patents acquires the
satellite signal using a locally generated C/A code and a locally generated L1 carrier.
After acquisition, the receiver synchronizes the locally generated C/A code and L1
carrier with the C/A code and L1 carrier in the received signal, using the narrow
correlators in a DLL and a punctual correlator in the carrier tracking loop The receiver
may then use the C/A code tracking information to track the L1 and/or L2 P-codes,
which have known timing relationships with the C/A code, and with each other.
In a newer generation of GPS satellites, the L2 carrier is also modulated by a
C/A code that is, in turn, modulated by a 10.23 MHz square wave. The square wave
modulated C/A code, which we refer to hereinafter as the "split C/A code," has

maximums in its power spectrum at offsets of ±10 MHz from the L2 carrier, or in
the nulls of the power spectrum of the P-code. The split C/A code can thus be
selectively jammed, as necessary, without jamming the L2 P-code.
The autocorrelation function associated with the split C/A code has an
envelope that corresponds to the autocorrelation of the 1.023MHz C/A code and
multiple peaks within the envelope that correspond to the autocorrelation of the
10.23 MHz square wave. There are thus 20 peaks within a two chip C/A code
envelope, or a square wave autocorrelation peak every 0.1 C/A code chips. The
multiple peaks associated with the square wave are each relatively narrow, and thus,
offer increased code tracking accuracy, assuming the DLL tracks the correct narrow
peak.
As discussed in United States Patent 6,184, 822 which is assigned to a
common Assignee and incorporated herein by reference, there are advantages to
acquiring and tracking the split-C/A code by separately aligning with the received
signal the phases of a locally-generated 10.23 MHz square wave, which can be
thought of as a 20.46 MHz square-wave code, and a locally-generated 1.023 MHz
C/A code. The receiver first aligns the phase of the locally generated square-wave
code with the received signal, and tracks one of the multiple peaks of the split-C/A
code autocorrelation function. It then shifts the phase of the locally-generated C/A
code with respect to the phase of the locally-generated square-wave code, to align the
local and the received C/A codes and position the correlators on the center peak of
the split-C/A. The receiver then tracks the center peak directly, with a locally
generated split-C/A code.
The document "Analysis of L5/E5 Acquisition, Tracking and Data
Demodulation Thresholds", F. Bastide, O. Julien, C. Macabiau, B. Roturier, in
Proceedings of the Institute of Navigation (ION), GPS (24-09-2002), pages 2196-
2207 discloses a receiver adapted to demodulate QPSK-modulated GPS L5 signals
(Quaternary Phase Shift Keying), or QPSK-modulated Galileo E5 A and E5B signals.
The European Commission and the European Space Agency (ESA) are
developing a GNSS known as Galileo. Galileo satellites will transmit signals in the
E5aband (1176.45MHz) andE5b band (1207.14MHz) as a composite signal with a
center frequency of 1195.795 MHz using a proposed modulation known as Alternate

Binary Offset Canier(AltBOC). The generation of the AltBOC signal is described in
the Galileo Signal Task Force document"Technical Annex to Galileo SRD Signal
Plans", Draft 1,18 July 2001, ref # STF-annexSRD-2001/003, which is incorporated
herein in its entirety by reference. Like the GPS satellites, the GNSS satellites each
transmit unique PRN codes and a GNSS receiver can thus associate a received signal
with a particular satellite. Accordingly, the GNSS receiver determines respective

