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departmentresearch group programmeprojectcoordinates)uuid:442fe5646bd645599edd48aac2c85c60Dhttp://resolver.tudelft.nl/uuid:442fe5646bd645599edd48aac2c85c60KAbsolute and relative orbit determination for the CHAMP/GRACE constellationMao, X. (TU Delft Astrodynamics & Space Missions); Visser, P.N.A.M. (TU Delft Astrodynamics & Space Missions); van den IJssel, J.A.A. (TU Delft Astrodynamics & Space Missions)Precise orbit determination was investigated for a satellite constellation comprised of two different missions, the CHAllenging Minisatellite Payload (CHAMP) satellite and the Gravity Recovery And Climate Experiment (GRACE) twin satellites. The orbital planes of these two missions aligned closely during March to May 2005, allowing precise baseline determinations between the associated three satellites based on their onboard BlackJack Global Positioning System (GPS) receivers. The GRACEA/B satellites fly in tandem formation with a baseline of around 220 km, whereas the baselines between CHAMP and the GRACE tandem vary from about 110 to 7500 km during 24h orbital arcs centered around the points of closest approaches. A number of factors had to be dealt with for orbit determinations, including the crosstalk between the CHAMP GPS main navigation and occultation antennas, the different levels of nongravitational accelerations, and the rapidly changing geometry that complicates the fixing of integer ambiguities for the GPS carrierphase observations. Quality assessments of the orbit solutions were based on comparisons with Satellite Laser Ranging (SLR) observations, best orbit solutions had a precision of typically 1.7 2.3 cm. Consistency checks between reduceddynamic and kinematic orbit solutions were done. For the GRACE baselines, the reduceddynamic/kinematic baseline consistency was typically better than 1 cm, with an ambiguity fixing success rate of around 94%. The agreement with the K/KaBand Radar Ranging (KBR) measurements was about 0.6 mm. For the CHAMP/GRACE pairs, the reduceddynamic/kinematic baseline consistency varied from 0.5 to 2.5 cm, where better consistency was obtained for shorter arcs.Antenna pattern; Highdynamic baseline; Integer ambiguity; Precise baseline determination; Precise orbit determination; Satellite constellationenjournal articleAccepted author manuscript
20210517)uuid:e3f7b9e6d4294684953e8ac9de32d20cDhttp://resolver.tudelft.nl/uuid:e3f7b9e6d4294684953e8ac9de32d20c?Highdynamic baseline determination for the Swarm constellationBaseline determination for the European Space Agency Swarm magnetic field mission is investigated. Swarm consists of three identical satellites A, B and C. The SwarmA and C form a pendulum formation whose baseline length varies between about 30 and 180 km. SwarmB flies in a higher orbit, causing its orbital plane to slowly rotate with respect to those of SwarmA and C. This special geometry results in short periods when the SwarmB satellite is adjacent to the other Swarm satellites. Ten 24hr periods around such close encounters have been selected, with baseline lengths varying between 50 and 3500 km. All Swarm satellites carry highquality, dualfrequency and identical Global Positioning System receivers not only allowing precise orbit determination of the single Swarm satellites, but also allowing a rigorous assessment of the capability of precise baseline determination between the three satellites. These baselines include the highdynamic baselines between SwarmB and the other two Swarm satellites. For all orbit determinations, use was made of an Iterative Extended Kalman Filter approach, which could run in single, dual, and triplesatellite mode. Results showed that resolving the issue of halfcycle carrier phase ambiguities (present in original release of GPS RINEX data) and reducing the code observation nois< e by the German Space Operations Center converter improved the consistency of reduceddynamic and kinematic baseline solutions for both the SwarmA/C pendulum pair and other combinations of Swarm satellites. All modes led to comparable consistencies between the computed orbit solutions and satellite laser ranging observations at a level of 2 cm. In addition, the consistencies with singlesatellite ambiguity fixed orbit solutions by the German Space Operations Center are at comparable levels for all the modes, with reduceddynamic baseline consistency at a level of 13 mm for the pendulum SwarmA/C formation and 35 mm for the highdynamic SwarmB/A and B/C satellite pairs in different directions.jAmbiguity fixing; Halfcycle ambiguity; Precise baseline determination; Precise orbit determination; Swarm
