IS-GPS-200F (811524), страница 7
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The two L1 carrier components modulated by the two separate bit trains (C/A-codeplus data and P(Y)-code plus data) shall be in phase quadrature (within ±100 milliradians) with the C/A signalcarrier lagging the P signal by 90 degrees. Referring to the phase of the P carrier when Pi(t) equals zero as the "zerophase angle", the P(Y)- and C/A-code generator output shall control the respective signal phases in the followingmanner: when Pi(t) equals one, a 180-degree phase reversal of the P-carrier occurs; when Gi(t) equals one, the C/Acarrier advances 90 degrees; when the Gi(t) equals zero, the C/A carrier shall be retarded 90 degrees (such that whenGi(t) changes state, a 180-degree phase reversal of the C/A carrier occurs).
The resultant nominal compositetransmitted signal phases as a function of the binary state of only the two modulating signals are as shown in Table3-IV.For Block IIR-M, IIF, and subsequent blocks of SVs, the two L2 carrier components shall be either in phasequadrature or in the same phase (within ±100 milliradians) - see paragraph 3.3.1.5.3 for additional information. Thecivil signal carrier component is modulated by any one of three (IIF) or four (IIR-M) different bit trains as describedin paragraph 3.2.3. The resultant composite transmitted signal phases will vary as a function of the binary state ofthe modulating signals as well as the signal power ratio and phase quadrature relationship.Beyond theseconsiderations, additional carrier components in Block IIR-M, IIF, and subsequent blocks of SVs will result incomposite transmitted signal phase relationships other than the nominal special case of Table 3-IV.
The currentphase relationship of the two L2 carrier components (L2C and L2 P(Y)) shall be indicated by means of bit 273 of theCNAV Type 10 Message (See section 30.3.3), where zero indicates phase quadrature, with the L2C lagging the L2P(Y) by 90 degrees, and one indicates that L2C and L2 P(Y) are in-phase. If the CNAV message is not available,then the L2C and L2 P(Y) shall be fixed in phase quadrature.16IS-GPS-200F21 Sep 20113.3.1.5.2 Phase Crosstalk. For Block IIF, the crosstalk between the C/A, when selected, and P(Y) signals shall notexceed -20 dB in the L1 and L2.
The crosstalk is the relative power level of the undesired signal to the desiredreference signal.3.3.1.5.3 Phase Continuity. While the satellite is broadcasting standard C/A, P(Y), and L2C codes with data thatindicates that C/A, P(Y), and L2C signal health (respectively) is OK, there will not be any commanded operationcausing an intentional phase discontinuity.This does not apply to phase discontinuities caused by signalmodulation. Prior to health data being available on L2C, satellites will be set unhealthy using the non-standardcode.3.3.1.6 User-Received Signal Levels.
The SV shall provide L1 and L2 navigation signal strength at end-of-life(EOL), worst-case, in order to meet the minimum levels specified in Table 3-V. Any combining operation done bythe SV and associated loss is compensated by an increase in SV transmitted power and thus transparent to the usersegment. The minimum received power is measured at the output of a 3 dBi linearly polarized user receivingantenna (located near ground) at worst normal orientation, when the SV is above a 5-degree elevation angle. Thereceived signal levels are observed within the in-band allocation defined in para. 3.3.1.1.The Block IIF SV shall provide L1 and L2 signals with the following characteristic: the L1 off-axis relative power(referenced to peak transmitted power) shall not decrease by more than 2 dB from the Edge-of-Earth (EOE) to nadir,nor more than 10 dB from EOE to 20 degrees off nadir, and no more than 18 dB from EOE to 23 degrees off nadir;the L2 off-axis power gain shall not decrease by more than 2 dB from EOE to nadir, and no more than 10 dB fromEOE to 23 degrees off nadir; the power drop off between EOE and ±23 degrees shall be in a monotonicallydecreasing fashion.The GPS III SV shall provide L1 and L2 signals with the following characteristic: the L1 off-axis relative power(referenced to peak transmitted power) shall not decrease by more than 2 dB from the Edge-of-Earth (EOE) to nadir;the L2 off-axis power gain shall not decrease by more than 2 dB from EOE to nadir; the power drop off betweenEOE and ±26 degrees shall be in a monotonically decreasing fashion.
Additional related data is provided assupporting material in paragraph 6.3.1.17IS-GPS-200F21 Sep 2011Table 3-IV.Composite L1 Transmitted Signal Phase ** (Block II/IIA and IIR SVs Only)Nominal Composite L1Signal Phase*0°-70.5°+109.5°180°**Code StatePC/A01010011*Relative to 0, 0 code state with positive angles leading and negative angles lagging.Based on the composite of two L1 carrier components with 3 dB difference in the power levels of the two.Table 3-Va.Received Minimum RF Signal Strength for Block IIA, IIR, IIR-M, IIF and III Satellites(20.46 MHz Bandwidth)SignalSV BlocksChannelP(Y)C/A or L2 CL1-161.5 dBW-158.5 dBWL2-164.5 dBW-164.5 dBWL1-161.5 dBW-158.5 dBWL2-161.5 dBW-160.0 dBWL1-161.5 dBW-158.5 dBWL2-161.5 dBW-158.5 dBWIIA/IIRIIR-M/IIFIIITable 3-Vb.Received Minimum RF Signal Strength for GPS III (30.69 MHz Bandwidth)SignalSV BlocksChannelP(Y)C/A or L2 CL1-161.5 dBW-158.5 dBWL2-161.5 dBW-158.5 dBWIII18IS-GPS-200F21 Sep 20113.3.1.6.1 Space Service Volume (SSV) User-Received Signal Levels.
