Markus Rothacher

Observation of the Earth System from Space

CHAMP CHAllenging Minisatellite Payload.- CHAMP Mission 5 Years in Orbit.- Remarks on CHAMP Orbit Products.- Harmonic Analysis of the Earths Gravitational Field from Kinematic CHAMP Orbits based on Numerically Derived Satellite Accelerations.- Earthquake Signatures in the Ionosphere Deduced from Ground and Space Based GPS Measurements.- Global Atmospheric Sounding with GPS Radio Occultation aboard CHAMP.- GRACE The Gravity Recovery and Climate Experiment.- Design and Operation of the GRACE ISDC.- De-aliasing of Short-term Atmospheric and Oceanic Mass Variations for GRACE.- Integrated Sensor Analysis GRACE.- Static and Time-Variable Gravity from GRACE Mission Data.- Gravity Field Recovery from GRACE-SST Data of Short Arcs.- Mapping Earths Gravitation Using GRACE Data.- High Frequency Temporal Earth Gravity Variations Detected by GRACE Satellites.- GOCE The Gravity Field and Steady-State Ocean Circulation Explorer.- From Kinematic Orbit Determination to Derivation of Satellite Velocity and Gravity Field.- Mission Simulation and Semi-analytical Gravity Field Analysis for GOCE SGG and SST.- GOCE Gravity Field Modeling: Computational Aspects - Free Kite Numbering Scheme.- An Integrated Global/Regional Gravity Field Determination Approach based on GOCE Observations.- High-Performance GOCE Gravity Field Recovery from Gravity Gradient Tensor Invariants and Kinematic Orbit Information.- The Impact of Temporal Gravity Variations on GOCE Gravity Field Recovery.- Quality Assessment of GOCE Gradients.- Color Figures.- IERS The International Earth Rotation and Reference Systems Service.- IERS Data and Information System.- IERS Analysis Coordination.- Analysis and Refined Computations of the International Terrestrial Reference Frame.- Combination of VLBI Analysis Results.- Towards a Rigorous Combination of Space Geodetic Observations for IERS Product Generation.- CONT02 Analysis and Combination of Long EOP Series.- Conventional and New Approaches for Combining Multi-Satellite Techniques.- GEOsensor Ring Laser for Seismology.- The GEOsensor Project: Rotations - a New Observable for Seismology.- Airborne Gravimetry.- Evaluation of Airborne Vector Gravimetry Using GNSS and SDINS Observations.- SAGS4 - StrapDown Airborne Gravimetry System Analysis.- Further Development of a High Precision Two-Frame Inertial Navigation System for Application in Airborne Gravimetry.

