G43C-1467
Computation of a Terrestrial Reference Frame based on homogeneously processed data of different space geodetic techniques
The computation of a Terrestrial Reference Frame (TRF) is done by combining time series derived from the space geodetic techniques VLBI, SLR, GPS and DORIS. To obtain a TRF of high accuracy, consistency of the different techniques is required, which was not completely achieved for the current ITRF2005 solution. To guarantee for consistency, the observations of the different techniques have to be analyzed using software packages, which are adapted concerning modelling and parameterization. The presented TRF solution is based on such homogeneously processed data, that were generated within the German GGOS-D project, a joint activity of four geodetic institutions. The applied combination model consists of two main parts. In the first one, the computation of multi-year solutions per technique is performed. It comprises the generation and analysis of the station position time series to identify systematic effects, e.g. jumps and periodical variations, the detection of outliers and finally the accumulation of the time series using the results of the time series analysis to model the temporal behavior of the station positions. In the second part the different techniques are combined to a terrestrial reference frame solution. The major issues of the combination process are: the selection of co- location sites, the estimation of relative weighting factors to consider differences between the variance levels of the techniques and the realization of the geodetic datum. The combination procedure is presented and the TRF results are discussed in terms of consistency and accuracy.
G43C-1468
Transformation of GPS Time Series to ITRF2005
More than 2 million station-days of GPS position estimates for 909 sites are used to examine the impact of various transformations into ITRF2005. Velocity estimates are obtained after 7, 6, and 3 parameter transformations. The traditional 7 parameter series uses the SLR geocenter and VLBI scale from ITRF2005. The 6 parameter series uses the SLR geocenter but keeps the GPS scale. The 3 parameter series uses both the GPS geocenter and scale. Results indicate that as GPS improves over time, less transformation parameters are necessary. http://sideshow.jpl.nasa.gov/mbh/series.html
G43C-1469
Latest evolutions of DORIS data processing at the LEGOS/CLS Analysis Center.
LEGOS/GRGS and CLS teams have been involved in DORIS data processing since the launch of Spot-2 in 1990. They jointly participate in the International DORIS Service (IDS) as Analysis Center. They also contribute to the GRGS Combination Research Center (CRC), which uses the GINS/DYNAMO software package in order to process observations of the astro-geodetic techniques DORIS, GPS, SLR and VLBI, and to perform the global combination of the geodetic parameters. The goal of this paper is to present the recent evolutions brought to DORIS processing. 1: a new set of models was taken into account in the orbit computation: the GRACE-derived gravity model EIGEN- GL04S which includes annual and semi-annual terms of the low coefficients; the latest ITRF2005; a priori tropospheric zenith delays, derived from ECMWF meteorological model. 2: the data processing is now fitted for a weekly delivery of the products requested by IDS and CRC. Improvements brought to the estimations of the coordinates times series, EOP, scale factor, geocenter will be discussed. In particular, the new configuration eliminates the systematic bias of the scale factor which appeared in our previous coordinates solutions.
G43C-1470
Towards an operational IDS combination
The International DORIS Service is in operation since 2003. Over the last four years, five instruments on the SPOT 2-4-5, ENVISAT and the JASON-1 satellites have provided DORIS Doppler data collected from a global network of about 50 stations. The number of Analysis Centers (AC) who have processed the data and have high level experience has progressively risen. Among them, two ACs : IGN/JPL using GYPSY/OASIS software and LEGOS/CLS using GINS/DYNAMO software now provide solutions of station coordinates and EOPs on a routine basis to the IGN and NASA CDDIS data centers. INASAN also processes DORIS data using GYPSY/OASIS software and submitted SINEX files to the ITRF2005 solution. The two newest DORIS analysis centers, include Geoscience Australia (GA) using the NASA GEODYN software and the Geodetic Observatory Pecny (GOP) using the BERNESE software. The GOP has adapted software not originally designed to process DORIS data. The performance reached by the new analysis centers in orbit and station positions determination is very encouraging. The availability of geodetic solutions from different algorithms and software packages allows us to efficiently contribute to cross-comparison of the solutions and to the improvement of the DORIS technique. The operational combinations of SINEX solutions contributed by the DORIS analysis centers will have to perform at least two functions. The first one is an internal validation of each individual AC time series. The second is a combination of all the AC weekly solutions in order to generate IDS combined products for station coordinates and EOPs. The paper presents the status of each AC processing (strategy, results, organization and future plan) and the combinations of solutions for the period of several years (2002-2006 as an objective). The combinations are realized with the IGN/LAREG CATREF software. Essential to the analysts, the station network event file (discontinuities, poor data span…) will be updated and listed. The complementary analysis of ITRF2005 for the new stations and a cumulative intra-technique solution will also be addressed. Finally, the future weekly CATREF combinations and processing standards will be specified and presented in the paper. The outputs such as transformation parameters and residuals, per station time series, to be available online at http://ids.cls.fr/ will also be discussed.
