Satellite Orbit and Ephemeris Determination using Inter Satellite Links
Satellite Orbit and Ephemeris Determination using Inter Satellite Links Satellite Orbit and Ephemeris Determination using Inter Satellite Links
Software DescriptionInter Satellite LinksThe first term accounts for the scale factor between synodal and sidereal rotation period. Thesecond term represents the actual sidereal time at the Greenwich meridian and is computedusing the following formulationϑ26 30 = 24110.54841+ 8640184.812866 ⋅ T0+ 0.093104 ⋅ T0− 6.2 ⋅10⋅ T0−Eq. 5.4-9where ϑ 0 is in seconds. T 0 is the interval between the standard epoch of J2000 and date ofobservation at 0 h UT.The third term accounts for nutation. While UT1 is a continuos time scale, coupled withearth's rotation, Universal Time Coordinated (UTC) is a realisation of UT1 using atomicclocks. The relationship between UT1 and UTC is expressed byUT 1 UTC + dUT1= Eq. 5.4-10The quantity dUT1 has an absolute value of less than 1 second and is determined by the IERS.If dUT1 gets larger than 0.9 seconds, a leap second is added to UTC.Page 86R. Wolf
Inter Satellite LinksSoftware Description5.5 Broadcast EphemerisThe broadcast ephemeris of a satellite is not obtained directly from the observations, butadjusted to the position vectors within a specified interval, i.e. the period of validity. Theposition vectors have been derived by propagating the satellites state from a known stateforward. This "known state" can be a deterministic initial state, if it is derived fromsimulation, or the best estimate at a certain time, derived from measurements. The latterwould be the case in an operational satellite navigation system.5.5.1 Adjustment of the Broadcast MessageThe broadcast message has to be adjusted to the precise ephemeris, determined and predictedby the orbit estimation process. The "observations" used to feed the adjustment process, are atime series of precise ephemeris position vectors. Figure 5-7 shows the basic adjustmentprocess.Precise Propagator=>PositionsBCE Propagator=> Positions+ -Least SquaresAdjustmentCompute DerivativesNew State forBCE-PropagatorPositionErrorNoYesBCE Message validfor fit intervalFigure 5-7 Broadcast Message AdjustmentR. Wolf Page 87
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Software Description<strong>Inter</strong> <strong>Satellite</strong> <strong>Links</strong>The first term accounts for the scale factor between synodal <strong>and</strong> sidereal rotation period. Thesecond term represents the actual sidereal time at the Greenwich meridian <strong>and</strong> is computed<strong>using</strong> the following formulationϑ26 30 = 24110.54841+ 8640184.812866 ⋅ T0+ 0.093104 ⋅ T0− 6.2 ⋅10⋅ T0−Eq. 5.4-9where ϑ 0 is in seconds. T 0 is the interval between the st<strong>and</strong>ard epoch of J2000 <strong>and</strong> date ofobservation at 0 h UT.The third term accounts for nutation. While UT1 is a continuos time scale, coupled withearth's rotation, Universal Time Coordinated (UTC) is a realisation of UT1 <strong>using</strong> atomicclocks. The relationship between UT1 <strong>and</strong> UTC is expressed byUT 1 UTC + dUT1= Eq. 5.4-10The quantity dUT1 has an absolute value of less than 1 second <strong>and</strong> is determined by the IERS.If dUT1 gets larger than 0.9 seconds, a leap second is added to UTC.Page 86R. Wolf
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