IS-GPS-200F (811524), страница 6
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The NSC, NSCM, NSCL, and NSY codes, used to protect the user from trackinganomalous navigation signals, are not for utilization by the user and, therefore, are not defined in this document.3.2.2 NAV Data. The NAV data, D(t), includes SV ephemerides, system time, SV clock behavior data, statusmessages and C/A to P (or Y) code handover information, etc. The 50 bps data is modulo-2 added to the P(Y)- andC/A- codes; the resultant bit-trains are used to modulate the L1 and L2 carriers. For a given SV, the data train D(t),if present, is common to the P(Y)- and C/A- codes on both the L1 and L2 channels. The content and characteristicsof the NAV data, D(t), are given in Appendix II of this document for legacy NAV (LNAV) data transmitted by SVsassigned to the lower set of PRN numbers (PRN 1-32) and Appendix IV of this document for LNAV datatransmitted by SVs assigned to the upper set of PRN numbers (PRN 33-63).For Block IIR-M, Block IIF, and subsequent blocks of SVs, civil navigation (CNAV) data, DC(t), also includes SVephemerides, system time, SV clock behavior, status messages, etc.
The DC(t) is a 25 bps data stream which isencoded by a rate ½ convolutional encoder. When selected by ground command, the resulting 50 sps symbol streamis modulo-2 added to the L2 CM-code; the resultant bit-train is combined with L2 CL-code using chip by chip timedivision multiplexing method (i.e. alternating between L2 CM ⊕ data and L2 CL chips); the multiplexed bit-train isused to modulate the L2 carrier. The content and characteristics of the CNAV data, DC(t), are given in Appendix IIIof this document.During the initial period of Block IIR-M SVs operation, prior to Initial Operational Capability of L2 C signal, BlockIIR-M may modulo-2 add the NAV data, D(t), to the L2 CM-code instead of CNAV data, DC(t).
Moreover, theNAV data, D(t), can be used in one of two different data rates which are selectable by ground command. D(t) with adata rate of 50 bps can be commanded to be modulo-2 added to the L2 CM-code, or D(t) with a symbol rate of 50symbols per second (sps) (rate ½ convolutional encoding of 25 bps NAV data) can be commanded to be modulo-2added to the L2 CM-code. The resultant bit-train is combined with L2 CL-code using chip by chip time-divisionmultiplexing method (i.e.
alternating between L2 CM ⊕ data and L2 CL chips). This multiplexed bit-train is used tomodulate the L2 carrier.12IS-GPS-200F21 Sep 20113.2.3 L1/L2 Signal Structure. The L1 consists of two carrier components which are in phase quadrature with eachother. Each carrier component is bi-phase shift key (BPSK) modulated by a separate bit train. One bit train is themodulo-2 sum of the P(Y)-code and NAV data, D(t), while the other is the modulo-2 sum of the C/A-code and theNAV data, D(t).
For Block II/IIA and IIR, the L2 is BPSK modulated by only one of those two bit trains; the bittrain to be used for L2 modulation is selected by ground command. A third modulation mode is also selectable onthe L2 channel by ground command: it utilizes the P(Y)-code without the NAV data as the modulating signal. For aparticular SV, all transmitted signal elements (carriers, codes and data) are coherently derived from the same onboard frequency source.For Block IIR-M, Block IIF, and subsequent blocks of SVs, the L2 consists of two carrier components. One carriercomponent is BPSK modulated by the bit train which is the modulo-2 sum of the P(Y)-code with or without NAVdata D(t), while the other is BPSK modulated by any one of three other bit trains which are selectable by groundcommand.
The three possible bit trains are: (1) the modulo-2 sum of the C/A-code and D(t); (2) the C/A-code withno data and; (3) a chip-by-chip time multiplex combination of bit trains consisting of the L2 CM-code with DC(t)and the L2 CL-code with no data. The L2 CM-code with the 50 sps symbol stream of DC(t) is time-multiplexedwith L2 CL-code at a 1023 kHz rate as described in paragraph 3.2.2.The first L2 CM-code chip startssynchronously with the end/start of week epoch.During the initial period of Block IIR-M SVs operation, prior to Initial Operational Capability of L2 C signal, BlockIIR-M may modulo-2 add the NAV data, D(t), to the L2 CM-code instead of CNAV data, DC(t).In suchconfiguration, the data rate of D(t) may be 50 bps (i.e. without convolution encoding) or it may be 25 bps.