pseudoranges based on the difference between the times the satellites transmit the
signals and then times the receiver receives the AltBOC signals.
A standard binary offset carrier (BOC) modulates a time domain signal by a
sine wave sin(wot), which shifts the frequency of the signal to both an upper sideband
and a corresponding lower sideband. The BOC modulation accomplishes the frequency
shift using a square wave, or sign(sin(wot)), and is generally denoted as BOC(fs,fc),
where fs is the subcarrier (square wave) frequency and fc is the spreading code chipping
rate. The factors of 1.023 MHz are usually omitted from the notation for clarity so a
BOC(15.345MHz, 10.23 MHz) modulation is denoted BOC(15,10). The BOC
modulation, which produces, for example, signals that are similar to the split C/A code
discussed above, allows a single spreading, or PRN, code on each of the in-phase and
quadrature carriers.
The modulation of a time domain signal by a complex exponential ewot shifts
the frequency of the signal to the upper sideband oniy. The goal of the AltBOC
modulation is to generate in a coherent manner the E5a and E5b bands, which are
respectively modulated by complex exponentials, or subcarriers, such that the signals
can be received as a wideband "BOC-like signal." The E5a and E5b bands each have
associated in-phase and quadrature spreading, or PRN, codes, with the E5a codes
shifted to the lower sideband and the E5b codes shifted to the upper sideband. The
respective E5a and E5b quadrature carriers are modulated by dataless pilot signals, and
the respective in-phase carriers are modulated by both PRN codes and data signals. A
GNSS receiver may track either the E5a codes or the E5b codes in a manner that is
similar to the tracking of the split C/A code discussed above.
There are, however, advantages in both multipath mitigation and tracking
accuracy associated with tracking the composite E5a and E5b signals, that is, tracking
the wideband AltBOC coherent signal. The respective inphase and quadrature carriers
of the composite signal are modulated by complex spreading codes, and thus, the in-
phase and quadrature channels each include contributions from both real and imaginary
signal components of the E5a and E5b codes. Theoretical analyses of the composite
tracking operations have been made using high level mathematics. Accordingly, the
associated receivers, which essentially reproduce the high-level mathematical
operations, are expected to be both complicated and costly

One proposed receiver produces local versions of the AltBOC composite codes
using the same look-up tables that the Galileo satellites use to generate the signals for
transmission, that is, the tables that correspond to the underlying phase shift keying
(PSK) spreading codes. The proposed receiver must thus not only maintain large look-
up tables for each of the codes transmitted by the respective Galileo satellites, the
receiver must also operate complex circuitry that controls entry to the look-up tables
each time a new code chip is received. The tables are even larger and entering them
more complicated when different pilot codes are used on the E5a and E5b bands, as is
now contemplated.
SUMMARY
The invention is (a GNSS receiver that tracks the AltBOC (15,10), or composite )
E5a and E5b, codes (using hardware that locally generates the complex composite signal
by combining separately generated real and the imaginary components of the complex
signal.) To track the dataless composite pilot code signals that are on the quadrature
channel of the AltBOC signal, for example, the receiver produces a local version of the
composite pilot code as a combination of the locally generated real and imaginary pilot
signal components. The receiver thus operates PRN code generators that produce
replica E5a and E5b PRN codes and square wave generators that generate the real and
imaginary components of the upper and lower subcarriers.
The receiver removes the complex composite code from the received signal by
multiplying the received signal, which has been downconverted to baseband I and Q
signal components, by the locally generated complex composite code. The receiver
then uses the results, which are correlated I and Q prompt signal values, to estimate the
center frequency carrier phase angle tracking error. The error signal is used to control a
numerically controlled oscillator that operates in a conventional manner, to correct the
phase angle of the locally generated center frequency carrier. The receiver also uses
early and late versions of the locally generated complex composite pilot code in a DLL,
and aligns the locally generated composite pilot code with the received composite pilot
code by minimizing the corresponding DLL error signal.
Once the receiver is tracking the composite pilot code, the receiver determine its
pseudorange and global position in a conventional manner Further, as discussed in