20210327)uuid:b87d25053c8f4e4d8ed6bb36c8f29c73Dhttp://resolver.tudelft.nl/uuid:b87d25053c8f4e4d8ed6bb36c8f29c73Characterization of Thermospheric Vertical Wind Activity at 225 to 295km Altitude Using GOCE Data and Validation Against Explorer MissionsVisser, T. (TU Delft Astrodynamics & Space Missions); March, G. (TU Delft Astrodynamics & Space Missions); Doornbos, E.N. (TU Delft Astrodynamics & Space Missions); de Visser, C.C. (TU Delft Control & Simulation); Visser, P.N.A.M. (TU Delft Astrodynamics & Space Missions)Recently, the horizontal and vertical cross wind at 225 to 295km altitude were derived from linear acceleration measurements of the Gravity field and steadystate Ocean Circulation Explorer satellite. The vertical component of these wind data is compared to wind data derived from the mass spectrometers of the Atmosphere Explorer C and E and Dynamics Explorer 2 satellites. From a statistical analysis of the 120s movingwindow standard deviation of the vertical wind ((V<sub>z</sub>)), no consistent discrepancy is found between the accelerometerderived and the mass spectrometerderived data. The validated Gravity field and steadystate Ocean Circulation Explorer data are then used to investigate the influence of several parameters and indices on the vertical wind activity. To this end, the probability distribution of (V<sub>z</sub>) is plotted after distributing the data over bins of the parameter under investigation. The vertical wind is found to respond strongly to geomagnetic activity at high latitudes, although the response settles around a maximum standard deviation of 50m/s at an Auroral Electrojet index of 800. The dependence on magnetic local time changes with magnetic latitude, peaking around 4:30 over the polar cap and around 01:30 and 13:30 in the auroral oval. Seasonal effects only become visible at low to middle latitudes, revealing a peak wind variability in both local summer and winter. The vertical wind is not affected by the solar activity level.Atmosphere Explorers; Dynamics Explorer 2; Gravity field and steadystate Ocean Circulation Explorer (GOCE); thermospheric vertical wind)uuid:fc40039296624b0f9675568ae26d591bDhttp://resolver.tudelft.nl/uuid:fc40039296624b0f9675568ae26d591baThe impact of GPS receiver modifications and ionospheric activity on Swarm baseline determinationThe European Space Agency (ESA) Swarm mission is a satellite constellation launched on 22 November 2013 aiming at observing the Earth geomagnetic field and its temporal variations. The three identical satellites are equipped with highprecision dualfrequency Global Positioning System (GPS) receivers, which make the constellation an ideal test bed for baseline determination. From October 2014 to August 2016, a number of GPS receiver modifications and a new GPS Receiver Independent Exchange Format (RINEX) converter were implemented. Moreover, the onboard GPS receiver performance has been influenced by the ionospheric scintillations. The impact of these factors is assessed for baseline determination of the pendulum formation flying SwarmA and C satellites. In total 30 months of data  from 15 July 2014 to the end of 2016  is analyzed. The assessment includes analysis of observation residuals, success r< ate of GPS carrier phase ambiguity fixing, a consistency check between the socalled kinematic and reduceddynamic baseline solution, and validations of orbits by comparing with Satellite Laser Ranging (SLR) observations. External baseline solutions from The German Space Operations Center (GSOC) and Astronomisches Institut  Universitt Bern (AIUB) are also included in the comparison. Results indicate that the GPS receiver modifications and RINEX converter changes are effective to improve the baseline determination. This research eventually shows a consistency level of 9.3/4.9/3.0 mm between kinematic and reduceddynamic baselines in the radial/alongtrack/crosstrack directions. On average 98.3% of the epochs have kinematic solutions. Consistency between TU Delft and external reduceddynamic baseline solutions is at a level of 1 mm level in all directions.xAntenna patterns; GPS receiver modifications; Ionospheric scintillation; Precise baseline determination; Swarm satellite
