G41A-01
Combination of Space Geodetic Techniques at the Measurement Level: Methodological Issues
Fundamental geodetic products such as the International Terrestrial Reference Frame (ITRF) and the Earth Orientation Parameters (EOPs) are currently computed by combinations of space-geodetic technique solutions. In the framework of the French Groupe de Recherche en Géodésie Spatiale (GRGS) activities, we directly combine the techniques (VLBI, SLR, GPS, DORIS) at observation level using the same models and software for all the individual data processing. The purpose is to better use all the information provided by the different techniques by reducing the number of steps of the combination processing to the minimum. In this work, we first study different ways to define the combined terrestrial reference frame. Indeed, each technique has its own sensitivity to the frame parameters and we must clearly define the multi-technique frame during the combination. Then, we reinforce this combination by using local ties between collocated instruments and common parameters for all techniques: EOPs and tropospheric delays. In order to use tropospheric delays as common parameters, we have extended the use of the ECMWF data (maps of wet and dry zenithal tropospheric delays for radiometric techniques) to the non-radiometric technique, such as SLR. Finally, we provide some prospects for data weighting and future improvements.
G41A-02 INVITED
Station Coordinates Combination Status and Contribution to ITRF2005
Under the auspices of the International GNSS Service (IGS), the Reference Frame Working Group combines at Natural Resources Canada (NRCan) an accurate and consistent set of station coordinates, velocities and Earth Rotation Parameters (ERP) that are updated weekly. The consistency of these parameters is ensured by simultaneously combining them using their full variance-covariance information. The IGS Analysis Centers (ACs) (cod, emr, esa, gfz, jpl, mit, ngs, sio) provide the solutions used in the IGS weekly combination. The Global Network Associates Analysis Centers (GNAAC) (mit, ncl) also generate weekly combined solutions that include consistent station coordinates and ERPs. Those GNAAC solutions are used for comparisons and quality control of the IGS weekly solution. In recent weeks each AC has been contributing between about 50 and 250 station coordinates estimates. The combined product currently approaches 300 stations. Of those, between 40 and 110 are used for the current IGS reference frame realization of the ITRF2005 in each AC solution. The quality of the AC station coordinates solutions has improved significantly since they have started contributing in early 1996. This is due to the densification of the network of stations along with gradual improvements in station hardware and processing software. For recent solutions, the estimated noise (std. dev.) level between the AC weekly solutions and the different combined solutions varies between 1-3mm horizontally and 2-10mm vertically. More details about the statistics, their evolution and differences between the ACs will be presented. IGS weekly station coordinates and ERP combined products have also been contributed to the ITRF2005. Those weekly solutions were all recombined from the original AC contributions, going back to the beginning of 1996. The recombined solutions were edited for known problems (e.g. incorrect antenna heights and outliers). Several stations with short time span were also removed. A list of station coordinates discontinuities was also prepared, in collaboration with several individuals and contributed to the IERS. Since November 5, 2006 (GPS week 1400), the IGS has implemented a realization of ITRF2005. It includes mainly the change from relative to absolute antenna phase centers and an update of the selected reference frame stations for the new realization. An expected change of scale from about 3 ppb to -0.7 ppb caused by the phase center shift was observed.
G41A-03 INVITED
The International Terrestrial Reference Frame: Lessons from ITRF2005 and Future Developments
Up to ITRF2000, global long-term solutions were submitted by the analysis centers of the four techniques (VLBI, SLR, GPS, DORIS) for the ITRF formation. Starting with the ITRF2005, time series of station positions (weekly from satellite techniques and daily from VLBI) and Earth Orientation Parameters (EOPs) are the current ingredients of the ITRF construction. The main advantages of using time series as input data are twofold: (1) to allow monitoring station non-linear motion and all kinds of discontinuities; and (2) to examine the temporal behavior of the frame physical parameters, namely the origin and the scale. The paper starts by a brief introduction of the main features of the combination model used in both the stacking of time series and the combination of long-term global solutions. A particular focus will be given to the main lessons learnt from the ITRF2005, including scale discrepancy between VLBI and SLR solutions, local tie uncertainties, the Z-translation drift of 1.8 mm/yr between ITRF2000 and ITRF2005, consistency of Earth Rotation Parameters and the impact of GPS antenna phase center variation correction. The discussion will be illustrated by the most up to date analysis of the available time series of the 4 techniques. http://itrf.ensg.ign.fr/ITRF_solutions/2005/ITRF2005.php
G41A-04
At What Exact Velocity is Earth's Center Moving?
