IGS Analysis Center Workshop - IGS - NASA

ecause of the above averaging. This confirms that:x~ (t) = ~ k ix i () t ,i=lalso satisfies the equation of motion provided that the weights ki are constant, ~ki = 1 and(Xi (t) - X~ (t)) are small.The second and more elegant approach is to combine initial satellite state vectors and associatedsolar pressure parameters. Under certain conditions this approach is equivalent to the introductionof individual orbit series as pseudo-observations into an orbit improvement program. This is theapproach proposed in Beutler et al. (1993) and further pursued in Section 3. The advantage of thisorbit combination method is that orbital dynamics is maintained. Furthermore, arcs longer than 24hours are possible (with less discontinuities), reliability is increased (even when all Centers arebiased on the same day a correct solution, compatible with the neighboring days, is still possible)and this approach is directly compatible with a future rigorous/distributed processing. Thedisadvantages are the increased complexity and the requirement for a different algorithm for clockprocessing.For both approaches it is essential that small systematic differences in coordinate and timereference frames are reconciled before any weighting is applied to ensure unbiased estimation in aconsistent reference frame. This is a complex problem which requires a close cooperation withIERS and for the time with BIPM (Bureau International des Poids et Mesures). For orbitcombinations it can approximately and implicitly be accomplished through a set of constrainedtracking station positions, which are well determined and maintained by IERS to be compatiblewith IERS EOP. This is, in fact the practice already in effect. Additionally, the individual CenterEOP solutions can be used here for removing further small differential rotations around X- and Y-coordinate axes, assuming that the corresponding orbits and EOP are consistent which may not betrue for some Centers (Springer and Beutler, 1993): Small unexplained systematic differences canalso be determined from previous and current orbit comparisons/ determinations along with longterm statistics. These can be used for apriori parameters and weighting in an orbit combinationadjustment. In this way, in an average sense, the proper reference frame would be maintained andused and no EOP parameters would be required. Another possibility is to use, for the apriori X andY rotation parameters, the average pole offsets as determined by IERS for each Center. The timereference ideally should be compatible with UTC as determined by BIPM. In fact BIPMinvolvement in IGS would be mutually beneficial, as IGS could provide clock transfer and timesystem consistency better than one ns, which is almost an order of magnitude improvement withrespect to the current state of the art time transfer capability and reference time maintenance. BIPMin return could provide precise time reference for IGS.Operationally, a combination of the above two methods maybe desirable. The first approach canprovide a quick look and feed back, it can also handle the reference frame transformation,appropriate weighting, blunder detection and rejections, etc. Then, the weighted average orbit canbe introduced into the second approach, providing additional quality check and producing the finalIGS orbit consistent with satellite dynamics. The quality of the proposed IGS combined orbits istested in Section 4 by processing several continental baselines. Finally, in Section 5, a combinationof satellite clock solutions is considered, which is similar to the orbit combination of Section 2.23