20200320Astrodynamics & Space Missions)uuid:5a9cd3ef66ae49be8cd105a5d80d21b8Dhttp://resolver.tudelft.nl/uuid:5a9cd3ef66ae49be8cd105a5d80d21b8DLaser and radio tracking for planetary science missions a comparisonDirkx, D. (TU Delft Astrodynamics & Space Missions); Prochazka, Ivan (Czech Technical University); Bauer, Sven (VolcanoTectonics Junior Research Group); Visser, P.N.A.M. (TU Delft Astrodynamics & Space Missions); Noomen, R. (TU Delft Astrodynamics & Space Missions); Gurvits, L. (TU Delft Astrodynamics & Space Missions; Joint Institute for VLBI ERIC); Vermeersen, L.L.A. (TU Delft Physical and Space Geodesy; TU Delft Astrodynamics & Space Missions)At present, tracking data for planetary missions largely consists of radio observables: rangerate (Doppler), range and angular position (VLBI/ DOR). Future planetary missions may use Interplanetary Laser Ranging (ILR) as a tracking observable. Twoway ILR will provide range data that are about 2 orders of magnitude more accurate than radiobased range data. ILR does not produce Doppler data, however. In this article, we compare the relative strength of radio Doppler and laser range data for the retrieval of parameters of interest in planetary missions, to clarify and quantify the science case of ILR, with a focus on geodetic observables. We first provide an overview of the nearterm attainable quality of ILR, in terms of both the realization of the observable and the models used to process the measurements. Subsequently, we analyse the sensitivity of radio Doppler and laser range measurements in representative mission scenarios for parameters of interest. We use both an analytical approximation and numerical analyses of the relative sensitivity of ILR and radio Doppler observables for more general cases. We show that mmprecise range normal points are feasible for ILR, but mmlevel accuracy and stability in the full analysis chain are unlikely to be attained, due to a combination of instrumental and model errors. We find that ILR has the potential for superior performance in observing signatures in the data with a characteristic period of greater than 0.33 1.65 hours (assuming 2 10 mm uncertainty for range and 10 m/s at 60s for Doppler). This indicates that Doppler tracking will typically remain the method of choice for gravity field determination and spacecraft orbit determination in planetary missions. ILR data will be able to supplement the orbiter tracking data used for the estimation of parameters with a onceperorbit signal. Laser ranging data, however, are shown to have a significant advantage for the retrieval of rotational and tidal characteristics from landers. Similarly, laser ranging data will be superior for the construction of planetary ephemerides and the improvement of solar system tests of gravitation, both for orbiter and for lander missions.@Interplanetary laser ranging; Planetary missions; Radio tracking)uuid:3e2024aa3ed54e8fbaa8b74a86d2a94cDhttp://resolver.tudelft.nl/uuid:3e2024aa3ed54e8fbaa8b74a86d2a94c.Torque model verication for the GOCE satelliteVisser, T. (T< U Delft Astrodynamics & Space Missions); Doornbos, E.N. (TU Delft Astrodynamics & Space Missions); de Visser, C.C. (TU Delft Control & Simulation); Visser, P.N.A.M. (TU Delft Astrodynamics & Space Missions); Fritsche, Bent (Hyperschall Technologie Gttingen)
20201101)uuid:a33716dddea54320a99b198a8878bd4bDhttp://resolver.tudelft.nl/uuid:a33716dddea54320a99b198a8878bd4buCHAMP, GRACE, GOCE and Swarm density and wind characterization with improved gassurface interactions modelling (PPT)March, G. (TU Delft Astrodynamics & Space Missions); Doornbos, E.N. (TU Delft Astrodynamics & Space Missions); Visser, P.N.A.M. (TU Delft Astrodynamics & Space Missions)otherPPT)uuid:5d79b7d04b8447bca7246d0b1e1ae228Dhttp://resolver.tudelft.nl/uuid:5d79b7d04b8447bca7246d0b1e1ae228DUpdate on thermospheric density products from satellite observationsXMarch, G. (TU Delft Astrodynamics & Space Missions); Visser, T. (TU Delft Astrodynamics & Space Missions); Doornbos, E.N. (TU Delft Astrodynamics & Space Missions); Iorfida, E. (TU Delft Astrodynamics & Space Missions); van den IJssel, J.A.A. (TU Delft Astrodynamics & Space Missions); Visser, P.N.A.M. (TU Delft Astrodynamics & Space Missions)poster)uuid:a5ef399865f440b8a6d031d9dd954729Dhttp://resolver.tudelft.nl/uuid:a5ef399865f440b8a6d031d9dd954729QUsing the GOCE star trackers for validating the calibration of its accelerometers:Visser, P.N.A.M. (TU Delft Astrodynamics & Space Missions)>A method for validating the calibration parameters of the six accelerometers on board the Gravity field and steadystate Ocean Circulation Explorer (GOCE) from star tracker observations that was originally tested by an endtoend simulation, has been updated and applied to real data from GOCE. It is shown that the method provides estimates of scale factors for all three axes of the six GOCE accelerometers that are consistent at a level significantly better than 0.01 compared to the a priori calibrated value of 1. In addition, relative accelerometer biases and drift terms were estimated consistent with values obtained by precise orbit determination, where the first GOCE accelerometer served as reference. The calibration results clearly reveal the different behavior of the sensitive and lesssensitive accelerometer axes.[Accelerometer; Bias; Bias drift; Calibration; GOCE; Gradiometer; Scale factor; Star tracker)uuid:e6481676502b45258eb07621b4cfbe2eDhttp://resolver.tudelft.nl/uuid:e6481676502b45258eb07621b4cfbe2e0Quantifying deformation in North Borneo with GPS>Mustafar, Mohamad Asrul (Universiti Teknologi MARA); Simons, W.J.F. (TU Delft Astrodynamics & Space Missions); Tongkul, Felix (Universiti Malaysia Sabah); Satirapod, Chalermchon (Chulalongkorn University); Omar, Kamaludin Mohd (Universiti Teknologi Malaysia); Visser, P.N.A.M. (TU Delft Astrodynamics & Space Missions)The existence of intraplate deformation of the Sundaland platelet along its eastern edge in North Borneo, SouthEast Asia, makes it an interesting area that still is relatively understudied. In addition, the motion of the coastal area of NorthWest Borneo is directed toward a frontal foldandthrust belt and has been fueling a long debate on the possible geophysical sources behind it. At present this foldandthrust belt is not generating significant seismic activity and may also not be entirely active due to a decreasing shelfal extension from south to north. Two sets of Global Positioning System (GPS) data have been used in this study; the first covering a time period from 1999 until 2004 (ending just before the Giant Sumatra Andaman earthquake) to determine the continuous Sundaland tectonic plate motion, and the second from 2009 until 2011 to investigate the current deformations of North Borneo. Both absolute and relative positioning methods were carried out to investigate horizontal and vertical displacements. Analysis of the GPS results indicates a clear trend of extension along coastal regions of Sarawak and Brunei in North Borneo. On the contrary strain rate tensors in Sabah reveal that only insignificant an< d inconsistent extension and compression occurs throughout NorthWest Borneo. Moreover, station velocities and rotation rate tensors on the northern part of North Borneo suggest a clockwise (microblock) rotation. The first analysis of vertical displacements recorded by GPS in NorthWest Borneo points to low subsidence rates along the western coastal regions of Sabah and inconsistent trends between the Crocker and Trusmadi mountain ranges. These results have not been able to either confirm or reject the hypothesis that gravity sliding is the main driving force behind the local motions in North Borneo. The ongoing Sundaland Philippine Sea plate convergence may also still play an active role in the presentday deformation (crustal shortening) in North Borneo and the possible clockwise rotation of the northern part of North Borneo as a microblock. However, more observations need to be collected to determine if the northern part of North Borneo indeed is (slowly) moving independently.KGPS; Intraplate deformation; North Borneo; Sundaland; Tectonic deformation)uuid:08b8f5467b3442319bfd4aa61835d88eDhttp://resolver.tudelft.nl/uuid:08b8f5467b3442319bfd4aa61835d88e7Sentinel1A  First precise orbit determination resultsPeterContesse, H. (PosiTim