Axel Rülke

Validation of Precipitable Water Vapor within the NCEP/DOE Reanalysis Using Global GPS Observations from One Decade

Abstract In contrast to previous studies validating numerical weather prediction (NWP) models using observations from the global positioning system (GPS), this paper focuses on the validation of seasonal and interannual variations in the water vapor. The main advantage of the performed validation is the independence of the GPS water vapor estimates compared to studies using water vapor datasets from radiosondes or satellite microwave radiometers that are already assimilated into the NWP models. Tropospheric parameters from a GPS reanalysis carried out in a common project of the Technical Universities in Munich and Dresden were converted into precipitable water (PW) using surface pressure observations from the WMO and mean atmospheric temperature data from ECMWF. PW time series were generated for 141 globally distributed GPS sites covering the time period from the beginning of 1994 to the end of 2004. The GPS-derived PW time series were carefully examined for their homogeneity. The validation of the NWP mo...

Assessing the Current Evolution of the Greenland Ice Sheet by Means of Satellite and Ground-Based Observations

The present study utilises different satellite and ground-based geodetic observations in order to assess the current evolution of the Greenland Ice Sheet (GIS). Satellite gravimetry data acquired by the Gravity Recovery and Climate Experiment are used to derive ice-mass changes for the period from 2003 to 2012. The inferred time series are investigated regarding long-term, seasonal and interannual variations. Laser altimetry data acquired by the Ice, Cloud, and land Elevation Satellite (ICESat) are utilised to solve for linear and seasonal changes in the ice-surface height and to infer independent mass-change estimates for the entire GIS and its major drainage basins. We demonstrate that common signals can be identified in the results of both sensors. Moreover, the analysis of a Global Positioning System (GPS) campaign network in West Greenland for the period 1995–2007 allows us to derive crustal deformation caused by glacial isostatic adjustment (GIA) and by present-day ice-mass changes. ICESat-derived elastic crustal deformations are evaluated comparing them with GPS-observed uplift rates which were corrected for the GIA effect inferred by model predictions. Existing differences can be related to the limited resolution of ICESat. Such differences are mostly evident in dynamical regions such as the Disko Bay region including the rapidly changing Jakobshavn Isbræ, which is investigated in more detail. Glacier flow velocities are inferred from satellite imagery yielding an accelerated flow from 1999 to 2012. Since our GPS observations cover a period of more than a decade, changes in the vertical uplift rates can also be investigated. It turns out that the increased mass loss of the glacier is also reflected by an accelerated vertical uplift.

Mass Variation Signals in GRACE Products and in Crustal Deformations crustal deformation from GPS: A Comparison

Geophysical surface mass variations are reflected both in gravity field variations and in load deformations of the solid Earth . These two signatures may be observed by GRACE and by GPS , respectively. This article reports about a comparison between both. Concerning GPS-derived deformations, a meaningful geophysical interpretation requires both homogeneously processed observations and a stable realization of the terrestrial reference system. Here we use results from a reprocessing of a global GPS network with consistent use of the latest processing and modelling strategies. This reprocessing includes the estimation of low-degree deformation terms. We directly compare them to respective GRACE results and find good agreement. Our main results concern the comparison of site displacement time series obtained from GPS, on the one hand, and from GRACE gravity variations converted to load deformations, on the other hand. We do this comparison both for the GRACE background models of short-term variations and for the final monthly GRACE solutions. For vertical deformations, we find good agreement. In contrast, the agreement is poor for the horizontal directions. The differences between GPS and GRACE contain some components which appear to have large-scale correlated patterns in space and seasonal patterns in time. More detailed analyses indicate that residual errors in the GPS solutions are likely the dominant cause of these differences. Analysing internal deformations of regional subnetworks is a way to circumvent some of the large-scale systematics of the GPS solution. Indeed, regional analyses show reasonable agreement between GPS and GRACE even in the horizontal components. Overall, our results demonstrate the progress and challenges of combining independent satellite geodetic observations within the Global Geodetic Observing System.

Effects of Different Antenna Phase Center Models on GPS-Derived Reference Frames

In GPS week 1400, the International GNSS Service (IGS) switched from a relative antenna phase center model (APCM) for receiver antennas only to an absolute model including receiver and satellite antenna corrections. At the same time the International Terrestrial Reference Frame 2005 (ITRF2005) was adopted. These changes had a significant influence on the terrestrial reference frame (TRF). In order to study the influence of different APCMs on GPS-derived TRFs, four TRF solutions have been computed from 11 years of homogeneously reprocessed GPS data. The processing strategy for the four solutions is completely identical except for the APCM applied. The following models have been used: (1) the relative model IGS01 used by the IGS till GPS week 1400, (2) the new absolute IGS model IGS05 including radome calibrations, (3) IGS05 ignoring the radome calibrations for the receiver antennas, and (4) IGS05 including azimuth-dependent satellite antenna phase center variations (PCVs). Station coordinates and velocities have been estimated simultaneously with daily pole coordinates. Consistent time series of station coordinates have been generated using the corresponding reference frames for datum definition. This paper compares the station coordinates and velocities as well as the station coordinate time series arising from the four different reference frames.

A Precise Reference Frame for Antarctica from SCAR GPS Campaign Data and Some Geophysical Implications

The SCAR GPS Campaigns have been carried out since 1995. Based on these data a densification of the Terrestrial Reference Frame (ITRF) in Antarctica has been performed. The results can be used for precise positioning, navigation and validation purposes. Furthermore, the station velocities were obtained. Based on these vectors the kinematics of the Antarctic plate could be determined and an active rifting of 7 mm/year across the Bransfield Strait was measured. Vertical rates are important for the validation of models on glacial isostatic adjustment. The activities are being continued and extended during the International Polar Year within the POLENET project.

Practical Aspects of the Unification of Height System Realizations in Europe

Three different approaches for the unification of height reference frames have been used to compute datum offsets between national vertical reference frames in Europe: the oceanographic approach, the spirit leveling approach and the gravity field approach. All three methods are discussed and advantages and drawbacks are evaluated. A set of tide gauge locations is used to compute height datum offsets between national height reference frames in Europe based on all three approaches. The results agree on a level of 5–10 cm. In summary, the gravity field approach is the most flexible approach. The ESA satellite gravity mission GOCE ensures a uniform global level in the range of 1–2 cm. Hence, the gravity field approach is a suitable approach for future realizations of the European Vertical Reference Frame as well as for the establishment of a World Height System.

Height System Unification Based on GOCE Gravity Field Models: Benefits and Challenges

In addition to the traditional way of realizing height systems based on spirit levelling and local gravity observations methods based on gravity field models and GNSS observations become more important. This contribution validates recent global gravity field models (GGM) with independent GNSS/levelling data in Germany. A European GNSS/levelling data set and a GOCO02S and EGM2008 combined GGM are used to unify the national European height systems. The comparison of the results to the traditional approach results based on the United European Levelling Network (UELN) confirms the high potential of this method although in most cases the satellite only GGMs need to be densified by additional terrestrial observations.

Earth and space observation at the German Antarctic Receiving Station O’Higgins

The German Antarctic Receiving Station (GARS) O’Higgins at the northern tip of the Antarctic Peninsula is a dual purpose facility for earth observation and has existed for more than 20 years. It serves as a satellite ground station for payload data downlink and telecommanding of remote sensing satellites as well as a geodetic observatory for global reference systems and global change. Both applications use the same 9 m diameter radio antenna. Major outcomes of this usage are summarised in this paper. The satellite ground station O’Higgins (OHG) is part of the global ground station network of the German Remote Sensing Data Centre (DFD) operated by the German Aerospace Centre (DLR). It was established in 1991 to provide remote sensing data downlink support within the missions of the European Remote Sensing Satellites ERS-1 and ERS-2. These missions provided valuable insights into the changes of the Antarctic ice shield. Especially after the failure of the on-board data recorder, OHG became an essential downlink station for ERS-2 real-time data transmission. Since 2010, OHG is manned during the entire year, specifically to support the TanDEM-X mission. OHG is a main dump station for payload data, monitoring and telecommanding of the German TerraSAR-X and TanDEM-X satellites. For space geodesy and astrometry the radio antenna O’Higgins significantly improves coverage over the southern hemisphere and plays an essential role within the global Very Long Baseline Interferometry (VLBI) network. In particular the determination of the Earth Orientation Parameters (EOP) and the sky coverage of the International Celestial Reference Frame (ICRF) benefit from the location at a high southern latitude. Further, the resolution of VLBI images of active galactic nuclei (AGN), cosmic radio sources defining the ICRF, improves significantly when O’Higgins is included in the network. The various geodetic instrumentation and the long time series at O’Higgins allow a reliable determination of crustal motions. VLBI station velocities, continuous GNSS measurements and campaign-wise absolute gravity measurements consistently document a vertical rate of about 5 mm/a. This crustal uplift is interpreted as an elastic rebound due to ice loss as a consequence of the ice shelf disintegration in the Prince Gustav Channel in the late 1990s. The outstanding location on the Antarctic continent and its year-around operation make GARS O’Higgins in future increasingly attractive for polar orbiting satellite missions and a vitally important station for the global VLBI network. Future plans call for the development of an observatory for environmentally relevant research. That means that the portfolio of the station will be expanded including the expansion of the infrastructure and the construction and operation of new scientific instruments suitable for long-term measurements and satellite ground truthing.

Vertical Crustal Deformation in Dronning Maud Land, Antarctica: Observation versu s Model Prediction

During the last decade a variety of geodetic observations have been carried out in central Dronning Maud Land, East Antarctica, in order to investigate geodynamic and glaciologic phenomena. Of special interest is the interaction of recent and historic ice mass changes and the vertical crustal deformation, which is characterized by the rheological properties of the Earth, especially of the crust and the upper mantle. The geodetic information may help to constrain the recent isostatic uplift pattern, the recent and — using additional age information — past ice sheet configuration in central Dronning Maud Land. Coupling the geodetic data with these additional constraints on recent and past ice mass changes allows a self-consistent glacial load history to be investigated. A spectrum of viable load histories will be examined and the respective isostatic deformation signature will be computed, using a flat earth approach. First model computations will be presented and discussed, aiming to reconcile the modelled vertical uplift signature and the observations.

CODE Analysis center: Technical Report 2015

Applications of the Global Navigation Satellite Systems (GNSS) to Earth Sciences are numerous. The International GNSS Service (IGS), a voluntary federation of government agencies, universities and research institutions, combines GNSS resources and expertise to provide the highest–quality GNSS data, products, and services in order to support high–precision applications for GNSS–related research and engineering activities. This IGS Technical Report 2015 includes contributions from the IGS Governing Board, the Central Bureau, Analysis Centers, Data Centers, station and network operators, working groups, pilot projects, and others highlighting status and important activities, changes and results that took place and were achieved during 2015.