The D(t)of 25 bps shall be convolutionally encoded resulting in 50 sps.The different configurations and combinations of codes/signals specified in this section are shown in Table 3-III.13IS-GPS-200F21 Sep 2011Table 3-III.Signal ConfigurationL1L2**SV BlocksIn-Phase*Block II/IIA/IIRP(Y) ⊕ D(t)Quadrature-Phase*In-Phase*Quadrature-Phase*C/A ⊕ D(t)P(Y) ⊕ D(t)orP(Y)orC/A ⊕ D(t)Not ApplicableBlock IIR-M***P(Y) ⊕ D(t)C/A ⊕ D(t)P(Y) ⊕ D(t)orP(Y)Block IIR-M/IIF/and GPS IIIP(Y) ⊕ D(t)C/A ⊕ D(t)P(Y) ⊕ D(t)orP(Y)L2 CM ⊕ D(t) with L2 CLorL2 CM ⊕ D′(t) with L2 CLorC/A ⊕ D(t)orC/AL2 CM ⊕ DC(t) with L2 CLorC/A ⊕ D(t)orC/ANotes: 1) The configuration identified in this table reflects only the content of Section 3.2.3 and does notshow all available codes/signals on L1/L2.2) It should be noted that there are no flags or bits in the navigation message to directly indicatewhich signal option is broadcast for L2 Civil (L2 C) signal.⊕ = “exclusive-or” (modulo-2 addition)D(t) = NAV data at 50 bpsD′(t) = NAV data at 25 bps with FEC encoding resulting in 50 spsDC(t) = CNAV data at 25 bps with FEC encoding resulting in 50 sps*Terminology of “in-phase” and “quadrature-phase” is used only to identify the relative phasequadrature relationship of the carrier components (i.e.
90 degrees offset of each other).** The two carrier components on L2 may not have the phase quadrature relationship. They may bebroadcast on same phase (ref. Section 3.3.1.5).*** Possible signal configuration for Block IIR-M only during the initial period of Block IIR-M SVsoperation, prior to Initial Operational Capability of L2 C signal. See paragraph 3.2.2.14IS-GPS-200F21 Sep 20113.3 Interface Criteria. The criteria specified in the following define the requisite characteristics of the SS/USinterface for the L1 and L2.3.3.1 Composite Signal. The following criteria define the characteristics of the composite signals.3.3.1.1 Frequency Plan.
For Block IIA, IIR, IIR-M, and IIF satellites, the requirements specified in this IS shallpertain to the signal contained within two 20.46 MHz bands; one centered about the L1 nominal frequency and theother centered about the L2 nominal frequency (see Table 3-Vb).
For GPS III and subsequent satellites, therequirements specified in this IS shall pertain to the signal contained within two 30.69 MHz bands; one centeredabout the L1 nominal frequency and the other centered about the L2 nominal frequency (see Table 3-Vc). Thecarrier frequencies for the L1 and L2 signals shall be coherently derived from a common frequency source withinthe SV.
The nominal frequency of this source -- as it appears to an observer on the ground -- is 10.23 MHz. The SVcarrier frequency and clock rates -- as they would appear to an observer located in the SV -- are offset to compensatefor relativistic effects. The clock rates are offset by ∆f/f = -4.4647E-10, equivalent to a change in the P-codechipping rate of 10.23 MHz offset by a ∆f = -4.5674E-3 Hz. This is equal to 10.2299999954326 MHz.
Thenominal carrier frequencies (f0) shall be 1575.42 MHz, and 1227.6 MHz for L1 and L2, respectively.3.3.1.2 Correlation Loss. The correlation loss is defined as the difference between the signal power received in thebandwidth defined in 3.3.1.1 (excluding signal combining loss) and the signal power recovered in an idealcorrelation receiver of the same bandwidth using an exact replica of the waveform within an ideal sharp-cutoff filterbandwidth, whose bandwidth corresponds to that specified in 3.3.1.1 and whose phase is linear over that bandwidth.The total allowable correlation loss due to SV modulation and filtering imperfections, which is a function of signal,shall be:CodeCorrelation LossCorrelation Loss(IIF and prior SVs)(III SVs)C/A & L2C0.6 dB0.3 dBL1P(Y) & L2P(Y)0.6 dB0.6 dB15IS-GPS-200F21 Sep 20113.3.1.3 Carrier Phase Noise.
The phase noise spectral density of the unmodulated carrier shall be such that a phaselocked loop of 10 Hz one-sided noise bandwidth shall be able to track the carrier to an accuracy of 0.1 radians rms.3.3.1.4 Spurious Transmissions. In-band spurious transmissions, from the SV, shall be at or below -40 dBc over therespective bands specified in 3.3.1.1. In-band spurious transmissions are defined as transmissions within the bandsspecified in 3.3.1.1 which are not expressly components of the L1 and L2 signals.3.3.1.5 Signal Component Phasing.3.3.1.5.1 Phase Quadrature.
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