UG); Jggi, Adrian (University of Bern); Fernndez, JJ (GMV AD); Escobar, D. (Deutsches Zentrum fr Luft und Raumfahrt e.V. (DLR)); Ayuga, F. (Deutsches Zentrum fr Luft und Raumfahrt e.V. (DLR)); Arnold, D (University of Bern); Wermuth, M. (Deutsches Zentrum fr Luft und Raumfahrt e.V. (DLR)); Hackel, S. (Deutsches Zentrum fr Luft und Raumfahrt e.V. (DLR)); Otten, M. (European Space Agency (ESA)); Simons, W.J.F. (TU Delft Astrodynamics & Space Missions); Visser, P.N.A.M. (TU Delft Astrodynamics & Space Missions); Hugentobler, U. (Technical University of Munich); Fmnias, P. (European Space Agency (ESA))bSentinel1A is the first satellite of the European Copernicus programme. Equipped with a Synthetic Aperture Radar (SAR) instrument the satellite was launched on April 3, 2014. Operational since October 2014 the satellite delivers valuable data for more than two years. The orbit accuracy requirements are given as 5. cm in 3D. In order to fulfill this stringent requirement the precise orbit determination (POD) is based on the dualfrequency GPS observations delivered by an eightchannel GPS receiver.The Copernicus POD (CPOD) Service is in charge of providing the orbital and auxiliary products required by the PDGS (Payload Data Ground Segment). External orbit validation is regularly performed by comparing the CPOD Service orbits to orbit solutions provided by POD expert members of the Copernicus POD Quality Working Group (QWG). The orbit comparisons revealed systematic orbit offsets mainly in radial direction (approx. 3. cm). Although no independent observation technique (e.g. DORIS, SLR) is available to validate the GPSderived orbit solutions, comparisons between the different antenna phase center variations and different reduceddynamic orbit determination approaches used in the various software packages helped to detect the cause of the systematic offset. An error in the given geometry information about the satellite has been found. After correction of the geometry the orbit validation shows a significant reduction of the radial offset to below 5. mm. The 5. cm orbit accuracy requirement in 3D is fulfilled according to the results of the orbit comparisons between the different orbit solutions from the QWG.cCopernicus; GPS; Orbit validation; Phase center variations; Precise orbit determination; Sentinel1)uuid:7e7b99f4c4974291b1528d82d1258dc3Dhttp://resolver.tudelft.nl/uuid:7e7b99f4c4974291b1528d82d1258dc3IHorizontal and Vertical Wind Measurements from GOCE Angular AccelerationsVisser, T. (TU Delft Astrodynamics & Space Missions); Doornbos, E.N. (TU Delft Astrodynamics & Space Missions); de Visser, C.C. (TU Delft Control & Simulation); Visser, P.N.A.M. (TU Delft Astrodynamics & Space Missions)UBecause of the highly accurate accelerometers, the GOCE mission has proven to b< e a unique source of thermosphere neutral density and crosswind data. In the current methods, in which only the horizontal linear accelerations are used, the vertical winds cannot be obtained. In the algorithm proposed in this paper, angular accelerations derived from the individual gradiometer accelerations are used to obtain the vertical wind speeds as well. To do so, the measured angular rate and acceleration are combined to find a measurement of the torque acting on the spacecraft. This measurement is then corrected for modeled control torque applied by the magnetic torquers, aerodynamic torque, gravity gradient torque, solar radiation pressure torque, the torque caused by the misalignment of the thrust with respect to the center of gravity, and magnetic torque caused by the operation of several different subsystems of the spacecraft bus. Since the proper documentation of the magnetic properties of the payload were not available, a least squares estimate is made of one hard and one softmagnetic dipole pertaining to the payload, on an aerodynamically quiet day. The model for aerodynamic torque uses moment coefficients from MonteCarlo Test Particle software ANGARA. Finally the neutral density, horizontal crosswind, and vertical wind are obtained from an iterative process, in which the residual forces and torques are minimized. It is found that, like horizontal wind, the vertical wind responds strongly to geomagnetic storms. This response is observed over the whole latitude range, and