more detail below, the receiver uses a separate set of correlators to align locally generated
versions of the in-phase composite PRN codes with the in-phase channel codes in the
received signal, and thereafter, recover the data that is modulated thereon.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention description below refers to the accompanying drawings, of
which:
Fig. 1 depicts the frequency spectrum of the AltBOC (15,10) quadrature
channel sequence;
Fig. 2 depicts the normalized autocorrelation function associated with the signal
depicted in Fig. 1;
Fig. 3 is a functional block diagram of one channel for GNSS receiver;
Fig. 4 is a functional block diagram of a local code generator that is included in
the receiver of Fig. 3,
Fig. 5 is a functional block diagram of a correlator subsystem that is included in
the receiver of Fig. 4;
Fig. 6 depicts an autocorrelation function associated with the AltBOC in phase
channel;
Fig. 7 is a chart of idealized autocorrelation values;
Fig. 8 depicts another autocorrelation function associated with the AltBOC in
phase channel;
Fig. 9 is a functional block diagram of a local code generator;
Fig. 10 is a functional block diagram of a correlator subsystem; and
Fig. 11 is a functional block diagram that combines the correlator subsystems of
Figs. 5 and 10.
DETAILED DESCRIPTION OF AN ILLUSTRATIVE
EMBODIMENT
The Galileo AltBOC modulation scheme generates an AltBOC (15,10) signal
that is a "BOC(15,10)-like" signal with the E5a and E5b bands having their own
respective spreading, or PRN, codes on their in-phase and quadrature carriers. The
AltBOC (15,10) signal has a center carrier frequency of 1191.795 HMz and a

subcarrier frequency of 15.345 MHz, with the E5a band (1176.45 MHz) as the lower
sideband and the E5b band (1207.14 MHz) as the upper sideband.
The AltBOC (15,10) signal is generated on the satellite as a constant envelope
signal that includes on the in-phase channel a composite of the E5a and E5b spreading,
or PRN, codes and data, on the quadrature channel and a composite of the E5a and E5b
dataless PRN, or pilot, codes. Fig. 1 depicts the frequency spectrum of an AltBOC
PRN quadrature channel sequence.
The idealized normalized autocorrelation function for the AltBOC (15, 10)
signal is shown in Fig. 2. The envelope 100 of the autocorrelation function 111 is the
autocorrelation function of a 10.23 MHz chipping rate signal and the multiple peaks of
the autocorrelation function 111 are associated with the 15.3454 MHz; subcarrier, which
can be thought of as a complex square wave code.
The operations of a GNSS receiver 10 in tracking the AltBOC (15, 10) Galileo
satellite signals are discussed below. In Section 1, the operations for tracking the
quadrature dataless pilot codes as a composite code are discussed. In Section 2 the
operations for recovering the E5a and E5b data from the in-phase composite data codes
are discussed. The discussion below assumes that the receiver has acquired the center
frequency carrier using a conventional carrier tracking loop (not shown).
Section 1. Tracking the Composite Pilot Code
The AltBOC signal is given by:

where c1 is the in-phase E5b code, c2 is the in-phase E5a code, c3 is the quadrature E5b
code and c4 is the quadrature E5a code, and the E5b and E5a spreading codes are
modulated, respectively, on the upper carrier er(t) and the lower carrier er*(t), which is
the complex conjugate of er(l). The upper carrier er(t) is:

where cr(t) = sign(cos(2π fst)), sr(t) = sign(sin7(2π fst)) and fs is the subcarrier frequency.
The lower carrier er*(t) is.

The composite pilot code, which is on the quadrature channel of the AltBOC (15,10)
signal, includes the E5a and E5b quadrature codes c4(t) and c3(1:). An expression Xq(t)
for the quadrature channel signal is derived from equation 1 by setting the in-phase
channel codes to zero and substituting the expressions for the carriers:

The receiver produces the local version of the complex composite pilot code by
combining locally generated real and imaginary signal components, as discussed in
more detail with reference to Fig. 4. The receiver then correlates the locally generated
composite pilot code with the corresponding composite pilot code in the received
signal, as discussed in more detail with reference to Fig. 5 below. The receiver then
determines associated pseudoranges and its global position in a conventional manner.
Referring now to Fig. 3, a GNSS receiver 10 receives over an antenna 12 a
signal that includes the AltBOC composite codes transmitted by all of the satellites that
are in view. The received signal is applied to a downconverter 14 that, in a
conventional manner, converts the received signal to an intermediate frequency ("IF")
signal that has a frequency which is compatible with an analog-to-digital converter 18.
The IF signal is next applied to an IF bandpass filter 16 that has a bandpass at
the desired center carrier frequency. The bandwidth of the filter 16 should be
sufficiently wide to allow the primary harmonic of the AltBOC composite pilot code to
pass, or approximately 1192 MHz. The wide bandwidth results in relatively sharp bit
transitions in the received code, and thus, fairly well defined correlation peaks.
The analog-to-digital converter 18 samples the filtered IF signal at a rate that
satisfies the Nyquist theorem and produces corresponding digital inphase (I) and
quadrature (Q) signal samples in a known manner. The 1 and Q digital signal samples
are supplied to a Doppler removal processor 20 that operates in a known manner, to
/ produce baseband Ibaseband and Qbaseband samples by rotating the signals in accordance

