IS-GPS-800D (797938), страница 9
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Integrity properties of the IAURANEDare specified with respect to the scaled (multiplied by either 4.42 or 5.73 as appropriate) upperbound values of the URANED0 index, URANED2 index, and URANED2 index (see 3.5.3.10.1).URANED0 accounts for zeroth order SIS contributions to user range error which include, but arenot limited to, the following: LSB representation/truncation error; the net effect of clockcorrection polynomial error and code phase error in the transmitted signal for single-frequencyL1C/A or single-frequency L2C users who correct the code phase as described in Section3.5.3.9; the net effect of clock parameter, code phase, and inter-signal correction error for dualfrequency L1/L2 and L1/L5 users who correct for group delay and ionospheric effects asdescribed in Section 3.5.3.9; radial ephemeris error; anisotropic antenna errors; and signaldeformation error.
URANED does not account for user range contributions due to the inaccuracyof the broadcast ionospheric data parameters used in the single-frequency ionospheric model orfor other atmospheric effects.The transmitted URANED1 index is an integer value in the range 0 to 7. The URANED1 index hasthe following relationship to the URA NED1 value:1NURANED1 = 2 (meters/second)whereN = 14 + URANED1 Index.The transmitted URANED2 index is an integer value in the range 0 to 7.
URANED2 index has thefollowing relationship to the URANED2:1NURANED2 = 2 (meters/second2)whereN = 28 + URANED2 Index.3.5.3.9 Group Delay Correction ParametersBits 527 through 565 of subframe 2 provide the group delay differential correction terms for L1Csignal users. The following algorithms shall apply when interpreting the correction parameters52IS-GPS-800D24 Sep 2013in the message.
The bit lengths, scale factors, ranges, and units of these parameters are given inTable 3.5-1.The bit string of “1000000000000” shall indicate that the group delay value is not available. Therelated algorithm is given in paragraphs 3.5.3.9.1 and 3.5.3.9.2.The broadcast group delay differential correction terms apply to the pseudorange measurementsproduced by an ideal correlation receiver with a bandwidth of 20.46 MHz centered at thereference waveform nominal frequency whose phase is linear over that bandwidth using an exactreplica of the reference waveform in an early-late discriminator having a correlator spacing of97.75 nanoseconds.
The group delay differential correction terms may not apply to pseudorangemeasurements produced by different methods due to potential distortion of the transmittedwaveform. Users employing pseudorange measurement methods different from the definedmethod must account for the potential inapplicability of the group delay differential correctionterms.3.5.3.9.1 Inter-Signal CorrectionThe correction terms, TGD, ISCL1CP, and ISCL1CD, are initially provided by the CS to account forthe effect of SV inter-signal biases between L1 P(Y) and L2 P(Y), between L1 P(Y) and L1CP,and between L1 P(Y) and L1CD, respectively, based on measurements made by the SVcontractor during SV manufacture.
The values of TGD and ISCs for each SV may besubsequently updated to reflect the actual on-orbit group delay differential. For maximumaccuracy, the single frequency L1CP user must use the correction terms to make furthermodifications to the code phase offset in paragraph 20.3.3.3.3.1 of IS-GPS-200 with theequation:(∆tSV)L1CP = ∆tSV - TGD + ISCL1CPwhere TGD (see paragraph 20.3.3.3.3.2 of IS-GPS-200) and ISCL1CP are provided to the user assubframe 2 data. For maximum accuracy, the single frequency L1CD user must use thecorrection terms to make further modifications to the code phase offset given by:(∆tSV)L1CD = ∆tSV - TGD + ISCL1CDwhere, ISCL1CD is provided to the user as subframe 2 data.The values of ISCL1CP and ISCL1CD are measured values that represent the mean SV group delaydifferential between the L1 P(Y)-code and the L1CP-code or L1CD-code respectively as follows,ISCL1CP = tL1P(Y) - tL1CPISCL1CD = tL1P(Y) - tL1CD,53IS-GPS-800D24 Sep 2013where tL1x is the GPS time of the L1 frequency x signal (a specific epoch of the signal)transmitted from the SV antenna phase center.3.5.3.9.2 L1/L2 Ionospheric CorrectionThe two frequency (L1CP and L2C) user shall correct for the group delay and ionospheric effectsby applying the relationshipPR =(PR L2C − γ12 PR L1CP ) + c (ISCL2C − γ12 ISCL1CP )− c TGD1 − γ12.The two frequency (L1CD and L2C) user shall correct for the group delay and ionospheric effectsby applying the relationshipPR =(PR L2C − γ12 PR L1CD ) + c (ISCL2C − γ12 ISCL1CD )− c TGD1 − γ12For the