IS-GPS-200F (811524), страница 26
Текст из файла (страница 26)
Integrity properties of the IAURANED are specified with respect to the scaled (multiplied by either4.42 or 5.73 as appropriate) upper bound values of the URANED0 index, URANED1 index, and URANED2 index (see30.3.3.1.1).URANED0 accounts for zeroth order SIS contributions to user range error which include, but are not limited to, thefollowing: LSB representation/truncation error; the net effect of clock correction polynomial error and code phaseerror in the transmitted signal for single-frequency L1C/A or single-frequency L2C users who correct the code phaseas described in Section 30.3.3.3.1.1.1; the net effect of clock parameter, code phase, and inter-signal correction errorfor dual-frequency L1/L2 and L1/L5 users who correct for group delay and ionospheric effects as described inSection 30.3.3.3.1.1.2; radial ephemeris error; anisotropic antenna errors; and signal deformation error.
URANEDdoes not account for user range contributions due to the inaccuracy of the broadcast ionospheric data parametersused in the single-frequency ionospheric model or for other atmospheric effects.The transmitted URANED1 index is an integer value in the range 0 to 7. The URANED1 index has the followingrelationship to the URANED1 value:1NURANED1 = 2 (meters/second)whereN = 14 + URANED1 Index167IS-GPS-200F21 Sep 2011The transmitted URANED2 index is an integer value in the range 0 to 7.URANED2 index has the followingrelationship to the URANED2:1NURANED2 = 2 (meters/second2)whereN = 28 + URANED2 Index.30.3.3.3 Message Type 30 Ionospheric and Group Delay Correction Parameters.30.3.3.3.1 Message Type 30 Ionospheric and Group Delay Correction Parameter Content.Message type 30provides SV clock correction parameters (ref.
Section 30.3.3.2) and ionospheric and group delay correctionparameters. Bits 128 through 192 of message type 30 provide the group delay differential correction terms for L1,L2, and L5 signal users. Bits 193 through 256 provide the ionospheric correction parameters for single frequencyusers. The following algorithms shall apply when interpreting the correction parameters in the message.The broadcast group delay differential correction terms apply to the pseudorange measurements produced by anideal correlation receiver with a bandwidth of 20.46 MHz centered at the reference waveform nominal frequencywhose phase is linear over that bandwidth using an exact replica of the reference waveform in an early-latediscriminator having a correlator spacing of 97.75 nanoseconds.
The group delay differential correction terms maynot apply to pseudorange measurements produced by different methods due to potential distortion of the transmittedwaveform. Users employing pseudorange measurement methods different from the defined method must accountfor the potential inapplicability of the group delay differential correction terms.30.3.3.3.1.1 Estimated L1-L2 Group Delay Differential.The group delay differential correction terms, TGD,ISCL1C/A, ISCL2C for the benefit of single frequency L1 P, L1 C/A, L2 P, L2 C users and dual frequency L1/L2 usersare contained in bits 128 through 166 of message type 30 (see Figure 30-3 for complete bit allocation).
The bitlength, scale factors, ranges, and units of these parameters are given in Table 30-IV.The bit string of“1000000000000” shall indicate that the group delay value is not available. The related algorithm is given inparagraphs 30.3.3.3.1.1.1 and 30.3.3.3.1.1.2.168IS-GPS-200F21 Sep 2011Table 30-IV.ParameterNo. ofBits**Group Delay Differential Parameters ****Scale Factor(LSB)EffectiveRange***Units-35secondsTGD13*2ISCL1C/A13*2-35secondsISCL2C13*2-35secondsISCL5I513*2-35secondsISCL5Q513*2-35seconds*Parameters so indicated are two’s complement with the sign bit (+ or -) occupying the MSB;**See Figure 30-3 for complete bit allocation in Message type 30;***Effective range is the maximum range attainable with indicated bit allocation and scale factor;**** The bit string of “1000000000000” will indicate that the group delay value is not available.30.3.3.3.1.1.1 Inter-Signal Correction.
The correction terms, TGD, ISCL1C/A and ISCL2C, are initially provided by theCS to account for the effect of inter-signal biases between L1 P(Y) and L2 P(Y), L1 P(Y) and L1 C/A, and betweenL1 P(Y) and L2 C, respectively, based on measurements made by the SV contractor during SV manufacture. Thevalues of TGD and ISCs for each SV may be subsequently updated to reflect the actual on-orbit group delaydifferential.