G43C-1471
The new position and velocity solution DGF07P01 of IGS Regional Network Associate Analysis Centre SIRGAS (IGS RNAAC SIR)
Since 1996 the German Geodetic Research Institute (DGFI) acts as the Regional Network Associate Analysis Centre for the Geocentric Reference System of the Americas (SIRGAS) within the International GNSS Service (IGS RNAAC SIR). It is processing all permanent GPS stations (IGS and regional stations) of South America, Central America, the Caribbean and surrounding areas. Each week a coordinate solution is delivered to the IGS data centres. After the decision of the IGS to introduce the new ITRF2005 (IGS05) and the implementation of absolute phase centre variations since GPS week 1400 the IGS RNAAC SIR at DGFI will reprocess all weekly solutions since GPS week 1860 (July 1996). A new position and velocity solution of IGS RNAAC SIR will be presented containing all reprocessed weeks so far. This paper reports about the station network, the used input data, and the strategy of processing. Comparisons with the latest ITRF2005 solution will be shown (coordinates and velocities). Some remarks will be given on future plans of IGS RNAAC SIR.
G43C-1472
Accuracy and Stability of the Terrestrial Reference Frame Realized by the Future Network of VLBI2010 Stations
To design the future global geodetic network of VLBI2010 stations, the IVS is performing simulations to determine optimal network antenna locations, VLBI station specifications, and antenna observing strategies. The desired goal is a system that will determine the terrestrial reference frame (TRF) with an accuracy of 1 mm and stability of 0.1 mm/yr. We first describe the simulation procedure and its validation using observed data. Validation involves verifying that measures of geodetic precision (e.g., repeatability) derived using simulated geodetic delay data are at the level obtained from analysis of observed data. The current global coverage with VLBI antennas suffers from poor Southern Hemisphere coverage. By analyzing results from simulated networks with different global distributions, one can investigate the systematic effects due to non-uniform geographical distribution of VLBI sites. For instance, one can determine how the precision of TRF scale and Earth orientation of a VLBI network depends on the number and distribution of VLBI antennas. Preliminary analysis shows that Earth orientation parameter and TRF scale precision improves with network size by a factor of about 2 as the network size increases from 8 sites, which is the typical size of current operational VLBI networks, to a network of 32 sites. We use simulation analysis to determine the tradeoffs between number of antennas, VLBI station specifications (e.g., antenna slewing rate and sensitivity), and delay observable precision in realizing the desired TRF accuracy and stability goals.
G43C-1473
Sensibility of the Reference Frame Definition in a Regional GNSS Network
The EUREF Permanent Network (EPN) includes about 200 GNSS stations in Europe. Today, the EPN is tied to the IGS05 by using minimal constraints on more than 20 EPN stations also included in the IGS05. This means that only regional stations are used for the reference frame definition which consequently can be a major error source. We have therefore investigated the sensibility of the station positions by adding data from carefully chosen IGS stations located on other continents to the processed network. For that purpose, we have done two processing. In the first one, only 45 regional EPN stations were considered and in the second one, this regional network is extended with the data of 37 well-selected IGS05 reference stations to obtain a global network. The data processing was done with Bernese V5.0, using absolute antenna phase center corrections and IGS orbits, starting from GPS week 1400 (Nov 2006) and covering almost a year. We have also looked into the effect of using the ITRF2005 instead of the IGS05 to determine the reference frame. Each time the reference frame is determined by applying a Helmert transformation under minimal constraints using the Bernese or Catref software. The paper will show the sensibility of the obtained station positions with respect to the underlying reference frame (global/regional and ITRF2005/IGS05) and the software used to perform the minimal constraints (Catref/Bernese).