Estimates of site velocity are determined relative to Earth's center; therefore the velocity of Earth's center is fundamental to geodesy and geoscience. I present and compare different means by which to define Earth's center. Earth's center can be defined to be either (CM) the mass center of Earth, oceans, and atmopshere, (CE) the mass center of solid Earth, or (CF) the mean position of Earth's surface. The velocity of (CM) the mass center of Earth, oceans, and atmosphere differs between ITRF2000 and ITRF2005 by nearly 2 millimeters per year, suggesting that the velocity of CM estimated using SLR is not tightly constrained. Using four space techniques I estimate the velocity of CE using two different assumptions. First, I assume that places on plate interiors are moving vertically relative to CE as predicted by a model of Earth's response to unloading of the ice sheets 10 to 20 thousand years ago. Second, I assume that places on plate interiors neither beneath nor along the margins of the former ice sheets are not moving horizontally relative to CE. The two estimates of the velocity of CE are similar and nearer the ITRF2000 estimate of the velocity of CM than the ITRF2005 estimate of the velocity of CM. My estimate of the velocity of CE is likely nearer the true velocity of CM than the estimates of the velocity of CM determined using SLR. First, no phenomenon is believed to sustain a velocity between CM and CE for many years. Second, the velocity of CE is constrained tightly by the four space techniques.
G41A-05 INVITED
The Origin and Scale of the Terrestrial Reference Frame From Laser Ranging
The International Terrestrial Reference Frame (ITRF) provides the foundation for most of the space- and ground- based Earth observations of global change. A reference frame needs by definition to exhibit high accuracy and stability over extended periods of time. Space geodesy is the prime group supporting the establishment and maintenance of the ITRF for decades now. Through various techniques over the years, it strives to maintain continuity, improved accuracy and ever-increasing stability in the updated realizations of the International Terrestrial Reference System that also undergoes refinements over the decades. Most of these refinements are the direct result of our improved understanding of the Earth system and the continuous and complex interactions of its components. This quest for higher accuracy and stability requires the close collaboration of space geodesy with all other scientific disciplines that study and monitor the Earth system components. As we gain improved understanding of these components, space geodesy adjusts its underlying modeling of the system to better and more completely describe it. This in turn results in an improved geodetic product (ITRF) for the scientific community and all other users. Laser Ranging (LR) supports this process since the very first years of space geodesy. With the proliferation of space techniques, LR and Satellite Laser Ranging (SLR) in particular, focused on the unique strength of the technique, in providing unbiased and very precise observations of the origin and scale of the ITRF. Origin and scale stability are fundamental in determining vital global change parameters, such as mean sea level changes, Earth rotation variations, and mass redistribution amongst the components of the Earth system. SLR has contributed the most accurate observations yet, demonstrating few-millimeter level accuracy for weekly averages over the past decade. Considering the goals of ongoing international efforts such as GEO, the Global Earth Observation, the level of accuracy of the ITRF in the very near future must be significantly improved (<1mm and 0.1 mm/y). Additionally, all of the observed inconsistencies between the contributing techniques must be resolved and reconciled. We examine some of the current practice in SLR data analysis and discuss the possible improvements in the areas that have the most serious and direct effect on the development of the ITRF in light of future improvements of the ground segment, target design, and satellite missions.