with an estimate of the center frequency carrier phase angle. The estimate of the carrier
phase angle is based in part on the signals produced by a carrier numerically controlled
oscillator ("carrier NCO") 30, which is adjusted in accordance with carrier phase error
tracking signals produced by a correlation subsystem 22. The operations of the
correlator subsystem are discussed below with reference to Fig. 5.
The Ibaseband and Qbaseband samples are next supplied to the correlator subsystem
22, which makes correlation measurements by multiplying the samples by early,
prompt and late or early minus late versions of locally generated composite pilot
codes produced by a composite code generator 24. The operations of the composite
code generator and the correlator subsystem are discussed below with reference to Figs.
4 and 5, respectfully. The I and Q correlation measurements associated with the early,
prompt and late or early minus late versions of the local-composite pilot code are
supplied to an integrate and dump circuit 26, which separately accumulates the
respective I and Q measurements over predetermined intervals. At the end of each
interval, the integrate and dump circuit 26 supplies the results of the respective I and Q
accumulations, that is, the I and Q correlation signals, to a controller 40. The controller
then controls the carrier NCO 30 and the composite code generator 24, to align the
locally generated composite pilot code with the corresponding composite code in the
received signal.
The GNSS receiver 10 tracks the AltBOC (15, 10) signals using a locally
generated composite pilot code that is generated from locally produced real and
imaginary composite signal components. The operations performed by the composite
code generator 24 to produce the locally generated composite pilot code components
are now discussed in detail with reference to Fig. 4.
The composite code generator 24 includes c3 and c4 PRN code generators 242
and 243 'that produce, respectively, local versions of the E5a and E5b PRN codes for a
given GNSS satellite. The code generator 24 further includes two square wave
generators 244 and 245 that produce values of cr and sr that correspond to the real and
imaginary components of the upper and lower carriers er(t) and er*(t). As discussed in
more detail below with reference to Fig. 5, the controller uses the correlation signals to
control the relative timing of the locally produced c3 and c4 code chips and the

transitions of the cr and sr square waves, which can be thought of as respective code
patterns of 0, 1,0...., and so forth.
The composite code generator 24 adds together the c3 and c4 code chips in an
adder 240 and multiplies the sum in multiplier 246 by the value of cr, which is
sign(cos(πfst), to produce the real component of the locally generated composite pilot
code. The real component of the composite pilot code is hereinafter referred to as
"Ipilot" The composite code generator produces the imaginary component of the
composite pilot code by inverting the c3 code chips in an inverter 248, adding the
inverted c3 code chip to the corresponding c4 code chip in an adder 2.50 and, in
multiplier 252, multiplying the result by sr, which is sign(sin(πfssf). The imaginary
component of the locally generated composite pilot code is hereinafter referred to as
"Qpilot" The local replica of the composite pilot code is then the sum:

As discussed in more detail below, the correlator subsystem 22 multiplies the received
composite pilot code by the locally generated composite pilot code. Based on the
results, the controller 40 adjusts the PRN code and square wave generators 242-245 to
align the local code to the received code.
Referring now to Fig. 5, the operations of the correlator subsystem 22 are
explained in terms of the operations involving the prompt version of the locally
generated composite pilot code. The receiver includes similar circuits for early and
late or early-minus-late versions of the locally generated composite pilot code that
operate as part of a delay lock loop, or DLL, which operates in a known manner to
produce an associated DLL error signal.
The correlation subsystem 22 multiplies together two complex signals, namely,
the locally generated composite pilot code and the received composite signal. The
correlator subsystem thus performs the operation:

As depicted in Fig. 5, the correlation subsystem manipulates the baseband
signals Ibaseband and Qbaseband provided by the Doppler removal processor 20 and the
locally-generated real and imaginary signal components Ipilot and Qpilot provided by the
code generator 24, to produce the real and imaginary components of the correlation
signal. The correlation subsystem 22 multiples the Ibaseband signal by the Ipilot signal in
multiplier 502, and the Qbaseband signal by the Qpilot signal in multiplier 510. An adder
506 then adds the two products together and provides the result to an integrate and
dump circuit 516. The integrate and dump circuit 516 accumulates the sums produced
by the adder 506 and at appropriate times produces a corresponding real component, or
Iprompt, signal. To produce the imaginary components, the correlation subsystem
multiplies the Qbaseband signal by the Ipilot signal in a multiplier 508 and multiplies the
Ibaseband signal by the Qpilot signal in a multiplier 504. The product produced by the
multiplier 504 is inverted by inverter 512 and added to the product produced by the
multiplier 508 in an adder 514. The adder 514 then supplies the sums to the integrate
and dump circuit 518, which accumulates the sums and at appropriate times produces a
corresponding Qprompt signal.
The controller 40 (Fig. 3) manipulates the Iprompt and Qprompt signals, to
determine the center carrier tracking phase error as the arctangent of QPrompt/IPrompt.
The phase error signal is then used, in a known manner, to control the carrier NCO 30,
which in turn controls the Doppler removal processor 20.
As discussed above, the controller 40 also receives early and late or early-
minus-late I and Q correlation signals. Based on these signals the controller 40 adjusts
the generators 242-245 to align the local composite code in the received code and thus
minimize the associated DLL error signal
Section 2. Recovering the Data from the Composite In-Phase Signals
The AltBOC (15, 10) signals include both data and spreading codes on the E5a
in-phase and E5b in-phase channels The E5a in-phase channel will carry data that is
transmitted at a particular data rate and the E5b in-phase channel will carry different
data that is transmitted at a different data rate. The data transitions on the E5a and E5b
in-phase channels will, however, occur at corresponding times. The GNSS receiver 10
acquires and tracks the AltBOC (15,10) signal using the composite pilot code, as
discussed above. After the removal of the carrier, the receiver recovers the data from

the composite in-phase signal using a separate set of correlators, as discussed in more
detail below with reference to Fig. 10.
The AItBOC(15, 10) complex in-phase baseband signal, that is, the signal with
the quadrature pilot codes set to zero is:

where the subcarrier is included in the expression as a sinusoid instead of the
corresponding rectangular functions cr(t) ± sr(t) and assuming, for the moment, that
c1(t) and c2(t) are free of data. Note that the term c1(t) •ej2xf + c2(t)•e-j2xf is similar
to the expression for the quadrature spreading, or PRN, codes discussed in Section 1
above.
If the baseband in-phase signal is correlated with a local replica of the
composite in-phase spreading code, the result is.

where Rk denotes the auto-correlation function for signal k. The cross-terms will be
filtered over the predetection interval, particularly since the E5a and E5b in-phase
spreading codes are designed to have low cross-correlation values. Removing the
cross-terms and expanding the complex exponentials the expression becomes:

In the case of datafree signals, the autocorrelation functions R1 and R2 will be
equal and the expression simplifies to:

