IS-GPS-200H (797934), страница 25
Текст из файла (страница 25)
The values of TGD and ISCs for each SV may be subsequently updatedto reflect the actual on-orbit group delay differential. For maximum accuracy, the singlefrequency L1 C/A user must use the correction terms to make further modifications to the codephase offset in paragraph 20.3.3.3.3.1 with the equation:(∆tSV)L1C/A = ∆tSV - TGD + ISCL1C/A171IS-GPS-200H24 Sep 2013where TGD (see paragraph 20.3.3.3.3.2) and ISCL1C/A are provided to the user as Message Type30 data, described in paragraph 30.3.3.3.1.1.
For the single frequency L2 C user, the code phaseoffset modification is given by:(∆tSV)L2C = ∆tSV - TGD + ISCL2Cwhere, ISCL2C is provided to the user as Message Type 30 data.The values of ISCL1C/A and ISCL2C are measured values that represent the mean SV group delaydifferential between the 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) istransmitted from the SV antenna 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 group delay and ionosphericeffects 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,172IS-GPS-200H24 Sep 2013ISCi = inter-signal correction for the channel indicated by the subscript (see paragraph30.3.3.3.1.1),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)2.30.3.3.3.1.2 Ionospheric Data.The ionospheric parameters which allow the “L1 only” or “L2 only” user to utilize theionospheric model for computation of the ionospheric delay are contained in Message Type 30.The “one frequency” user should use the model given in paragraph 20.3.3.5.2.5 to make thiscorrection for the ionospheric effects.
The bit lengths, scale factors, ranges, and units of theseparameters 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 ableto upload the SVs. If the CS is unable to upload the SVs, the ionospheric data transmitted by theSVs may not be accurate.30.3.3.3.1.3 Data Predict Week Number.Bits 257-264 of Message Type 30 shall indicate the Data Predict Week Number (WNop) to whichthe Data Predict Time of Week (top) is referenced (see 30.3.3.1.1.3 and 30.3.3.2.1.2). The WNopterm consists of eight bits which shall be a modulo 256 binary representation of the GPS weeknumber to which the top is referenced.
The user must account for the truncated nature of WNop inall calculations in which WNop is used.30.3.3.4 Message Types 31, 12, and 37 Almanac Parameters.The almanac parameters are provided in any one of message types 31, 37, and 12. Message type37 provides Midi almanac parameters and the reduced almanac parameters are provided in eithermessage 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 tothe broadcast of the complete set of Midi almanac parameters (i.e. message type 37). Theparameters are defined below, followed by material 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 message types 31 and 37shall 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 amodulo-8192 binary representation of the GPS week number (see paragraph 6.2.4) to which the173IS-GPS-200H24 Sep 2013toa is referenced. Bits 52 through 59 of message type 12, and bits 141 to 148 of message types 31and 37 shall contain the value of toa, which is referenced to 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 of reduced almanacshall specify PRN number of the SV whose almanac or reduced almanac, respectively, isprovided in 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 type 37 and bits 29,30and 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.
For each health indicator, a “0”signifies that all signals on the associated frequency are okay and “1” signifies that some or allsignals on the associated frequency are bad. The predicted health data will be updated at thetime of upload when a new reduced almanac has been built by the CS. The transmitted healthdata may not correspond to the actual health of the transmitting SV or other SVs in theconstellation.30.3.3.4.5 Midi Almanac Parameter Content.Message type 37, Figure 30-10, provides Midi almanac data for a SV whose PRN number isspecified in the message. The number of bits, the scale factor (LSB), the range, and the units ofthe almanac parameters are given in Table 30-V.
The user algorithm is essentially the same asthe user algorithm used for computing the precise ephemeris as specified in Table 20-IV. Otherparameters appearing in the equations of Table 20-IV, but not provided by the Midi almanacwith the reference values, are set to zero for SV position determination. See paragraph20.3.3.5.2.3 for almanac time parameters.30.3.3.4.6 Reduced Almanac Parameter Content.Message type 31, Figure 30-4, provides SV clock correction parameters (ref. Section 30.3.3.2)and reduced almanac data packets for 4 SVs.
