European GNSS (Galileo) open service (797926), страница 12
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Galileo Clock Correction DatarP5.1.4. Satellite Time Correction AlgorithmFoEach satellite transmits time correction data. The predicted offset of the physical satellitesignal TOT relative to the satellite signal TOT in GST can be computed for the dual frequencysignal combination using the following formula:TOTc(X) = TOTm(X) - ΔtSV(X)Eq. 12where●● (X)=(f1,f2) is the dual frequency combination f1 and f2 used for the clock model●● TOTC(X) is the corrected satellite TOT in GST for the signal combination X●● TOTm(X) is the physical satellite TOT for the signal combination X retrieved throughpseudo-range measurements●● ΔtSV(X) is the satellite time correction for the signal combination X computed by meansof the time correction data retrieved from the navigation message© European Union 2014Document subject to terms of use and disclaimers p.
i-iiOS SIS ICD, Issue 1.2, ?????????? 201443This satellite time correction (in seconds) is modelled through the following second orderpolynomial:( )where( )( )[( )[( )]( )]Eq. 13●● af0(X), af1(X), and af2(X) are defined in 5.1.3●● t0c(X) is the reference time for the clock correction as defined in 5.1.3tion●● t is the GST time in seconds●● Δtr , expressed in seconds, is a relativistic correction term, given byΔtr=F e A1/2 sin(E)with the orbital parameters (e, A1/2, E) as described in paragraph 5.1.1 andltaAFCoTnF = -2μ1/2/c2 = ‑4.442807309 x 10-10 s/m1/25.1.5. Broadcast Group DelaysuThe Broadcast Group Delay BGD(f1,f2) broadcast through the Galileo navigation messageis defined as follows:TR1 − TR2 f 1 − 1 f2 2Eq.
14DRBGD( f1 , f 2 ) =where●● f1 and f2 denote the carrier frequencies of two Galileo signalslic●● TR1 and TR2 are the group delays of the signals whose carrier frequencies arerespectively f1 and f2.A single frequency user receiver processing pseudo-ranges from the frequency f1 appliesthe following correction to the SV clock correction ΔtSV which is defined in paragraph 5.1.4ubΔt SV ( f1 ) = Δt SV ( f1 , f 2 ) − BGD( f1 , f 2 )Eq. 15rPA single frequency user receiver processing pseudo-ranges from the frequency f2 appliesthe following correction to the SV clock correction ΔtSV which is defined in paragraph 5.1.42 f Δt SV ( f 2 ) = Δt SV ( f1 , f 2 ) − 1 BGD( f1 , f 2 ) f2 Eq. 16FoA dual frequency user receiver processing pseudo-ranges from the two frequencies f1 andf2 does not apply any additional correction for group delay.
The Broadcast Group Delay iscoded according to the values stated in Table 62.ParameterDefinitionBitsScalefactorUnitBGD(E1,E5a)E1-E5a Broadcast Group Delay10*2-32sBGD(E1,E5b)E1-E5b Broadcast Group Delay10*2-32sTable 62. BGD Parameters*44Parameters so indicated are two’s complement, with the sign bit (+ or -) occupying the MSB.© European Union 2014Document subject to terms of use and disclaimers p.
i-iiOS SIS ICD, Issue 1.2, ?????????? 2014Each Galileo satellite broadcasts its own BGD data for all signals, through the relevantsignal according to Table 63.Message Type Type of Satellite ClocksBGD(f1,f2)ServicesF/NAV(E1,E5a)BGD(E1,E5a)Single-frequency E5aI/NAV(E1,E5b)BGD(E1,E5b)Single-frequency E1Single-frequency E5bTable 63. BGD Values Mapping on Messages and Servicestion5.1.6. Ionospheric CorrectionThe ionospheric model parameters include:●● the broadcast coefficients ai0, ai1 and ai2 used to compute the Effective Ionisation LevelAzltaAFCoTn●● the “Ionospheric Disturbance Flag” (also referred as “storm flag”), given for five differentregionsThese parameters are transmitted according to the characteristics stated in Table 64.ai0ai1SF1SF2SF3SF4BitsScalefactorUnitEffective Ionisation Level 1st order parameter112-2sfu**Effective Ionisation Level 2nd order parameter11*2-8sfu**/degreeEffective Ionisation Level 3rd order parameter14*2-15sfu**/degree2Ionospheric Disturbance Flag for region 11N/AdimensionlessIonospheric Disturbance Flag for region 21N/AdimensionlessIonospheric Disturbance Flag for region 31N/AdimensionlessIonospheric Disturbance Flag for region 41N/AdimensionlessIonospheric Disturbance Flag for region 51N/AdimensionlessDRai2DefinitionsuParameterlicSF5Total Ionosphere Correction Size41***ubTable 64.
Ionospheric Correction ParametersParameters so indicated are two’s complement, with the sign bit (+ or -) occupying the MSB.Note that ‘sfu’ (solar flux unit) is not a SI unit but can be converted as: 1 sfu = 10-22 W/(m2*Hz)rPThe effective ionisation level is computed according to the equation below where μ is themodified dip latitude MODIP.Az = ai0 + ai1 μ + ai2 μ2Eq.