The SV shall provide L1 and L2 navigationsignal strength at end-of-life (EOL), worst-case, in order to meet the minimum levels specified in Table 3-Vc. Theminimum received power is measured at the output of a 0 dBi right-hand circularly polarized (i.e. 0 dB axial ratio)user receiving antenna at normal orientation, at the off-nadir angles defined in Table 3-Vc. The received signallevels are observed within the in-band allocation defined in paragraph. 3.3.1.1.Table 3-Vc.Space Service Volume (SSV) Received Minimum RF Signal Strength for GPS III andSubsequent Satellites over the Bandwidth Specified in 3.3.1.1 – GEO Based AntennasSignalSV BlocksChannelOff Axis AngleRelative To NadirIII andSubsequentL123.5 deg-187.0 dBW*-184.0 dBW*L226.0 deg-186.0 dBW-183.0 dBWP(Y)C/A or L2 C* Over 99.5% of the solid angle inside a cone with its apex at the SV and measured from 0 degrees at the centerof the Earth19IS-GPS-200F21 Sep 20113.3.1.7 Equipment Group Delay.
Equipment group delay is defined as the delay between the signal radiated outputof a specific SV (measured at the antenna phase center) and the output of that SV's on-board frequency source; thedelay consists of a bias term and an uncertainty. The bias term is of no concern to the US since it is included in theclock correction parameters relayed in the NAV data, and is therefore accounted for by the user computations ofsystem time (reference paragraphs 20.3.3.3.3.1, 30.3.3.2.3).
The uncertainty (variation) of this delay as well as thegroup delay differential between the signals of L1 and L2 are defined in the following.3.3.1.7.1 Group Delay Uncertainty. The effective uncertainty of the group delay shall not exceed 3.0 nanoseconds(95% probability).3.3.1.7.2 Group Delay Differential. The group delay differential between the radiated L1 and L2 signals (i.e.
L1P(Y) and L2 P(Y), L1 P(Y) and L2C) is specified as consisting of random plus bias components. The meandifferential is defined as the bias component and will be either positive or negative. For a given navigation payloadredundancy configuration, the absolute value of the mean differential delay shall not exceed 15.0 nanoseconds. Therandom plus non-random variations about the mean shall not exceed 3.0 nanoseconds (95% probability), whenincluding consideration of the temperature and antenna effects during a vehicle orbital revolution. Corrections forthe bias components of the group delay differential are provided to the US in the NAV/CNAV message usingparameters designated as TGD (reference paragraph 20.3.3.3.3.2) and Inter-Signal Correction (ISC) (referenceparagraph 30.3.3.3.1.1).3.3.1.7.3 Space Service Volume Group Delay Differential.
The group delay differential between the radiated L1and L2 signals with respect to the Earth Coverage signal for users of the Space Service Volume are provided inhttp://www.igs.org/products/ssv.3.3.1.8 Signal Coherence. All transmitted signals for a particular SV shall be coherently derived from the same onboard frequency standard.
On the L1 carrier, the chip transitions of the modulating signals, C/A and L1 P(Y), andon the L2 carrier the chip transitions of L2 P(Y) and L2C, shall be such that the average time difference between thechips on the same carrier do not exceed 10 nanoseconds.The variable time difference shall not exceed 1nanosecond (95% probability), when including consideration of the temperature and antenna effect changes during avehicle orbital revolution. Corrections for the bias components of the time difference are provided to the US in theCNAV message using parameters designated as ISCs (reference paragraph 30.3.3.3.1.1).3.3.1.9 Signal Polarization.
The transmitted signal shall be right-hand circularly polarized (RHCP). For the angularrange of ±13.8 degrees from nadir, L1 ellipticity shall be no worse than 1.2 dB for Block IIA and shall be no worsethan 1.8 dB for Block IIR/IIR-M/IIF/GPS III SVs. L2 ellipticity shall be no worse than 3.2 dB for Block II/IIA SVsand shall be no worse than 2.2 dB for Block IIR/IIR-M/IIF and GPS III SVs over the angular range of ±13.8 degreesfrom nadir.20IS-GPS-200F21 Sep 20113.3.2 PRN Code Characteristics. The characteristics of the P-, L2 CM-, L2 CL-, and the C/A-codes are definedbelow in terms of their structure and the basic method used for generating them. Figure 3-1 depicts a simplifiedblock diagram of the scheme for generating the 10.23 Mbps Pi(t) and the 1.023 Mbps Gi(t) patterns (referred to as Pand C/A-codes respectively), and for modulo-2 summing these patterns with the NAV bit train, D(t), which isclocked at 50 bps.
The resultant composite bit trains are then used to modulate the signal carriers.3.3.2.1 Code Structure. For PRN codes 1 through 37, the Pi(t) pattern (P-code) is generated by the modulo-2summation of two PRN codes, X1(t) and X2(t - iT), where T is the period of one P-code chip and equals (1.023E7)-1seconds, while i is an integer from 1 through 37. This allows the generation of 37 unique P(t) code phases(identified in Table 3-Ia) using the same basic code generator.Expanded P-code PRN sequences, Pi(t) where 38 ≤ i ≤ 63, are described as follows:Pi(t) = Pi-37(t - T) where T will equal 24 hours)therefore, the equation isPi(t) = Pi-37x(t + i * 24 hours),where i is an integer from 64 to 210, x is an integer portion of (i-1)/37.As an example, the P-code sequence for PRN 38 is the same sequence as PRN 1 shifted 24 hours into a week (i.e.1st chip of PRN 38 at beginning of week is the same chip for PRN 1 at 24 hours after beginning of week).
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