shows seasonal variations.)uuid:6c53412db1fa4172856edb6979033321Dhttp://resolver.tudelft.nl/uuid:6c53412db1fa4172856edb6979033321)uuid:a7ffb975b4f2498b9864a7b076fabda8Dhttp://resolver.tudelft.nl/uuid:a7ffb975b4f2498b9864a7b076fabda8[1:1 Groundtrack resonance in a uniformly rotating 4th degree and order gravitational fieldFeng, J. (TU Delft Astrodynamics & Space Missions); Noomen, R. (TU Delft Astrodynamics & Space Missions); Hou, Xiyun (Nanjing University); Visser, P.N.A.M. (TU Delft Astrodynamics & Space Missions); Yuan, Jianping (Northwestern Polytechnical University)Using a gravitational field truncated at the 4th degree and order, the 1:1 groundtrack resonance is studied. To address the main properties of this resonance, a 1degree of freedom (1DOF) system is firstly studied. Equilibrium points (EPs), stability and resonance width are obtained. Different from previous studies, the inclusion of nonspherical terms higher than degree and order 2 introduces new phenomena. For a further study about this resonance, a 2DOF model which includes a main resonance term (the 1DOF system) and a perturbing resonance term is studied. With the aid of Poincar sections, the generation of chaos in the phase space is studied in detail by addressing the overlap process of these two resonances with arbitrary combinations of eccentricity (e) and inclination (i). Retrograde orbits, near circular orbits and near polar orbits are found to have better stability against the perturbation of the second resonance. The situations of complete chaos are estimated in the e i plane. By applying the maximum Lyapunov Characteristic Exponent (LCE), chaos is characterized quantitatively and similar conclusions can be achieved. This study is applied to three asteroids 1996 HW1, Vesta and Betulia, but the conclusions are not restricted to them.r1996 HW1; Asteroid; Betulia; Chaos; Equilibrium Points (EPs); Poincar sections; Resonance width; Stability; Vesta)uuid:73738044c41a4b8d958fc70d0fa2ceeeDhttp://resolver.tudelft.nl/uuid:73738044c41a4b8d958fc70d0fa2ceeegCHAMP, GRACE, GOCE and Swarm Thermosphere Density Data with Improved Aerodynamic and Geometry Modelling+Since 2000, accelerometers on board of the CHAMP, GRACE, GOCE and Swarm satellites have provided highresolution thermosphere density data, improving knowledge on atmospheric dynamics and coupling processes in the thermosphereionosphere layer. Most of the research has focused on relative changes in density. Scale differences between datasets and models have been largely neglected or removed < using ad hoc scale factors. The origin of these variations arises from errors in the aerodynamic modelling, specifically in the modelling of the satellite outer surface geometry and of the gassurface interactions. Therefore, in order to further improve density datasets and models that rely on these datasets, and in order to make them align with each other in terms of the absolute scale of the density, it is first required to enhance the geometry modelling. Once accurate geometric models of the satellites are available, it will be possible to enhance the characterization of the gassurface interactions, and to enhance the satellite aerodynamic modelling. This presentation offers an accurate approach for determining aerodynamic forces and torques and improved density data for CHAMP, GRACE, GOCE and Swarm. Through detailed high fidelity 3D CAD models and Direct Simulation Monte Carlo computations, flow shadowing and complex concave geometries can be investigated. This was not possible with previous closedform solutions, especially because of the low fidelity geometries and the incapability to introduce shadowing effects. This inaccurate geometry and aerodynamic modelling turned out to have relevant influence on derived densities, particularly for satellites with complex elongated shapes and protruding instruments, beams and antennae. Once the geometry and aerodynamic modelling have been enhanced with the proposed approach, the accelerometer data can be reprocessed leading to 81 higher fidelity density estimates. An overview of achieved improvements and dataset comparisons will be provided together with an introduction to the next gassurface interactions research phase.)uuid:416856937431441e9ad5368b132c308bDhttp://resolver.tudelft.nl/uuid:416856937431441e9ad5368b132c308b0Gravity