ILRS Analysis Working Group, Orbit Benchmark Study NERC Space Geodesy Facility Graham Appleby Presentation to AWG, 2002 Oct 3-4, HTSI, MD, USA

Orbit Comparisons Took analysis groups’ SolA, B and C geocentric ephemerides X and Xdot Formed differences, mainly from nerc, between values of X at each 2-minute epoch Resolved differences into along, across and radial directions. Plotted as functions of time

Orbit benchmark-comments (1) the initial orbits (solA) run off amongst themselves by between zero, 1m and 4m along-track; For ausg-jcet, the differences are essentially zero. For the second stage (solB): some groups (csr, crl, nerc, iaa) appear to solve for all orbital params; others (jcet, asi, ausg) probably only solve for corrections to the initial state vector and initial 4-day along-track accelerations; GEODYN groups agree with each other very well; clear discontinuities at regular (4-day) intervals in the iaa orbits.

Orbit benchmark-comments (2) For solC orbits: GEODYN groups probably not correcting their orbits as a result of the solutions for stations and EOPs; Some good agreements (nerc - csr, crl, iaa; ausg – jcet; nsd – jcet); Some cm-level diurnal periodicities present in some differences.

Comparison of EOPs in SolC.snx For those institutes that submitted SolC.snx files: Simple differencing of Xp, Yp and UTsat from a-priori C04 series given in sinex files Plots of results show: –asi, jcet apply loose constraints –dgfi apply strong constraints