IGS Analysis Center Workshop - IGS - NASA

eceive the freely estimated locations of the 3 common stations, since they provide a tool toprobe systematic orbit errors as they affect different geographical regions, Ourrecommendation to regional analysis centers is the same as for global analysis centers: the 3(or more) common stations should be constrained with an a priori standard deviation of 10meters, but the nominal values should be consistent with the ITRF at the level of 1 meter orb e t t e r ,3.2 Combination of regional and global coordinate solutionsWe have already outlined the fiducial-free approach and why it is important for thecombination of regional and global coordinate solutions. The implicit assumption is thatregional networks will add very little additional information to the determination of orbits orEOP. We should also note that it would be undesirable to adjust further the globallydetermined “common station coordinates” based on regional network solutions, because thesame data would be used twice. Therefore, in combining regional with global solutions,we recommend that the global estimates for the common stations and their covariancematrix elements remain unperturbed by the regional solution, and that the solution be onlyaugmented by those regional stations that are not common stations. Before augmentation,however, the common station estimates in the regional solution should be combined withthe global solution in order to ensure reference frame consistency. Many of the details ofthe combination should probably be left to the NAC’S to develop, since it is not an entirelytheoretical question and techniques will undoubtedly evolve as NAC’s gain moreexperience with real data, Some of the issues that need to be addressed include relativeweighting schemes, treatment of outliers or problematic solutions, minimal constraints,treatment of network distortion of common stations in regional solutions, etc. Morecomplicated schemes will be able to derive, by backsubstitution, transformations, etc., acomplete time-series of both global and regional station coordinates, and polar motion in aconsistent reference frame.What we do need to decide on is a parameterization and a format to submit fiducial-freesolutions. This is a difficult problem due to differences in software packages.3.3 Parameterization of submitted solutionsParameterization should be flexible given that different software packages work infundamentally different ways. Both the file name and file header will indicate what type ofparameterization is used. The file name is important since it is useful for scripts to handle.The file header is important as a consistency check for the reading program.(i) We should consider allowing both Cartesian (X, Y, Z) and spherical coordinates(spherical latitude, longitude, and radius). An alternative to this is to allow only Cartesiancoordinates (as recommended by IERS), which would mean that centers such as GFZ whouse spherical parameters would have to reformat their solutions. GFZ argues that sphericalparameters have the advantage that they are more natural for imposing certain a prioriconstraints (e.g., orientation). Cartesian parameters have the advantage that they havehomogeneous units, and are more natural for imposing certain a priori constraints (e.g.,translation).(ii) We shoyld consider allowing three types of solution representation.(a) The most basic representation has the parameter estimates x, and the variancecovariancematrix M = (A ~CA)- 1. The covariance matrix is more compactly written asthe correlation matrix (because elements are from -1 to +1), augmented with the..74