field models derived from Swarm GPS data?de Teixeira da Encarnacao, J. (TU Delft Astrodynamics & Space Missions); Arnold, Daniel (University of Bern); Bezdk, Alea (Czech Academy of Sciences and Arts); Dahle, Christoph (University of Bern); Doornbos, E.N. (TU Delft Astrodynamics & Space Missions); van den IJssel, J.A.A. (TU Delft Astrodynamics & Space Missions); Jggi, Adrian (University of Bern); MayerGrr, Torsten (Graz University of Technology); Sebera, Josef (Czech Academy of Sciences and Arts); Visser, P.N.A.M. (TU Delft Astrodynamics & Space Missions); Zehentner, Norbert (Graz University of Technology) It is of great interest to numerous geophysical studies that the time series of global gravity field models derived from Gravity Recovery and Climate Experiment (GRACE) data remains uninterrupted after the end of this mission. With this in mind, some institutes have been spending efforts to estimate gravity field models from alternative sources of gravimetric data. This study focuses on the gravity field solutions estimated from Swarm global positioning system (GPS) data, produced by the Astronomical Institute of the University of Bern, the Astronomical Institute (ASU, Czech Academy of Sciences) and Institute of Geodesy (IfG, Graz University of Technology). The three sets of solutions are based on different approaches, namely the celestial mechanics approach, the acceleration approach and the shortarc approach, respectively. We derive the maximum spatial resolution of the timevarying gravity signal in the Swarm gravity field models to be degree 12, in comparison with the more accurate models obtained from Kband ranging data of GRACE. We demonstrate that the combination of the GPSdriven models produced with the three different approaches improves the accuracy in all analysed monthly solutions, with respect to any of them. In other words, the combined gravity field model consistently benefits from the individual strengths of each separate solution. The improved accuracy of the combined model is expected to bring benefits to the geophysical studies during the period when no dedicated gravimetric mission is operational.EGRACE; Gravity field; Highlow satellitetosatellite tracking; Swarm)uuid:8d88e5fb02124f64a0536734c19dd397Dhttp://resolver< .tudelft.nl/uuid:8d88e5fb02124f64a0536734c19dd397 GOCE Aerodynamic Torque ModelingVisser, T. (TU Delft Astrodynamics & Space Missions); Doornbos, E.N. (TU Delft Astrodynamics & Space Missions); de Visser, C.C. (TU Delft Control & Simulation); Visser, P.N.A.M. (TU Delft Astrodynamics & Space Missions); Fritsche, B (Hyperschall Technologie Gttingen)In recent studies thermospheric densities and crosswinds have been derived from linear acceleration measurements of the gradiometer on board the GOCE satellite. Our current work is aimed at analyzing also the angular accelerations, in order to improve the thermosphere density and wind data by allowing for the estimation of more unknown parameters. On this poster an overview is provided of the modeling efforts involved in isolating the aerodynamic torque. The intermediate result is a comparison of modeled and measured torques. Each box contains a plot of the torque from a specific source, compared to the measured torque, on October 16th, 2013. A short description of the model for each torque is also provided.)uuid:156d6522a8db47c5a85b3341be2791afDhttp://resolver.tudelft.nl/uuid:156d6522a8db47c5a85b3341be2791afIModeling and analysis of periodic orbits around a contact binary asteroid0Feng, J.; Noomen, R.; Visser, P.N.A.M.; Yuan, J.The existence and characteristics of periodic orbits (POs) in the vicinity of a contact binary asteroid are investigated with an averaged spherical harmonics model. A contact binary asteroid consists of two components connected to each other, resulting in a highly bifurcated shape. Here, it is represented by a combination of an ellipsoid and a sphere. The gravitational field of this configuration is for the first time expanded into a spherical harmonics model up to degree and order 8. Compared with the exact potential, the truncation at degree and order 4 is found to introduce an error of less than 10 % at the circumscribing sphere and less than 1 % at a distance of the double of the reference radius. The