The corresponding correlation function is depicted in Fig 6. Note that this is similar to
the correlation function for the composite quadrature pilot code, which is shown in Fig.
2.
If the assumption that the E5a and E5b in-phase codes c1(t) and c2(t) are
datafree is removed, the maximum normalized ideal value of the individual correlation
functions R1 and R2, and their sum and differences are shown in the table of Fig. 7.
Accordingly, the original data sequence can be recovered if the receiver recovers both
(R1+R2) and
(R2-R1) from the composite in-phase signal,
The receiver may recover the (R1+R2) data directly from the in-phase I prompt
signal. However, the recovery of the (R2-R1) data is not as straight forward. As
depicted in Fig. 6, the R1+R2 component of the in-phase composite signal, namely,
(R1 (ι) + R2 (r)) • cos(2πfsτ) has an autocorrelation function that is similar to the
autocorrelation function of the composite quadrature channel signal. However, as
depicted in Fig. 8, the R1-R2 signal component of the in-phase composite signal
j (R2 (τ) - R1 (τ)) sin(2πfτ) does not have a similar autocorrelation function because
the sin term goes to zero when r goes to zero.
To compensate for the sin term going to zero when x goes to zero, a correlation
operation could be offset so that the operation tracks the correlation peak that
corresponds to j (R2(τ) -R1 (τ)) sin(2πfτ) and the (R2-R1) data sequence may then
be read from the Qprompt correlator, but with reduced power.
Alternatively, the circuitry that generates the local version of the composite
signal may instead generate a signal that produces for the (R2 -R1) term the
autocorrelation function (R2 (τ) - Rx (τ)) cos(2πfsτ) The local in-phase composite
signal thus becomes:
To demodulate the data using this method, the receiver locally produces two
combinations of the c1(t) and c2(t) spreading codes, namely, combinations that
correspond, respectively, to the R1 +R2 data and the R2-R1 data To recover the (R1+R2)
data the receiver produces the local signal:

Referring now to Fig. 9, the real and imaginary components of the R2-R1 and
R1 +R2 combinations are locally produced by a code generator 54, which may be part of
the local composite code generator 24 (Fig. 3). To produce the real component of the
R1 +R2 combination, the code generator adds together the c1 and c2 codes in an adder
540 and multiplies the result by the square wave code cr in a multiplier 546. To
produce the imaginary component of the R1 +R2 combination, the code generator adds
together the c1 code and an inverted c2 code in an adder 550 and multiples the result by
the square wave code sr in a multiplier 562. The generator also multiplies the sum c1 +
c2 produced by the adder 540 by the square wave code sr in a multiplier 560, to produce
the imaginary component of the R2-R1 combination. The generator further multiplies
the sum c1-c2 produced by the adder 550 by the square wave code cr in a multiplier 522,
to produce the real component of the R2 -R1 code
The receiver then uses the locally produced real and imaginary components of
the R1 +R2 and R2-R1 combinations to recover the data from the composite in-phase
code.
Referring now to Fig. 10, the system multiplies the in-phase baseband signal
Ibaseband by the real component of R1 +R2 in a multiplier 602, and the quadrature
baseband signal Qbaseband by the imaginary component of R1 +R2 in a multiplier 606.
The products produced by the multipliers 602 and 606 are then added together in an
adder 608 and the sum provided to an integrate and dump circuit 615. To produce the
correlation signal relating to R1-R2, the correlation subsystem multiplies the quadrature
baseband signal Qbaseband by the imaginary component of R2-R1 in a multiplier 610.
Further, the system multiplies the real component of the baseband signal Ibaseband by the
real component of R2-R1, in a multiplier 604 code. The two sums are added together in
an adder 612 and provided to an integrate and dump circuit 616. The integrate and
dump circuits 615 and 616 accumulate the correlation values produced by the adders

608 and 612, respectively, and at appropriate times produce the R1 + R2prompt and R1-
R2prompt signals. The results are then used to recover the data in accordance with the
chart of Fig. 7.
The circuitry of Figs. 5 and 10 can be combined, as depicted in Fig. 11, to
produce a system that tracks the quadrature AltBOC composite code and recovers E5a
and E5b data from the in phase AltBOC composite code.

