Интерфейсный документ GPS (1014597), страница 5
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ground transmitters).NOTE: There are many other available initial register states which can be used for other signaltransmitters including any additional SVs in future.IS-GPS-200D7 Dec 2004103.2.1.6Non-Standard Codes.The NSC, NSCM, NSCL, and NSY codes, used to protect the user from amalfunction in the SV's reference frequency system (reference paragraph 3.2.1), are not for utilization by the userand, 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)and C/A- codes; the resultant bit-trains are used to modulate the L1 and L2 carriers.
For a given SV, the data trainD(t), if present, is common to the P(Y)- and C/A- codes on both the L1 and L2 channels. The content andcharacteristics of the NAV data, D(t), are given in Appendix II of this document.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 iscoded by a rate ½ convolutional coder. When selected by ground command, the resulting 50 sps symbol stream ismodulo-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 encode of a 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.IS-GPS-200D7 Dec 2004113.2.3 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. Fora particular 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 configuration and combination of codes/signals specified in this section are shown in Table 3-III.IS-GPS-200D7 Dec 200412Table 3-III.L1SV BlocksIn-Phase*Block II/IIA/IIRBlock IIR-M***Block IIR-M/IIFSignal ConfigurationP(Y) ⊕ D(t)P(Y) ⊕ D(t)P(Y) ⊕ D(t)L2**Quadrature-Phase*In-Phase*Quadrature-Phase*C/A ⊕ D(t)P(Y) ⊕ D(t)orP(Y)orC/A ⊕ D(t)Not ApplicableC/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/AC/A ⊕ D(t)P(Y) ⊕ D(t)orP(Y)L2 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.IS-GPS-200D7 Dec 2004133.3 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. The signals shall be contained within two 20.46-MHz bands centered about L1 and L2.The carrier frequencies for the L1 and L2 signals shall be coherently derived from a common frequency sourcewithin the SV. The nominal frequency of this source -- as it appears to an observer on the ground -- is 10.23 MHz.The SV carrier frequency and clock rates -- as they would appear to an observer located in the SV -- are offset tocompensate for relativistic effects. The clock rates are offset by ∆ f/f = -4.4647E-10, equivalent to a change in theP-code chipping rate of 10.23 MHz offset by a ∆ f = -4.5674E-3 Hz. This is equal to 10.22999999543 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. Correlation loss is defined as the difference between the SV power received in a 20.46MHz bandwidth and the signal power recovered in an ideal correlation receiver of the same bandwidth. On the L1and L2 channels, the worst case correlation loss occurs when the carrier is modulated by the sum of the P(Y) codeand the NAV data stream. For this case, the correlation loss apportionment shall be as follows:1.SV modulation imperfections 0.6 dB2.Ideal UE receiver waveform distortion 0.4 dB(due to 20.46 MHz filter)3.3.1.3 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 shall be at least 40 dB below the unmodulated L1and L2 carriers over the allocated 20.46 MHz channel bandwidth.IS-GPS-200D7 Dec 2004143.3.1.5 Phase Quadrature. The two L1 carrier components modulated by the two separate bit trains (C/A-code plusdata and P(Y)-code plus data) shall be in phase quadrature (within ±100 milliradians) with the C/A signal carrierlagging the P signal by 90 degrees. Referring to the phase of the P carrier when Pi(t) equals zero as the "zero phaseangle", the P(Y)- and C/A-code generator output shall control the respective signal phases in the following manner:when Pi(t) equals one, a 180-degree phase reversal of the P-carrier occurs; when Gi(t) equals one, the C/A carrieradvances 90 degrees; when the Gi(t) equals zero, the C/A carrier shall be retarded 90 degrees (such that when Gi(t)changes state, a 180-degree phase reversal of the C/A carrier occurs). The resultant nominal composite transmittedsignal phases as a function of the binary state of only the two modulating signals are as shown in Table 3-IV.For Block IIR-M, IIF, and subsequent blocks of SVs, phase quadrature relationship between the two L2 carriercomponents can be the same as for the two L1 carrier components as described above.
However, for the L2 case,the civil signal carrier component is modulated by any one of three (IIF) or four (IIR-M) different bit trains asdescribed in paragraph 3.2.3. Moreover, the two L2 carrier components can be in same phase. The resultantcomposite transmitted signal phases will vary as a function of the binary state of the modulating signals as well asthe signal power ratio and phase quadrature relationship.Beyond these considerations, additional carriercomponents in Block IIR-M, IIF, and subsequent blocks of SVs will result in composite transmitted signal phaserelationships other than the nominal special case of Table 3-IV.For Block IIF, the crosstalk between the C/A, when selected, and P(Y) signals shall not exceed –20 dB in the L1 andL2.
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