System Earth via Geodetic-Geophysical Space Techniques

CHAMP and GRACE.- More Accurate and Faster Available CHAMP and GRACE Gravity Fields for the User Community.- The CHAMP/GRACE User Portal ISDC.- Improvements for the CHAMP and GRACE Observation Model.- The Release 04 CHAMP and GRACE EIGEN Gravity Field Models.- Orbit Predictions for CHAMP and GRACE.- Rapid Science Orbits for CHAMP and GRACE Radio Occultation Data Analysis.- Parallelization and High Performance Computation for Accelerated CHAMP and GRACE Data Analysis.- GRACE.- Improved GRACE Level-1 and Level-2 Products and Their Validation by Ocean Bottom Pressure.- The GRACE Gravity Sensor System.- Numerical Simulations of Short-Term Non-tidal Ocean Mass Anomalies.- Improved Non-tidal Atmospheric and Oceanic De-aliasing for GRACE and SLR Satellites.- Global Gravity Fields from Simulated Level-1 GRACE Data.- ITG-GRACE: Global Static and Temporal Gravity Field Models from GRACE Data.- Validation of GRACE Gravity Fields by In-Situ Data of Ocean Bottom Pressure.- Antarctic Circumpolar Current Transport Variability in GRACE Gravity Solutions and Numerical Ocean Model Simulations.- GOCE.- Gravity and Steady-State Ocean Circulation Explorer GOCE.- GOCE Data Analysis: From Calibrated Measurements to the Global Earth Gravity Field.- GOCE and Its Use for a High-Resolution Global Gravity Combination Model.- Spectral Approaches to Solving the Polar Gap Problem.- Regionally Refined Gravity Field Models from In-Situ Satellite Data.- Quality Evaluation of GOCE Gradients.- Validation of Satellite Gravity Field Models by Regional Terrestrial Data Sets.- Comparison of GRACE and Model-Based Estimates of Bottom Pressure Variations Against In Situ Bottom Pressure Measurements.- SEAVAR.- Sea Level Variations - Prospects from the Past to the Present (SEAVAR).- Radar Altimetry Derived Sea Level Anomalies - The Benefit of New Orbits and Harmonization.- Combining GEOSAT and TOPEX/Poseidon Data by Means of Data Assimilation.- Reanalysis of GPS Data at Tide Gauges and the Combination for the IGS TIGA Pilot Project.- Sea Level Rise in North Atlantic Derived from Gap Filled Tide Gauge Stations of the PSMSL Data Set.- Using ARGO, GRACE and Altimetry Data to Assess the Quasi Stationary North Atlantic Circulation.- A 15-Year Reconstruction of Sea Level Anomalies Using Radar Altimetry and GPS-Corrected Tide Gauge Data.- TIVAGAM.- Continental Water Storage Variations from GRACE Time-Variable Gravity Data.- Surface Mass VariabilitySurface mass variability from GRACE and Hydrological Models Hydrological model : Characteristic PeriodsPeriods characteristic and the Reconstruction of Significant SignalsReconstruction of significant signals .- Time-Space Multiscale AnalysisTime-Space Multiscale Analysis Multiscale analysis and Its Application to GRACE and Hydrology Data.- Mass Variation Signals in GRACE Products and in Crustal Deformations crustal deformation from GPS: A Comparison.- Monthly and Daily Variations of Continental Water Storage and Flows.- Calibration of a Global Hydrological Modelglobal hydrological model with GRACE Data.- NRT-RO.- Near-Real-Time Provision and Usage of Global Atmospheric Data from CHAMP and GRACE (NRT-RO): Motivation and Introduction.- Global Atmospheric Data from CHAMP and GRACE-A: Overview and Results.- Near-Real Time Satellite Orbit Determination for GPS Radio Occultation with CHAMP and GRACE.- The Operational Processing System for GPS Radio Occultation Data from CHAMP and GRACE.- Assimilation of CHAMP and GRACE-A Radio Occultation Data in the GME Global Meteorological Model of the German Weather Service.- MAGFIELD.- The Earths Magnetic Field at the CHAMP Satellite Epoch.- GGOS-D.- Integration of Space Geodetic Techniques as the Basis for a Global Geodetic-Geophysical Observing System (GGOS-D): An Overview.- GGOS-D Data Management - From Data to Knowledge.- GGOS-D Consistent, High-Accuracy Technique-Specific Solutions.- GGOS-D Global Terrestrial Reference Frame.- GGOS-D Consistent and Combined Time Series of Geodetic/Geophyical Parameters.- GGOS-D Integration with Low Earth Orbiters.

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...

The role of GPS in the study of global change

Abstract The Global Positioning System (GPS) may be used today as a mature technique in geodesy and geodynamics. Thanks to the orbits, the Earth orientation parameters, and the coordinates and velocities of about 100 IGS (International GPS Service for Geodynamics) stations, which are made available to the scientific community on a daily basis, GPS is a very powerful and serious contributor to all scientific questions related to high accuracy positioning on and near the Earths surface. In this article we first give a short characterization of the GPS (as compared to other space-geodetic techniques like VLBI and SLR). The difficulties related to GPS-determined station heights are subsequently discussed, in particular in view of the necessity to model tropospheric refraction. In studies related to Global Change as, e.g., sea level monitoring, and postglacial rebound, station heights and their development in time are of particular interest. It is known for a long time, however, that GPS-derived station heights are not of the same quality as GPS-derived horizontal positions. It is thus much more delicate to extract vertical (as opposed to horizontal) movements from GPS time series. When looking at station heights on a global scale, the motion of the Earths crust, defined by the tracking sites, relative to the center of mass of the Earth has to be taken into consideration in order to correctly interpret the relative movement of the crust and the sea level. This “motion of the geocenter” may have a size of up to a few centimeters. Solar radiation pressure acting on the GPS satellites is difficult to model due to the complicated shape of the spacecrafts and may affect the geocenter estimates derived from GPS data. GPS may contribute, however, to monitoring the time variations of the geocenter. The influence of the troposphere on the GPS signals is not just a nuisance, it may be used as a signal, too. GPS may be used as an accurate instrument to measure the integrated water vapor above the GPS sites. Long time series of water vapor values from a dense global network may eventually reveal trends in the water vapor content of the atmosphere. Last but not least, we discuss the importance of the IGS, its global products and the densification project, for Global Change investigations.