For maximum accuracy, the single frequency L1 C/A user must use the correction terms to makefurther modifications to the code phase offset in paragraph 20.3.3.3.3.1 with the equation:(∆tSV)L1C/A = ∆tSV - TGD + ISCL1C/Awhere TGD (see paragraph 20.3.3.3.3.2) and ISCL1C/A are provided to the user as Message Type 30 data, described inparagraph 30.3.3.3.1.1. For the single frequency L2 C user, the code phase offset modification is given by:(∆tSV)L2C = ∆tSV - TGD + ISCL2Cwhere, ISCL2C is provided to the user as Message Type 30 data.169IS-GPS-200F21 Sep 2011The values of ISCL1C/A and ISCL2C are measured values that represent the mean SV group delay differential betweenthe L1 P(Y)-code and the L1 C/A- or L2 C-codes respectively as follows,ISCL1C/A = tL1P(Y) - tL1C/AISCL2C = tL1P(Y) - tL2Cwhere, tLix is the GPS time the ith frequency x signal (a specific epoch of the signal) is transmitted from the SVantenna phase center.30.3.3.3.1.1.2 L1/L2 Ionospheric Correction.
The two frequency (L1 C/A and L2 C) user shall correct for the groupdelay and ionospheric effects by applying the relationship:PR =( PRL 2C − γ 12 PRL1C / A ) + c ( ISC L 2C − γ 12 ISC L1C / A )− c TGD1 − γ 12where,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 (see paragraph 30.3.3.3.1.1),TGD = see paragraph 20.3.3.3.3.2,c = speed of light,and where, 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)2.170IS-GPS-200F21 Sep 201130.3.3.3.1.2 Ionospheric Data.
The ionospheric parameters which allow the “L1 only” or “L2 only” user to utilizethe ionospheric model for computation of the ionospheric delay are contained in Message Type 30. The “onefrequency” user should use the model given in paragraph 20.3.3.5.2.5 to make this correction for the ionosphericeffects. The bit lengths, scale factors, ranges, and units of these parameters are given in Table 20-X.The ionospheric data shall be updated by the CS at least once every six days while the CS is able to upload the SVs.If the CS is unable to upload the SVs, the ionospheric data transmitted by the SVs may not be accurate.171IS-GPS-200F21 Sep 201130.3.3.4 Message Types 31, 12 and 37 Almanac Types. The almanac parameters are provided in any one ofmessage types 31, 37, and 12.
Message type 37 provides Midi almanac parameters and the reduced almanacparameters are provided in either message type 31 or type 12. The SV shall broadcast both message types 31(and/or 12) and 37. However, the reduced almanac parameters (i.e. message types 31 and/or 12) for the complete setof SVs in the constellation will be broadcast by a SV using shorter duration of time compared to the broadcast of thecomplete set of Midi almanac parameters (i.e. message type 37).
The parameters are defined below, followed bymaterial pertinent to the use of the data.30.3.3.4.1 Almanac Reference Week. Bits 39 through 51 of message type 12, and bits 128 through 140 of messagetypes 31 and 37 shall indicate the number of the week (WNa-n) to which the almanac reference time (toa) isreferenced (see paragraph 20.3.3.5.2.2). The WNa-n term consists of 13 bits which shall be a modulo-8192 binaryrepresentation of the GPS week number (see paragraph 6.2.4) to which the toa is referenced. Bits 52 through 59 ofmessage type 12, and bits 141 to 148 of message types 31 and 37 shall contain the value of toa, which is referencedto this WNa-n.30.3.3.4.2 Almanac Reference Time.
See paragraph 20.3.3.5.2.2.30.3.3.4.3 SV PRN Number. Bits 149 through 154 of message type 37 and bits 1 through 6 in each packet ofreduced almanac shall specify PRN number of the SV whose almanac or reduced almanac, respectively, is providedin the message or in the packet.30.3.3.4.4 Signal Health (L1/L2/L5). The three, one-bit, health indication in bits 155, 156, and 157 of message type37 and bits 29,30 and 31 of each packet of reduced almanac refers to the L1, L2, and L5 signals of the SV whosePRN number is specified in the message or in the packet.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