preceding equations, the following definitions apply:PR=pseudorange corrected for ionospheric effects,PRi=pseudorange measured on the channel indicated by the subscript,ISCi =inter-signal correction for the channel indicated by the subscript (seeparagraph 3.5.3.9.1, see paragraph 30.3.3.3.1.1 of IS-GPS-200 for ISCL2C),TGD=see paragraph 20.3.3.3.3.2 of IS-GPS-200,c=speed of light,and, denoting the nominal center frequencies of L1 and L2 as fL1 and fL2 respectively,γ12 = (fL1/fL2)2 = (1575.42/1227.6)2 = (77/60)2.3.5.3.9.3 L1/L5 Ionospheric Correction54IS-GPS-800D24 Sep 2013The two frequency (L1CP and L5 Q5) user shall correct for the group delay and ionosphericeffects by applying the relationshipPR =(PR L5Q5 − γ15PR L1CP ) + c (ISCL5Q5 − γ15ISCL1CP )1 − γ15− c TGD.The two frequency (L1CD and L5 Q5) user shall correct for the group delay and ionosphericeffects by applying the relationshipPR =(PR L5Q5 − γ15PR L1CD ) + c (ISCL5Q5 − γ15ISCL1CD )1 − γ15− c TGD.The two frequency (L1CP and L5 I5) user shall correct for the group delay and ionosphericeffects by applying the relationshipPR =(PR L5I5 − γ15PR L1CP ) + c (ISCL5I5 − γ15ISCL1CP )1 − γ15− c TGD.The two frequency (L1CD and L5 I5) user shall correct for the group delay and ionosphericeffects by applying the relationshipPR =(PR L5I5 − γ15PR L1CD ) + c (ISCL5I5 − γ15ISCL1CD )− c TGD1 − γ15.For the preceding equations, the following definitions apply:PR=pseudorange corrected for ionospheric effects,PRi=pseudorange measured on the channel indicated by the subscript,ISCi =inter-signal correction for the channel indicated by the subscript (seeparagraph 3.5.3.9.1, see paragraph 20.3.3.3.1.2.1 of IS-GPS-705 for ISCL5Q5 and ISCL5I5),TGD=see paragraph 20.3.3.3.3.2 of IS-GPS-200,c=speed of light,and, denoting the nominal center frequencies of L1 and L5 as fL1 and fL5 respectively,γ15 = (fL1/fL5)2 = (1575.42/1176.45)2 = (154/115)2.55IS-GPS-800D24 Sep 20133.5.3.10 Integrity AssuranceThe L1C message will contain information that allows users to operate when integrity is assured.This is accomplished using an integrity assured URA value in conjunction with an integritystatus flag.
The URA value is the RSS of URAED and URANED; URA is integrity assured to theenhanced level only when the integrity status flag is “1”.3.5.3.10.1 Integrity Status Flag (ISF)Bit 566 of subframe 2 shall be the Integrity Status Flag (ISF). A “0” in bit position 566 indicatesthat the conveying signal is provided with the legacy level of integrity assurance. That is, theprobability that the instantaneous URE of the conveying signal exceeds 4.42 times the upperbound value of the current broadcast URA index, for more than 5.2 seconds, without anaccompanying alert, is less than 1 x 10-5 per hour. A “1” in bit position 566 indicates that theconveying signal is provided with an enhanced level of integrity assurance.
That is, theprobability that the instantaneous URE of the conveying signal exceeds 5.73 times the upperbound value of the current broadcast URA index, for more than 5.2 seconds, without anaccompanying alert, is less than 1 x 10-8 per hour. The probabilities associated with the nominaland lower bound values of the current broadcast URA are not defined.In this context, an “alert” is defined as any indication or characteristic in the conveying signal, asspecified elsewhere in this document, which signifies that the conveying signal may be invalidand should not be used, such as, not Operational-Healthy, Non-Standard Code, parity error, etc.3.5.3.11 Data Predict Week Number.Bits 567-574 of Subframe 2 shall indicate the Data Predict Week Number (WNOP) to which theData Predict Time of Week (top) is referenced (see 3.5.3.3).
The WNOP term consists of eightbits which shall be a modulo 256 binary representation of the GPS week number to which the topis referenced.3.5.4 Subframe 3Subframe 3 provides users with other navigation data which is commutated over multiple pages.The contents of subframe 3 data are defined below.Every subframe 3 page begins with an 8-bit PRN number of the transmitting SV with a range of0 (00000000) to 255 (11111111). Each subframe 3 page is identified by a 6-bit page numberprovided in bits 9 through 14.3.5.4.1 Subframe 3, Page 1 - UTC & IONOAs depicted in Figure 3.5-2, subframe 3, page 1 contains the UTC and ionospheric correctionparameters. The contents of subframe 3, page 1, are defined below, and followed by materialpertinent to the use of the data.3.5.4.1.1 UTC Parameter Content56IS-GPS-800D24 Sep 2013Subframe 3, page 1 shall contain the parameters related to correlating UTC (USNO) time withGPS Time.
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