17FoThe Ionospheric Disturbance Flag has the following values: [0:No disturbance,1:Disturbance]in the region, where the regions are defined as:●●region 1: for the northern region(60°<MODIP≤90°)●●region 2: for the northern middle region(30°<MODIP≤60°)●●region 3: for the equatorial region(-30°≤MODIP≤30°)●●region 4: for the southern middle region(-60°≤MODIP<-30°)●●region 5: for the southern region(-90°≤MODIP<-60°)© European Union 2014Document subject to terms of use and disclaimers p. i-iiOS SIS ICD, Issue 1.2, ?????????? 2014455.1.7.
GST-UTC Conversion Algorithm and ParametersThe UTC time tUTC is computed through 3 different cases depending on the epoch of apossible leap second adjustment (scheduled future or recent past) given by DN, the day atthe end of which the leap second becomes effective, and week number WNLSF to which DNis referenced. “Day one” of DN is the first day relative to the end/start of week and WNLSFis the Galileo week number modulo 256.BitsScalefactorUnitA0Constant term of polynomial32*2-30sA11st order term of polynomial24*2-50s/sΔtLSLeap Second count before leap second adjustment8*1st0tUTC data reference Time of Week83600sWN0tUTC data reference Week Number81weekWNLSFWeek Number of leap second adjustment81weekDNDay Number at the end of which a leap secondadjustment becomes effective3**1dayΔtLSFLeap Second count after leap second adjustment8*1sTotal GST-UTC Conversion Size99sultationDefinitionAFCoTnParameterTable 65.
Parameters for the GST-UTC ConversionParameters so indicated are two’s complement, with the sign bit (+ or -) occupying the MSB.The value range of DN is from 1 (= Sunday) to 7 (= Saturday).DR***In addition to the parameters listed in Table 65, the following parameters are used inGST‑UTC conversion algorithm:lic●● tE is the GST as estimated by the user through its GST determination algorithm●● WN is the week number to which tE is referenced, modulo 256Case arPubWhenever the leap second adjustment time indicated by WNLSF and DN is not in the past(relative to the user’s present time) and the user’s present time does not fall in the timespan which starts six hours prior to the effective time (= DN+3/4) and ends six hours afterthe effective time at (= DN+5/4), tUTC is computed according to the following equations:((FoCase b])[Eq. 18())Whenever the user’s current time falls within the time span of six hours prior to the leapsecond adjustment time to six hours after the adjustment time, tUTC is computed accordingto the following equations (ΔtUTC as defined in case a):Case c([()[)]]Eq.
19Whenever the leap second adjustment time, as indicated by the WNLSF and DN values, isin the “past” (relative to the user’s current time) and the user’s present time does not fall46© European Union 2014Document subject to terms of use and disclaimers p.
i-iiOS SIS ICD, Issue 1.2, ?????????? 2014in the time span which starts six hours prior to the leap second adjustment time and endssix hours after the adjustment time, tUTC is computed according to the following equation:((])[(Eq. 20))The parameters for GST to UTC conversion are defined in Table 65.tion5.1.8.
GPS to Galileo System Time Conversion and ParametersThe difference between the Galileo and the GPS time scales, expressed in seconds, is givenby the equation below.[●● A0G constant term of the offset Δtsystems●● A1G rate of change of the offset Δtsystemssu●● t0G reference time for GGTO data●● tGalileo GST time (s)●● tGPS GPS time(s)●● WN GST Week NumberDREq. 21ltaAFCoTnwith])])[((●● WN0G Week Number of the GPS/Galileo Time Offset referenceThe GGTO parameters are formatted according to the values in Table 66.BitsScalefactorUnitConstant term of the polynomial describing the offsetΔtsystems16*2-35sRate of change of the offset Δtsystems12*2-51s/sReference time for GGTO data83600sWeek Number of GGTO reference61weekDefinitionA0GA1GWN0Gubt0GlicParameter42rPTotal GST-GPS Conversion SizeTable 66.
Parameters for the GPS Time to GST Offset ComputationFoWhen the GGTO is not available the GGTO parameters disseminated are: A0G (all ones -16bits), A1G (all ones - 12 bits), t0G (all ones - 8 bits), WN0G (all ones - 6 bits). When a userreceives all four parameters set to all ones the GGTO is considered as not valid.5.1.9. Service Parameters5.1.9.1. Satellite IDThe satellite Identification is coded with 6 bits and has the characteristics given in Table 67.ParameterDefinitionBitsScaleFactorUnitValuesSVIDSatellite Identification6N/Adimensionless0…63Table 67. Satellite ID© European Union 2014Document subject to terms of use and disclaimers p.
i-iiOS SIS ICD, Issue 1.2, ?????????? 201447Note: SVID = 0 is used in the broadcast almanac data to indicate unused almanac entries.SVID values 1 to 36 are defined in this OS SIS ICD. Higher values are reserved forfuture use.5.1.9.2. Issue Of DatationThe navigation data is disseminated in data batches each one identified by an Issue ofData. In nominal operation the navigation data (ephemeris, satellite clock correction andSISA) have limited validity duration depending on the data type. The identification of eachbatch by an Issue of Data (IOD) value enables:the users to distinguish the data in different batches received from each satellite●●to indicate to the user receiver the validity of the data (which have to be updatedusing new issue of navigation data)●●the user receiver to compute the full batch of data even if it misses some pagesor start receiving the data somewhere during the transmissionltaAFCoTn●●Two IODs are defined for (Table 68):the ephemeris, satellite clock correction parameters and SISA●●the almanacsBitsUnitEphemeris and Clock correction IODnav10dimensionlessAlmanacs IODa4dimensionlessDRData Typesu●●Table 68.
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