WE CLAIM :
1. A receiver for use with a global navigation satellite system that transmits Alternate binary offset
carrier, or AltBOC signals, the receiver comprising :
a local composite code generator (24) for producing real and imaginary code components
(Ipilot, Qpilot) of a local version of an AltBOC composite code obtained by combining locally
produced codes (c3, c4) with real and imaginary components (cr, sr) of upper and lower subcarriers (er,
er*);
a correlation subsystem (22, 26) for producing correlation signals (Iprompt, Qprompt)
resulting from correlating the locally produced composite code with the composite code in a received
AltBOC signal by combining products produced by multiplying baseband inphase and quadrature
components (Ibaseband, Qbaseband) of the received signal by the locally produced real and imaginary
composite code components; and
a controller (40) for adjusting the local composite code generator to align the local composite
code with the corresponding composite code in the received AltBOC signal based on the correlation
signals, the controller comprising means for determining a global position based on timing differences
between times the AltBOC composite codes are transmitted and the times the codes are received.
2. The receiver as claimed in claim 1, wherein the local composite code generator (24) comprises :
square wave code generators (244, 245) for producing real and imaginary components (cr, sr) of
the upper and lower carriers (er, er*),
a first PRN code generator (243) for producing a first code (c3) that is modulated on the upper
carrier (er),
a second PRN code generator (242) tor producing a second code (c4) that is modulated on the
lower carrier (er*), and
adders (240, 250) and multipliers (246, 248, 252) for combining the first and second codes (c3,
c4) with the real and imaginary components (cr, sr) of the upper and lower carriers (er, er*) to produce
the real and imaginary components (Ipilot, Qpilot) of the local composite code.

3. The receiver as claimed in claim 1 or 2, wherein the local composite code generated (Ipilot,
Qpilot) corresponds to a dataless composite pilot code.
4. The receiver as claimed in anyone of claims 1 to 3, wherein
the local composite code generator (24) comprises means for producing combinations (ki+R2,
QRI+R2, IR1-R2, QR1-R2) that correspond respectively to a sum and a difference of a first autocorrelation
function (R1) associated with a third code (c1) and a second autocorrelation function (R2) associated
with a fourth code (c2),
the correlation subsystem (22) comprises means for producing combination correlation signals
((Rl+R2)prompt, (Rl-R2)prompt) that correspond to the respective combinations, and
the controller (40) comprises means for recovering data from the combination correlation
signals.
5. The receiver as claimed in anyone of claims 2 to 4, wherein the adders (240, 248, 250) comprise
at least one inverter (248) for selectively inverting the codes (c3, c4).
6. The receiver as claimed in anyone of claims 3 to 5, wherein the local code generator comprises :
a third PRN code generator (542) for producing a third code (c1) that is modulated on the upper
carrier (er);
a fourth PRN code generator (543) for producing a fourth code (c2) that is modulated on the
lower carrier (er*);
adders (540, 548, 550) for combining the third and fourth codes to produce associated sums; and
multipliers (546, 552, 560. 562) for multipling each sum separately by the first and second
square waves (cr, sr) to produce real and imaginary components (IR1+R2, QR1+R2, IR1-R2, QR1-R2) of the
respective associated combination correlation signals ((Rl+R2)prompt, (Rl-R2)prompt)).
7. A method of determining global position from Alternate binary offset carrier, or AltBOC signals
received from a global navigation satellite system, the method comprising the steps of:

producing real and imaginary code components (Ipilot, Qpilot) of a local version of an
AltBOC composite code obtained by combining locally produced codes (c3, c4) with real and
imaginary components (cr, sr) of upper and lower subcarriers (er, er*) ;
producing in-phase and quadrature components of the received AltBOC signal;
correlating the locally produced composite code with the composite code in the received
AltBOC signal by combining products produced by multiplying the baseband inphase and quadrature
components (Ibaseband, Qbaseband) of the received AltBOC signal by the locally produced real and
imaginary composite code components to produce associated correlation signals (Iprompt, Qprompt) ;
adjusting the local composite code generator based on the correlation signals to align the local
composite code with the corresponding composite code in the received AltBOC signal; and
determining a global position based on timing differences between times the received AltBOC
composite codes are transmitted and the times the codes are received.
8. The method as claimed in claim 7, wherein the step of producing the local version of the
AltBOC composite code comprises steps of:
producing square waves (cr, sr) that correspond to real and imaginary components of upper and
lower carriers (er, er*),
producing a first code (c3) that is modulated on the upper carrier,
producing a second code (c4) that is modulated on the lower carrier, and
selectively combining the first and second codes and multiplying the results by the real and
imaginary components of the upper and lower carriers to produce the real and imaginary components
(Ipilot, Qpilot) of the local composite code.
9. The method as claimed in claim 8, wherein the step of selectively combining the first and
second codes comprises the steps of producing first and second sums that are associated respectively
with the real and imaginary components (Ipilot, Qpilot), the first sum corresponding to the addition of
the second code (c4) to an inverted first code (-c3) and the second sum corresponding to the addition of
the two codes.