G41A-06 INVITED
GPS-Only Terrestrial Reference Frame Based on a Global Reprocessing
The realization of the International Terrestrial Reference System (ITRS) with highest accuracy and stability is fundamental and crucial for applications in geodesy, geodynamics, geophysics and global change. In a joint effort TU Dresden and TU Munich/GFZ Potsdam reprocessed a global GPS network of more than 200 stations. As a contribution to an ITRS realization daily normal equations from 1994 to 2005 were rigorously combined in order to determine a global GPS-only reference frame (PDR05/Potsdam-Dresden-Reprocessing Reference Frame). We present a realization of the global terrestrial reference system which follows the center of mass approach in consideration of the load-induced deformation of the Earth's crust due to the redistribution of surface masses. The stability of our reference frame will be evaluated based on the obtained long-term trends of station coordinates, the load-induced deformation estimates and the homogeneous time series of station positions. We will compare our solution with other recent terrestrial reference system realizations and give some conclusions for future realizations of the ITRS.
G41A-07
Stability of VLBI, SLR, DORIS, and GPS positioning
The residual signal in VLBI, SLR, DORIS and GPS station motion, after a linear trend and seasonal components have been removed, is analysed to investigate site-specific and technique-specific error spectra. The study concentrates on 60 sites with dense observation history by two or more space geodetic techniques. The solutions analysed are single-analysis center solutions currently available. The GPS data are taken from the IGS files. Statistical methods include the Allan variance analysis and the three-cornered hat algorithm. The site-specific noise level is found to be in the range 0.5–3.5 mm in either horizontal direction and 1–4.5 mm in height for most sites. The distribution of site-specific noise type includes both white noise and flicker noise. White noise is predominant in the East direction. Both types of noise are found in the North direction, with no particular geographical clustering.Technique-specific noise characteristics are estimated in several ways, leading to a white noise diagnostic for VLBI and SLR in all three local directions. DORIS has also white noise in the horizontal directions, whereas GPS has a flicker noise spectrum. The vertical noise spectrum is indecisive for both DORIS and GPS. The three-dimensional noise levels for the one-year sampling time are 1.7 mm for VLBI, 2.5 mm for SLR, 5.2 mm for DORIS, and 4.1 mm for GPS. For GPS, the long-term analysis homogeneity has a strong influence. In the case of a test solution reanalysed in a fully consistent way, the noise level drops to the VLBI level in horizontal and to the SLR level in vertical. The three-dimensional noise level for a one-year sampling time decreases to 1.8 mm. In addition, the percentage of stations with flicker noise drops to only about 20% of the network.
G41A-08
ULR Re-analysed Global GPS Solution for Vertical Land Motion Correction at Tide Gauges
The presentation will review the recent results published by W\¨{o}ppelmann et al. (2007) in Global and Planetary Change. Geocentric sea-level trend estimates were derived from the global GPS analyses conducted at ULR consortium to correct a set of relevant tide gauges from the vertical motion of the land upon which they are settled. The exercise proved worthwhile. The results showed a reduced dispersion of the estimated sea level trends, either regionally or globally, after application of the GPS corrections compared to the corrections derived from the glacio-isostatic adjustment models of Peltier (2004). Here we will focus on two important issues that were not addressed in W\¨{o}ppelmann et al. (2007). The first issue concerns the noise content of our GPS solutions. Previous works have shown that GPS coordinate time series are subject to significant time-correlated (coloured) noise, with a large predominance of flicker noise (Zhang et al. 1997, Mao et al. 1999, Williams et al. 2004). The presence of coloured noise in a time series has a significant effect on the rate uncertainty, which may otherwise be underestimated by as much as an order of magnitude. We therefore carefully investigate the now 10-year long data set of reanalysed GPS solutions for noise content using the Allan variance technique (Feissel et al. 2007). Preliminary results show that the reanalysed solutions at ULR exhibit far less flicker noise than any other solution published so far in the literature available to us. The percentage of stations with flicker noise drops to only about 20%. These encouraging results advocate for a comprehensive reanalysis strategy with full coherent models over the entire observation data span. Moreover, the noise level reaches the best levels of other geodetic results recently published, namely the VLBI level in the horizontal component and the SLR level in the vertical component (Feissel et al. 2007). The second issue that we would like to address in the presentation relates to the reference frame realisation. This is indeed a long standing key issue in achieving the accuracy goal required by long term sea level studies from tide gauges. We investigate the impact of the recent transition from ITRF2000 to ITRF2005 in our previous and current sea level trend estimates.