Validation of GPS slant delays using water vapour radiometers and weather models

Slant delay data obtained from global positioning system (GPS) observations carry valuable meteorological information. The spatial distribution of the water vapour can be reconstructed from such slant delays. To estimate the quality of the GPS slant delays two validation studies were carried out. One study was based on the observations of a water vapour radiometer, a second on the analysis fields of a numerical weather model which were used to compute the corresponding GPS delays. Both studies yielded a high correlation between the available slant delays at higher elevation angles but showed deficiencies at low elevations. The mean bias between the GPS zenith delays and the radiometer data is 1.18 mm with a RMS of 6.0 mm. The corresponding bias and RMS of the GPS vs. model comparison are 3.3 mm and 2.9 mm.

Comparison of Water Vapor and Temperature Results From GPS Radio Occultation Aboard CHAMP With MOZAIC Aircraft Measurements

Global positioning system (GPS) radio occultation (RO) observations aboard low earth orbiting (LEO) satellites provide a powerful tool for global atmospheric sounding. Almost continuously activated since mid-2001, the challenging minisatellite payload (CHAMP) GPS RO experiment provides up to 200 vertical atmospheric profiles per day. In this paper, we intercompare CHAMP RO humidity results and analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF) with coinciding measurement of ozone and water vapor by airbus in-service aircraft (MOZAIC) data collected during aircraft ascents and descents. About 320 coinciding profiles with CHAMP were found from 2001 to 2006 (coincidence radius: 3 h, 300 km). Between about 650 and 300 hPa, the CHAMP-MOZAIC humidity bias is smaller than the ECMWF-MOZAIC bias. On the other hand, the standard deviation between MOZAIC and CHAMP humidity is slightly higher than that between MOZAIC and ECMWF through the entire altitude range. Apart from the water vapor validation (ascent and descent data), we also compare MOZAIC cruise data at an altitude of typically 10-11 km with CHAMP refractivity and temperature results (dry retrieval), and corresponding ECMWF analysis data. Whereas refractivity data from MOZAIC, CHAMP, and ECMWF show excellent agreement, the CHAMP temperature exhibits a cold bias of about 0.9 K in comparison to MOZAIC and ECMWF.

Ambiguity resolution strategies using the results of the International GPS Geodynamics Service (IGS)

Resolving the initial phase ambiguities of GPS carrier phase observations was always considered an important aspect of GPS processing techniques. Resolution of the so-called wide-lane ambiguities using a special linear combination of theL1 andL2 carrier and code observations has become standard. New aspects have to be considered today: (1) Soon AS, the so-called Anti-Spoofing, will be turned on for all Block II spacecrafts. This means that precise code observations will be no longer available, which in turn means that the mentioned approach to resolve the wide-lane ambiguities will fail. (2) Most encouraging is the establishment of the new International GPS Geodynamics Service (IGS), from where high quality orbits, earth rotation parameters, and eventually also ionospheric models will be available. We are reviewing the ambiguity resolution problem under these new aspects: We look for methods to resolve the initial phase ambiguities without using code observations but using high quality orbits and ionospheric models from IGS, and we study the resolution of the ”narrow-lane ambiguities” (after wide-lane ambiguity resolution) using IGS orbits.

The Tracking, Occultation and Ranging (TOR) instrument onboard TerraSAR-X and on TanDEM-X

TerraSAR-X, to be launched at the end of May, 2007, carries the tracking, occultation and ranging (TOR) Category A payload instrument package. The TOR consists of a high-precision dual-frequency GPS receiver, called integrated GPS occultation receiver (IGOR), for precise orbit determination and atmospheric sounding and a laser retro-reflector (LRR) serving as target for the global satellite laser ranging (SLR) ground station network. The TOR is supplied by the GeoForschungsZentrum Potsdam (GFZ) Germany, and the Center for Space Research (CSR), Austin, Texas. The objective of the German/US collaboration is twofold: provision of atmospheric profiles for use in numerical weather predictions and climate studies from the occultation data and precision SAR data processing based on precise orbits and atmospheric products. GFZ also supplies the TOR instrumentation for the follow-on mission TanDEM-X. For the scientific objectives of the TanDEM- X mission, i.e. bi-static SAR together with TerraSAR-X, the dual- frequency GPS receiver is of vital importance for the millimeter level determination of the baseline between the two spacecrafts. In the following, the TOR instrumentation is characterized by the features of and accuracies achievable with the GPS receiver and the LRR. The data flow is shown, as GFZ operates a high latitude data receiving station for fast data access, and runs a data system for preprocessing, archiving and retrieval. With this data system, higher level products are generated for TerraSAR- X, including near real-time orbits by GFZ and post-processed precise science orbits by GFZ and CSR. In the case of TanDEM- X, GFZ operationally determines the inter-satellite baseline on the millimeter level. CSR produces calibration orbits to support the GFZ operational activity. Orbit results in terms of latency and accuracy from current missions are given and achievable results for TerraSAR-X and TanDEM-X are discussed.