10. The method as claimed in anyone of claims 7 to 9, wherein the local composite code generated
(Ipilot, Qpilot) corresponds to a dataless composite pilot code.
11. The method as claimed in anyone of claims 7 to 10, comprising steps of:
producing a third code (c1) that is modulated on the upper carrier,
producing a fourth code (c2) that is modulated on the lower carrier,
producing two combinations of the third and fourth codes with the first and second square
waves to obtain real and imaginary components (IRI+R2, QR1+R2, IR1-R2, QR1-R2) of combinations R1 + R2
and R2 - Rl, where Rl is the autocorrelation function associated with the third code (cl), and R2 is the
autocorrelation function associated with the fourth code (c2),
correlating the combinations so as to obtain combination correlation signals ((Rl+R2)prompt,
(Rl-R2)prompt) for each combination, and
recovering data from the combination correlation signals.

There is disclosed a receiver for use with a global navigation satellite system that
transmits Alternate binary offset carrier, or AltBOC signals. The receiver includes a
local composite code generator (24) for producing real and imaginary code components
(Ipilot, Qpilot) of a local version of an AltBOC composite code obtained by combining
locally produced codes (c3, c4) with real and imaginary components (cr, sr) of upper
and lower subcarriers (er, er*) ; a correlation subsystem (22, 26) for producing
correlation signals (Iprompt, Qprompt) resulting from correlating the locally produced
composite code with the composite code in a received AltBOC signal by combining
products produced by multiplying baseband inphase and quadrature components
(Ibaseband, Qbaseband) of the received signal by the locally produced real and
imaginary composite code components ; and a controller (40) for adjusting the local
composite code generator to align the local composite code with the corresponding
composite code in the received AltBOC signal based on the correlation signals, the
controller comprising means for determining a global position based on timing
differences between times the AltBOC composite codes are transmitted and the times
the codes are received.

Documents:

2582-KOLNP-2005-CORRESPONDENCE.pdf

2582-KOLNP-2005-FORM 27.pdf

2582-KOLNP-2005-FORM-27.pdf

2582-kolnp-2005-granted-abstract.pdf

2582-kolnp-2005-granted-assignment.pdf

2582-kolnp-2005-granted-claims.pdf

2582-kolnp-2005-granted-correspondence.pdf

2582-kolnp-2005-granted-description (complete).pdf

2582-kolnp-2005-granted-drawings.pdf

2582-kolnp-2005-granted-examination report.pdf

2582-kolnp-2005-granted-form 1.pdf

2582-kolnp-2005-granted-form 18.pdf

2582-kolnp-2005-granted-form 3.pdf

2582-kolnp-2005-granted-form 5.pdf

2582-kolnp-2005-granted-pa.pdf

2582-kolnp-2005-granted-reply to examination report.pdf

2582-kolnp-2005-granted-specification.pdf

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Patent Number

225967

Indian Patent Application Number

2582/KOLNP/2005

PG Journal Number

49/2008

Publication Date

05-Dec-2008

Grant Date

03-Dec-2008

Date of Filing

13-Dec-2005

Name of Patentee

EUROPEAN SPACE AGENCY

Applicant Address

8-10 RUE MARIO-NIKIS, F-75015 PARIS

Inventors:

#	Inventor's Name	Inventor's Address
1	GEREIN, NEIL	112 THORSON CRESCENT, OKOTOKS, ALBERTA T 1S 1C9

PCT International Classification Number

G01S 1/04

PCT International Application Number

PCT/CA2003/001548

PCT International Filing date

2003-10-09

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	60/487,180	2003-07-14	U.S.A.