Message type 12, Figure 30-11, contains reducedalmanac data packets for 7 SVs.30.3.3.4.6.1 Reduced Almanac Data.Message type 31 or 12 contains reduced almanac data and SV health words for SVs in theconstellation. The reduced almanac data of a SV is broadcast in a packet of 31 bits long, asdescribed in Figure 30-15.
The reduced almanac data are a subset of the almanac data whichprovide less precise ephemeris. The reduced almanac data values are provided relative to prespecified reference values. The number of bits, the scale factor (LSB), the range, and the units of174IS-GPS-200H24 Sep 2013the reduced almanac parameters are given in Table 30-VI. The algorithms and other materialrelated to the use of the reduced almanac data are given in Section 30.3.3.4.6.2.The reduced almanac parameters shall be updated by the CS at least once every 3 days while theCS is able to upload the SVs. If the CS is unable to upload the SVs, the accuracy of the reducedalmanac parameters transmitted by the SVs will degrade over time.A 6-bit value of “000000” in the PRNa field shall indicate that no further Status Words arecontained in the remainder of the data block.
In this event, all subsequent bits in the data blockfield shall be filler bits, i.e., alternating ones and zeros beginning with one.30.3.3.4.6.2 Reduced Almanac Packet.The following shall apply when interpreting the data provided in each packet of reduced almanac(see Figure 30-15).30.3.3.4.6.2.1 Reduced Almanac.The reduced almanac data is provided in bits 7 through 28 of each packet. The data from apacket along with the reference values (see Table 30-VI) provide ephemeris with further reducedprecision. The user algorithm is essentially the same as the user algorithm used for computingthe precise ephemeris from the parameters of the message types 10 and 11 (see paragraph30.3.3.1.3 and Table 30-II).
Other parameters appearing in the equations of Table 30-II, but notprovided by the reduced almanac with the reference values, are set to zero for SV positiondetermination.175IS-GPS-200H24 Sep 2013Table 30-V. Midi Almanac ParametersParameterNo. ofBits**ScaleFactor(LSB)EffectiveRange***Unitstoa8212602,112secondse112-16dimensionlessδi****11*2-14semi-circles11*2-33semi-circles/secA172-4Ω016*2-15semi-circlesω16*2-15semi-circlesM016*2-15semi-circlesaf011*2-20secondsaf110*2-37sec/sec•Ωmeters* Parameters so indicated shall be two's complement with the sign bit (+ or -) occupying the MSB;** See Figure 30-10 for complete bit allocation in message type 37;*** Unless otherwise indicated in this column, effective range is the maximum range attainable withindicated bit allocation and scale factor;**** Relative to i0 = 0.30 semi-circles.176IS-GPS-200H24 Sep 201331 BITS171522δAPRN a6 BITSΩ8 BITS29 300Φ07 BITS7 BITS31L1 HEALTHL2 HEALTH*See Figures 30 -4 and 30 -11 for complete bit allocation in therespective messages.L5 HEALTHFigure 30-15.
Reduced Almanac Packet ContentTable 30-VI.ParameterNo. of BitsReduced Almanac Parameters *****Scale Factor (LSB)Effective Range **Units+9**metersδA ***8*2Ω07*2-6**semi-circles7*-6**semi-circlesΦ0 ****2*Parameters so indicated shall be two’s complement with the sign bit (+ or -) occupying the MSB;**Effective range is the maximum range attainable with indicated bit allocation and scale factor;***Relative to Aref = 26,559,710 meters;****Φ0 = Argument of Latitude at Reference Time = M0 + ω;*****Relative to following reference values:e=0δi = +0.0056 semi-circles (i = 55 degrees)•Ω = -2.6 x 10-9 semi-circles/second.30.3.3.5 Message Type 32 Earth Orientation Parameters (EOP).The earth orientation parameters are provided in message type 32.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