G43C-1474
The New Generation GEOSAT Follow-On Precise Orbit Ephemeris Using ITRF2005 and Other Improvements
Launched in 1998, the U.S. Navy GEOSAT Follow-On spacecraft (GFO) began continuous radar altimeter coverage of the oceans in 2000 and is still operating, although currently with reduced capability. By providing high quality altimeter data GFO can supplement Jason, TOPEX/POSEIDON (T/P), and Envisat, providing a different synoptic sampling of the oceans with its 17-day ground track repeat cycle. GFO altimeter data is also currently being used to aid in the monitoring of lake and reservoir levels worldwide under a USDA funded project. Altimeter crossover analysis suggests GFO is capable of POSEIDON class altimetry, showing crossover residuals averaging below 7.5 cm, with 5-cm orbit error the largest contributor to the altimeter error budget. Non- conservative forces are believed to be the largest single contributor to orbit error. Satellite laser ranging (SLR), especially in combination with altimeter crossover data, offer the only means of determining high-quality precise orbits. SLR tracking is also augmented by Doppler (Tranet style) beacons. This study evaluates possible improvements to the current GFO GDR orbits. POD model improvements include using model standards consistent with the latest generation of GSFC reprocessed TOPEX and Jason orbits (for example GGM02C GRACE gravity, and time-varying gravity models) as well as a new high-resolution radiation pressure model developed by University College London. We also document the impact of the new ITRF2005 reference frame on the GFO GDR orbit products. Orbit accuracy is evaluated using GFO tracking data and altimeter crossover and collinear sea surface height residuals, and analysis of coefficients used to adjust GFO data into the TOPEX frame.
G43C-1475
Station Coordinates, low Degree Harmonics, and Earth Rotation Parameters From an Integrated GPS/CHAMP/GRACE Processing
Time series of geodetic Earth system parameters as part of a global geodetic reference system have been derived for the year 2004 by means of dynamic satellite orbit adjustment. The procedure applied is the integrated approach where the GPS satellites and the Low Earth Orbiters (LEOs) CHAMP and GRACE are processed simultaneously with common standards. The set of estimated Earth system parameters includes coordinates of ground stations, low degree harmonic coefficients of the Earth's gravity field, and Earth rotation parameters. Selected parameter time series are presented and compared to external series and products for validation. The particular advantage of the integrated approach is the high sensitivity of the LEO to the geocenter in combination with the stability from the GPS constellation geometry. Also, the dense tracking coverage by GPS measurements allows for a high temporal resolution of the parameters solved for. In order to demonstrate the superior performance of the integrated approach, comparisons to the commonly applied two-step processing are presented.
G43C-1476
Mantle dynamics and Length of Day variations
The geological evolution of the Length of Day (LOD) variations is mainly controlled by the frictional tidal torque responsible for the secular deceleration of the Earth rotation velocity and for the removal of the Moon. Superimposed to this variation existing from the early history of the planet, there are, at shorter time-scale (less than one Myr), LOD perturbations induced by the glaciation-deglaciation cycles. Here we investigate the influence of the mantle dynamics on the LOD at the geological time-scale. Using the complete non-linear equations governing the rotational dynamics of a viscous planet, we compute the effects induced by upwelling domes and subducted plates sinking into the mantle on the velocity of the rotation, that is to say on the LOD, and we also discuss the geological variation in the orientation of the rotational axis with respect to a fixed terrestrial frame.
G43C-1477
A Rigorous Combination of Space Geodetic Techniques: A Bayesian Approach
The IERS has encouraged the development of rigorous combination techniques of the ITRF, EOPs, and ICRF in recent years. These techniques use space geodetic data at the observation level, e.g. fundamental site coordinates and velocities, and quasar coordinates and motion. The standard approach to combining such data is the use of normal equations with minimal constraints and the rejection of outlier data, usually using an iterative solution. We propose an alternative technique using Bayesian inference to estimate EOPs. This poster gives an outline of the technique being developed and discusses the benefits of Bayesion inference in the handling of constraints and outlier data using the prior probability distribution function. The poster concludes with some preliminary results of EOPs using GPS and VLBI data.
G43C-1478
Production of Earth orientation parameter series by Kalman filter-based combination
The Kalman Earth Orientation Filter (KEOF) developed at the Jet Propulsion Laboratory (JPL) is an operational software system that generates real-time and retrospective estimates as well as predictions of the Universal Time and Polar Motion (UTPM) parameters used in Earth orientation calibrations to support all JPL interplanetary flight projects. KEOF accomplishes this task by timely assimilation of specific UTPM measurements using Kalman filter and smoother algorithms. The behaviors of KEOF are governed by the stochastic models for dynamics of the three UTPM parameters as well as the observation models that relate the dynamical variables with the measurements and quantify the measurement uncertainties. Since KEOF began operations, the structures of these stochastic and observation models and their parameter values have undergone steady updates, due to advances in scientific understanding of the forces that affect Earth's rotation and orientation as well as improvements in accuracy of the instruments that measure these phenomena. We describe various versions of the stochastic and observation models including those used currently in KEOF operations.
