HR: 11:05h
AN: G22A-04    [Abstracts]
TI: Evaluation of Colocation Ties Relating the VLBI and GPS Frames
AU: * Altamimi, Z
EM: altamimi@ensg.ign.fr
AF: Institut Geographique National, LAREG, 6-8 Avenue Blaise Pascal, Champs-sur-Marne, 77455 France
AU: Ray, J
EM: jimr@ngs.noaa.gov
AF: National Geodetic Survey, NOAA, 1315 East-West Hwy, Silver Spring, MD 20910 United States
AB: We have compared the VLBI and GPS terrestrial frames, realized using five years of time series observations of station positions and polar motion, with surveyed colocation tie vectors for 25 sites. The goal was to assess the overall quality of the ties and to determine whether a subset of colocation sites might be found with VLBI-GPS ties that are self-consistent within a few mm. Our procedure was designed to guard against internal distortion of the two space geodetic networks and takes advantage of the reduction in tie information needed with the time-series combination method by using the very strong contribution due to colocation of the daily pole of rotation. The general quality of the available ties is somewhat discouraging in that most seem to have residuals, compared to the space geodetic frames, at the level of 1 to 2 cm. However, by a careful selection process we have identified a subset of nine local VLBI-GPS ties that are consistent with each other and with space geodesy to better than 4 mm (RMS) in each component. While certainly promising, it is not possible to confidently assess the reliability of this particular subset without new information to verify the absolute accuracy of at least a few of the individual ties. Particular care must be taken to demonstrate that possible systematic errors within the VLBI and GPS systems have been properly accounted. A minimum of two (preferably three or four) ties must be measured with accuracies of 1 mm or better in each component, including any potential systematic effects. If this can be done, then the VLBI and GPS frames can be globally aligned to less than 1 mm in each Helmert component using our subset of nine ties. In any case, the X and Y rotations are better determined, to about 0.5 mm, due to the contribution of colocated polar motion.
DE: 1229 Reference systems
DE: 1243 Space geodetic surveys
DE: 1247 Terrestrial reference systems
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting