G31B-0793 0800h
Atmospheric Loading Blue-Sky Effects on SLR Station Coordinates
Satellite laser ranging (SLR) station coordinates are found to be dependent on local atmospheric pressure through a precise 5-year orbit analysis of LAGEOS-I and LAGEOS-II satellites. The loading coefficients of the station heights are estimated to be mostly around -0.3 to -0.5 mm/hPa. This result indicates that the site displacement due to the atmospheric pressure loading effect is detected for the first time by the SLR technique. Furthermore, due to its weather restriction, a -0.4 to -1.3 mm offset is theoretically predicted in the height of SLR stations when it is compared to all-weather microwave-based geodetic techniques like GPS and VLBI.
G31B-0794 0800h
Alternative ITRF Combination Formulation Based on the Strength of Different Space Geodesy Techniques
The evolution of space geodesy techniques (e.g. SLR, VLBI, GPS, DORIS) and their contribution to the reference frame realization has a marked impact on the geophysical and geodetic research over the past decade. Most of these studies rely on the International Terrestrial Reference Frame (ITRF) which is defined as a combination of each individual solution of space geodesy techniques that defines its own terrestrial reference frame (TRF). In this process, it is desirable that each of the techniques should contribute to the ITRF realization with independent geodetic information as well as in those areas where they provide the best available geodetic measurement. However, the current ITRF combination model does not differentiate explicitly the strength of each technique and the independence of the geodetic information they provide. The objective of this presentation is to demonstrate an alternative ITRF combination model formulation which takes into consideration the inherent properties of SLR which is sensitive to the center of mass variations of the Earth, VLBI which provides accurate earth orientation information and GPS which generates accurate scale through baseline measurements. This study also investigates and quantifies the impact of the new formulation on the realization of ITRF.
G31B-0795 0800h
Use of Pulsar Timing at Nancay 's Radiotelescope for the Construction of the Dynamical Reference Frame
Pulsars, discovered by radioastronomers in 1967, not only offer the opportunity to study fundamental physics in conditions not reachable on Earth, but also can play an essential role in modern astrometry. Indeed, pulsar timing analysis is directly dependent on the way by which the position of the earth on its orbit is modelized. Indeed, the arrival times of the pulses are shifted by the Doppler effect due to the relative speed of the Earth on its orbit, whose direction and amplitude vary along the year with respect to the line of sight. Therefore, it provides precious information about the location of the ecliptic, i.e. a dynamical frame related to the revolution of the Earth., with respect to a frame related to the pulsars, as galactic objects. Thus we explain how radio pulses observations of pulsars carried out at Nancay's radiotelescope can participate actively to the construction of both a very accurate catalogue of the coordinates of pulsars, which should give birth to a pulsar Reference Frame, and to a very accurate positioning of the ecliptic with respect to inertial directions. A set of about 20 pulsars are regularly observed at Nancay, among which PSR1937+21 and PSR 1821-21 have been subject to a very intensive campaign for 15 years. We show the advantage of all these observations to accomplish the objectives above.
G31B-0796 0800h
ITRS Combination Center at DGFI: TRF Realisations and Accuracy Evaluation
DGFI serves as an ITRS Combination Center within the International Earth Rotation and Reference Systems Service (IERS). The poster presents the combination methodology and the terrestrial reference frame realisations computed at DGFI. These are the TRF realisation based on multi-year VLBI, SLR, GPS and DORIS solutions, and a TRF computation based on epoch (e.g. weekly/daily) normal equations of the different space techniques. This new approach has major advantages, as for instance the detection of discontinuities and non-linear effects of site motions and TRF datum parameters (e.g., origin, scale). Combination methods and parameterisations for modelling all the observed non-linear effects and discontinuities have to be improved. The performed TRF computations provide valuable results to assess the current accuracy of the terrestrial reference frame. The combined intra-technique solutions were used to evaluate the TRF accuracy by comparing the space geodetic solutions with local ties, and the velocity estimations of co-located instruments. The current accuracy is not satisfying for many co-location sites. Remaining biases between the contributing space techniques are a major error source. Furthermore, the poster presents the comparison of the DGFI TRF realisation with ITRF2000, which provides an optimal basis for a validation and a first "quasi-independent" accuracy assessment.
G31B-0797 0800h
Crustal Deformation Velocities From Episodic Regional Measurements at Canadian Base Network Sites
Initiated in 1994, the Canadian Base Network (CBN) is a network of pillar monuments with forced-centering mounts for Global Positioning System (GPS) receiver antennae. Accurately positioned three-dimensionally with GPS, the CBN can serve as a monitoring network for deformation studies of the Canadian landmass. By combining nearly ten years of repeated multi-epoch (episodic) GPS measurements, we estimate velocities at the CBN sites to provide an increased spatial sampling of crustal deformation throughout Canada. To determine individual station velocities, we systematically combine regional CBN solutions for each measurement epoch into a single Canada-wide, multi-epoch cumulative solution. In order to generate time series of consistent, high-accuracy coordinates for velocity estimation, it is necessary to ensure consistency in the realization of the reference frame. We accomplish this by aligning each of the individual CBN solutions to the IGS realization of ITRF using a subset of stations from a recent IGS cumulative solution for the IGS global network. Fortunately, there are many IGS stations in Canada and most were included in each regional CBN solution to strengthen the realization of the reference frame and ensure consistency between epochs. We also ensure consistent and realistic weighting of the individual CBN solutions though the estimation of variance components relative to the IGS global solution. After the individual CBN solutions are aligned and weighted, they are combined together in a simultaneous cumulative solution for velocities at each site. Preliminary results from the analysis of CBN time series in eastern Canada exhibit a spatially coherent pattern of uplift consistent with the expected post-glacial rebound (PGR) signal. For this region the highest observed uplift rates are in the vicinity of James Bay through to southwestern Labrador; the rates then decrease to the south and towards the coastal Atlantic margins. Indeed, the Nouveau Quebec-Labrador region was the site of one of the major ice domes of the Laurentide Ice Sheet and the surrounding region is predicted to experiencing contemporary postglacial rebound. We compare these preliminary site-by-site results with those from our simultaneous cumulative solution, as well as those predicted by PGR models. In order to densify the velocity field in the Arctic and Great Lakes regions, future investigations will include additional, recent continuous and episodic GPS measurements in these areas of interest as well as continuous GPS measurements at selected CORS sites in the northern U.S. states.