G43C-1479
Ultra-rapid UT1 measurement by e-VLBI
Using e-VLBI technology, we carried out several ultra-rapid UT1 measurements using the baselines Kashima- Onsala and Kashima-Metsähovi. The observational data were correlated with the Kashima VLBI software correlator in near real-time. After the end of the observing sessions, we performed standard VLBI analysis and we obtained the UT1 results within 30 minutes in the fastest case. The previous record for near real-time UT1 results was 4.5 hours, achieved by Koyama et al., (2004) on the Kashima-Westford baseline. The regular intensive sessions conducted by the International VLBI Service for Geodesy and Astrometry (IVS) usually obtain the UT1 results about a few days after the observations. Thus, with our measurements we greatly improved the latency of UT1 results. We also compared our UT1 results with the combined solution of the International Earth Rotation Service (IERS) Bulletin-A (rapid/predicted values) and Bulletin-B (final values). Our results proved to be consistent with the Bulletin-B values, and the average difference was only 32 microseconds. However, there were large differences between our results and the Bulletin-A predictions. On average the difference was 571 microseconds. For the experiment on 2 May 2007 the difference was more than 1.5 milliseconds, since the experiment was observed just before the next Bulletin-A was published. These comparisons show that it is difficult to use the Bulletin-A UT1 values for near real-time applications. On the other hand we think that our ultra-rapid UT1 results can fulfill quasi-real-time demands. Based on these results we will discuss the application of the results of ultra-rapid UT1 measurements.
G43C-1480
Current Results of the Earth Orientation Parameters Prediction Comparison Campaign
The precise transformation between the celestial (ICRF) and terrestrial (ITRF) reference frames is needed for many advanced geodetic and astronomical tasks. To perform this transformation for the time moment of observation the precise EOP data and their predictions have to be known. This poster presents the current status of the Earth Orientation Parameters Prediction Comparison Campaign (EOP PCC), which started in October 2005 under the umbrella of the IERS (International Earth rotation and Reference systems Service). The ultra-short term, short term and medium term EOP predictions submitted since then by different groups/algorithms were evaluated by means of the same statistical analysis. The mean prediction errors of the EOP together with their error bars for each group/algorithm were computed with respect to IERS C04_IAU2000.62-now data to show the performance in each prediction category. The contribution of each group/algorithm for computation of the combined EOP predictions is shown. http://www.cbk.waw.pl/EOP_PCC/
G43C-1481
The Impact on EOP Predictions of AAM Forecasts From the ECMWF and NCEP
In support of the operational tracking and navigation of interplanetary spacecraft, a Kalman filter has been used at the Jet Propulsion Laboratory (JPL) for nearly two decades to combine independent measurements of the Earth's orientation and to predict its future evolution. Changes in the Earth's orientation can be described as a randomly excited stochastic process; consequently, between measurements, the uncertainty in our knowledge of the Earth's orientation grows and rapidly becomes much larger than the uncertainty in the measurements. Thus, measurements of the Earth's orientation must be taken frequently and processed rapidly in order to meet the demanding accuracy requirements of the spacecraft navigation teams. Short-term predictions of the Earth's changing orientation have been shown to be improved by using the dynamical model-based analyses and forecasts of the axial component of atmospheric angular momentum (AAM) as proxy length-of-day measurements and forecasts. Such AAM analyses and forecasts are produced as part of the numerical weather prediction activities at governmental agencies like the National Centers for Environmental Prediction (NCEP) and the European Centre for Medium-Range Weather Forecasts (ECMWF). For example, the accuracy of JPL's predictions of the Earth's rate of rotation can be improved by nearly a factor of 2 when AAM analysis and forecast data from NCEP are used. Here we compare the results we get using NCEP forecasts with those we get using ECMWF forecasts. We find that both NCEP and ECMWF 5-day wind AAM forecasts agree extremely well with LOD during 19 March 2004 to 22 July 2004, with the ECMWF having a slightly higher correlation with the LOD observations but with the NCEP explaining a bit more of the LOD variance. We find that the 7-day UT1 prediction accuracy is slightly better with the NCEP forecasts. Finally we find that adding oceanic angular momentum from an ocean general circulation model (ECCO/JPL kf066b) to the AAM forecasts improves the accuracy of the UT1 prediction only slightly.