Status of GNSS reflectometry related receiver developments and feasibility studies within the German Indonesian Tsunami Early Warning System

In the frame of the German Indonesian Tsunami Early Warning System (GITEWS) project a multi-frequency Global Navigation Satellite System (GNSS) Occultation & Reflectometry & Scatterometry (GORS) space receiver is developed. It is based on commercial off-the-shelf (COTS) GNSS receiver technology, as the core instrument for a future tsunami detection constellation of small low Earth orbit (LEO) satellites. For use in reflectometry, scatterometry and radio-occultation measurements as well as high-precision navigation applications, specific adaptations of the GNSS receiver firmware are desirable, which require a close interaction between scientists and the receiver manufacturer. Within the GITEWS project GFZ has set up a team consisting of GFZ, DLR and JAVAD GNSS (JAVAD) to adapt and extend their new generation GNSS receivers for advanced scientific space applications. Specific adaptations address the improvement of the cold start time-to-first-fix, the selection of optimal tracking loop parameters and channel slaving for monitoring of reflected signals. Besides pseudorange, phase and signal-to-noise measurements, the modified receiver allows output of in-phase (I) and quadrature-phase (Q) accumulations at 5 msec intervals (200 Hz). As a major step forward compared to current space receivers, the new receiver supports tracking of the civil L2C signal of the GPS constellation. An overview of the current status is given and first results are discussed. Within GITEWS the feasibility of a tsunami detection mission is studied, including the constellation mission design, the options for operating the system and the ways to develop an end-to-end system for the quick response to tsunami events. In parallel simulation studies of the GNSS signals reflected to a LEO satellite are carried out. This will be realised by a Zavorotny and Voronovich scattering model with a two-scale model approach using an Elfouhaily sea wave spectrum. An overview of the current activities is given and first results are discussed.

CONT02 Analysis and Combination of Long EOP Series

This report is divided into two parts: the first part gives an overview of the combination studies performed by the Forschungseinrichtung Satellitengeodasie TU Munchen (FESG) and the Deutsches Geodatisches Forschungsinstitut (DGFI) based on the data of the continuous IVS campaign CONT02. The close cooperation of the two institutions established the basis for a detailed adaption of the GPS and the VLBI software concerning models and parameterization to avoid systematic differences between the technique contributions. Special attention was payed to parameters with a high temporal resolution; in this study tropospheric parameters and Earth rotation parameters (ERP) are considered. Including the troposphere parameters offered a good possibility to study the correlation between troposphere parameters and station coordinates. It was found that this interaction can deliver a very important contribution to validate the available local tie information. For comparison of the troposphere results derived for the 14-days campaign CONT02, long time series for VLBI and GPS were used as well, and it turned out that the results are in good agreement. Regarding the sub-daily Earth rotation parameters it can be shown that a combination of the space techniques improves the results compared to single-technique solutions. Furthermore, it is illustrated that UT1-UTC can be combined from VLBI together with the satellite techniques. All in all, the presented results demonstrate the high potential of a combination of VLBI, GPS and SLR data. The second part is devoted to the combination of long sub-daily EOP time series from VLBI and GPS. Space geodetic techniques like the Global Positioning System (GPS) and Very Long Baseline Interferometry (VLBI) can provide Earth Orientation Parameter (EOP) time series with very high sampling rates. This offers the opportunity to study sub-daily tidal excitations and the influences of high-frequency or episodic geophysical effects on Earth rotation. Therefore we need sub-daily time series as consistent and homogeneous as possible. Based on the Combined Smoothing method of Vondrak and Cepek (2000), we developed a new combination scheme for sub-daily EOPs to obtain a new sub-daily time series which benefits from the longterm stability of VLBI and the continuity of GPS. Furthermore we can remove the weakness of UT1 estimations of the satellite techniques. We analysed the combination using spectral imaging methods and the results from sub-daily tidal harmonic estimation.