G22A-01 INVITED 10:20h
Combination of the Space-Geodetic Techniques for Monitoring the Earth's System
The combination of all major space-geodetic techniques into an global geodetic observing system (GGOS) must be a primary goal in geodesy in the next years. A combination is important and beneficial because (1) it helps to distinguish genuine geodetic/geophysical signals from technique-specific systematic biases, (2) it makes use of the complementarity of the individual techniques to strengthen the solutions, (3) it allows to benefit from observing instruments co-located at the same site/satellite, (4) it is the only way to account for the fact that there is, e.g., only one orientation of the Earth's rotation axis for all techniques, only one tropospheric and ionospheric refraction for VLBI, GPS and other microwave techniques, only one orbit for a multi-technique satellite, etc., and to ensure the consistency of the resulting geodetic products, and, finally (5) this approach is crucial to get a more detailed view and understanding of the complexity of the "System Earth" and its geophysical processes. Benefits for geophysical interpretations and Earth system monitoring, but also problems resulting from combination approaches will be discussed and illustrated with various examples.
G22A-02 INVITED 10:35h
Enhancing Earth Rotation Studies by Combining Earth Rotation Measurements
The Earth rotates about its axis once a day, but does not do so uniformly. Instead, the rate of rotation fluctuates by up to a millisecond per day, and the Earth wobbles as it rotates. These changes in the Earth's orientation occur over a wide range of time scales, from subdaily to decadal and longer, reflecting the wide variety of geophysical processes that are causing the Earth's orientation to change. A number of different techniques have been and are currently being used to measure the Earth's changing orientation in space, including lunar occultation, optical astrometry, lunar and satellite laser ranging, very long baseline interferometry, and the global positioning system. These different measurement techniques have their own unique strengths and weaknesses. They may be sensitive to a different subset and/or linear combination of the Earth orientation parameters, the averaging time for their determination may be different, as may be their duration, the interval between observations, and the precision with which they can be determined. By combining Earth orientation measurements that have been determined by these different techniques, a series of the Earth's orientation can be obtained that is potentially more accurate than that determined by any single technique and that spans a time interval greater than that spanned by any single technique. The improved accuracy and longer duration of such combined Earth orientation series enhance scientific studies of the Earth's rotation, enabling greater understanding of its causes and consequences.
G22A-03 10:50h
Is there Utility in Rigorous Combinations of VLBI and GPS EOPs?
Combinations of station positions and velocities from independent space geodetic techniques have long been the standard method to realize global terrestrial reference frames. In principle, the particular strengths of one observing method can compensate for weaknesses in others if the combination is properly constructed, suitable weights are found, and accurate colocation ties are available. More recently, the methodology has been extended to combine time series of results at the normal equation level. This allows Earth orientation parameters (EOPs) to be included in a fully consistent way. While the utility of such multi-technique combinations is generally recognized for the reference frame, its benefit for the EOPs is still to be accurately assessed, though it is indispensable for better alignment with the frame. We have studied test combinations of VLBI and GPS time series solutions to assess the effects on combined EOP measurements. One expects any effects to be small, considering that GPS dominates the polar motion estimates due to its relatively dense and uniform global network coverage, high precision, daily sampling, and homogeneity, while VLBI solely observes UT1. Presently, we see no practical method to include the GPS estimates of length-of-day variations due to significant time-varying biases. Nevertheless, there are indications that the stronger reference frame from GPS contributes slightly to improved UT1 results. The situation with polar motion is less clear. The VLBI data contribute only slightly, increasing coherence with geophysical excitations in some spectral bands while decreasing it in others. The effects differ between the X and Y components but are small in both cases. Variations in the combination strategy also affect the assessments at a similar level. Further research is needed to determine an optimal combination strategy that yields the highest quality EOP estimates.
G22A-04 11:05h
Evaluation of Colocation Ties Relating the VLBI and GPS Frames
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.
