IGS Analysis Center Workshop - IGS - NASA

Figure 6 shows the activities of a NAC. Feedback on problems with orbits or referenceframe are sent to the data center, but it may also be sent directly to GAC’S and RAC’Sthrough an IGS electronic mail box. The combination frame solutions submitted byNAC’S, along with the primary and secondary frame solutions submitted by GAC’S andRAC’S, are ultimately submitted to the IGS and IERS Central Bureaus for incorporationinto the ITRF and for producing an annual Standard Frame (which is to be consistent withITRF, but is based enfirely on ~GS solutions).1, Network Analysis Centers1Combination Algorithm–~~~dSubmit Compare with Combine into kinematic SubmitWeeklyframes.solution; apply constraints AnnuallyG!!EMw~ Annual NetworkAnalysis Cente?combined reference frame,and assessment of GAC, RAC,Figure 6. NetworkAnalysis Center Flowchart3. Technical AspectsIn this paper we have focused on the concepts rather than the technical details. Completingthe details will take a considerable amount of further work; however, there are sometechnical aspects that we have felt important to clarify in this section.3.1 Fiducial-free approach and application of constraintsGlobal _‘ . The fiducial-free approach fixes no station coordinates when deriving thesolution [Herring et al., 1991; HefZin et al,, 1992]. The scale is well defined by fixing thespeed of light and GM to standard values, If both orbits and station coordinates areestimated simultaneously, the origin is by definition the Earth center of mass if the gravityfield harmonics C 10=C 11=S 11=0, If, in addition to orbits and station locations, the poleposition is estimated, then “loose” a priori constraints (to be defined below) should be,72