G22A-05 INVITED 11:20h
Improvements in regional crustal deformation monitoring using ultra high rate GPS and seismic data
The development of regional GPS and seismic networks has occurred with virtually no overlap, although both share many of the same geophysical objectives such as geologic fault detection, fault plane modeling during earthquakes, and real-time seismic hazards mitigation. Meanwhile, the detection spectrum of geodesy has pushed steadily towards measuring dynamic transients previously accessible only with seismology, as well as measuring transient deformation not detected by seismic methods such as slow earthquakes, fault creep, and transient fault slip. Recent developments in GPS instantaneous positioning with ultra high rate GPS sampling (10-20 Hz) further demonstrate certain weaknesses in the current operations of regional GPS networks. We describe efforts in southern California to collocate seismic and GPS sensors and to develop a GPS/seismic displacement meter at the software level, which would optimally combine real-time seismic and ultra-high-rate GPS network data, for the purposes of better quantifying fault slip and fault/earthquake parameter detection, and improved seismic early warning.
G22A-06 INVITED 11:35h
Integrating tropospheric parameters from different observing techniques
Co-location of independent space geodetic and remote sensing techniques allows to combine and integrate the independently derived results for tropospheric parameters. The combined results promise to provide a robust and consistent basis for possible applications in climate related research and may give complementary information to the results derived from traditional meteorological techniques. For our study we use the independent and co-located techniques of geodetic VLBI, GPS and microwave radiometry at the Onsala Space Observatory and aditionally radiosonde observations taken at the Landvetter airport in 38 km distance. We combine and integrate the atmospheric water vapor content that is derived from the different techniques on time scales up to more than two decades. We apply different statistical approaches and investigate for possible trends in the amount of atmospheric water vapor. The derived results are compared to a climate model.
G22A-07 11:50h
Multi-Instrument Investigations of Space Weather Storm Fronts
Trans-ionospheric signal propagation anomalies impact a wide variety of GPS system users. Solar outbursts drive ionosphere/magnetosphere disturbances which launch space weather storm fronts which sweep across the Americas from equatorial to polar latitudes. We investigate the characteristics and causes of these storm-time disturbances using a combination of ground and space-based observing techniques. During the November 20, 2003 superstorm event, total electron content (TEC) over the continental USA approached 300 TECu, some 10 times the normal value. Steep spatial gradients in TEC (in excess of 100 TECu per degree of latitude) were observed over the heavily-populated northeast. We use distributed ground-based imagery of ionosphere/magnetosphere TEC derived from GPS observations to produce high-resolution spatial and temporal maps of the intensity and evolution of these dynamic space weather features. During strong disturbances, a ridge of SED (storm enhanced density, greatly elevated TEC) forms across mid latitudes in the post-noon ionosphere. The evolution of continuous SED plumes stretching from the US East Coast, across Canada, and from noon to midnight across high polar latitudes is revealed using the ground-based GPS TEC observations. The MIT Millstone Hill incoherent scatter radar (Massachusetts) has been used to probe the altitude structure of the ionosphere in and around the SED plume, and quantifies its rapid sunward (westward) motion. Overflights with the Defense Meteorological Satellite Program (DMSP) satellites locate the plume with respect to auroral particle precipitation and electric fields, further clarifying the processes leading to the formation of this global space weather feature. Correlating the ground-based and low-altitude observations with space-based imagery of the high-altitude plasmasphere (from the NASA IMAGE spacecraft) reveals that these SED features result from the erosion of the outer layers of Earth's plasmasphere by intense sub-auroral electric fields. The SED features observed over the USA extend many Earth radii into space, spanning our atmosphere from the lower ionosphere to the outer limits of the magnetosphere.
G22A-08 12:05h
Surface Mass Variations From GPS/Ocean Bottom Pressure Model and GRACE
We study seasonal surface mass variations and the resulting load-induced deformation as well as time-variable gravity. Monthly global GPS data and an altimetry-assimilated relative ocean bottom pressure model are inverted for spherical harmonic surface mass variations. The concurrent low-degree spherical harmonic time series are compared with corresponding GRACE-derived series. Geographic comparison of the independent surface mass variation results is carried out after applying a Gaussian spatial filter. Good agreements are seen in both spherical harmonic and geographical domains. We also obtain a joint solution of surface mass harmonics using GPS and GRACE data combination. The two independent data sets are highly complementary to each other. Combining them can achieve a more complete picture of the surface mass variations by including robust degree-1 surface mass estimates, which reflect the longest-wavelength hemispherical mass exchange, but are not included in the gravity solution. The joint solution also improves long-wavelength estimates and serves to cross-validate the two techniques from their overlapping strengths.