Hamiltonian taking into account harmonics up to degree and order 4 is developed. After double averaging of this Hamiltonian, the model is reduced to include zonal harmonics only and frozen orbits are obtained. The tesseral terms are found to introduce significant variations on the frozen orbits and distort the frozen situation. Applying the method of Poincar sections, phase space structures of the singleaveraged model are generated for different energy levels and rotation rates of the asteroid, from which the dynamics driven by the 44 harmonics model is identified and POs are found. It is found that the disturbing effect of the highly irregular gravitational field on orbital motion is weakened around the polar region, and also for an asteroid with a fast rotation rate. Starting with initial conditions from this averaged model, families of exact POs in the original nonaveraged system are obtained employing a numerical search method and a continuation technique. Some of these POs are stable and are candidates for future missions.qcontact binary asteroid; spherical harmonics; averaging method; frozen orbits; poincar sections; periodic orbitsSpringerAerospace EngineeringSpace Engineering)uuid:9e71e73e0026488da012d1328e5c82bbDhttp://resolver.tudelft.nl/uuid:9e71e73e0026488da012d1328e5c82bbRuimtevaart in bewegingVisser, P.N.A.M.
Intreeredepublic lectureDelft University of Technology)uuid:a48a4b5cec07468098b2bcf2659e5451Dhttp://resolver.tudelft.nl/uuid:a48a4b5cec07468098b2bcf2659e5451[Calibration and validation of individual GOCE accelerometers by precise orbit determination(Visser, P.N.A.M.; Van den IJssel, J.A.A.The European Space Agency Gravity field and steadystate Ocean Circular Explorer (GOCE) carries a gradiometer consisting of three pairs of accelerometers in an orthogonal triad. Precise GOCE science orbit solutions (PSO), which are based on satellitetosatellite tracking observations by the Global Positioning System and which are claimed to be at the few cm precision level, can be used to calibrate and vali< date the observations taken by the accelerometers. This has been done for each individual accelerometer by a dynamic orbit fit of the time series of position coordinates from the PSOs, where the accelerometer observations represent the nongravitational accelerations. Since the accelerometers do not coincide with the center of mass of the GOCE satellite, the observations have to be corrected for rotational and gravity gradient terms. This is not required when using the socalled commonmode accelerometer observations, provided the center of the gradiometer coincides with the GOCE center of mass. Dynamic orbit fits based on these commonmode accelerations therefore served as reference. It is shown that for all individual accelerometers, similar dynamic orbit fits can be obtained provided the abovementioned corrections are made. In addition, accelerometer bias estimates are obtained that are consistent with offsets in the gravity gradients that are derived from the GOCE gradiometer observations.iGOCE; accelerometer; calibration; validation; precise orbit determination; bias; drift; gravity gradients)uuid:b1ae5c0cac8c4650a53f361887f8b31aDhttp://resolver.tudelft.nl/uuid:b1ae5c0cac8c4650a53f361887f8b31a_Antarctic outlet glacier mass change resolved at basin scale from satellite gravity gradiometryaBouman, J.; Fuchs, M.; Ivins, E.; Van der Wal, W.; Schrama, E.J.O.; Visser, P.N.A.M.; Horwath, M.The orbit and instrumental measurement of the Gravity Field and Steady State Ocean Circulation Explorer (GOCE) satellite mission offer the highest ever resolution capabilities for mapping Earth's gravity field from space. However, past analysis predicted that GOCE would not detect changes in ice sheet mass. Here we demonstrate that GOCE gravity gradiometry observations can be combined with Gravity Recovery and Climate Experiment (GRACE) gravity data to estimate mass changes in the Amundsen Sea Sector. This refined resolution allows land ice changes within the Pine Island Glacier (PIG), Thwaites Glacier, and Getz Ice Shelf drainage systems to be measured at respectively ?67??7, ?63??12, and ?55??9 Gt/yr over the GOCE observing period of November 2009 to June 2012. This is the most accurate pure satellite gravimetry measurement to date of current mass loss from PIG, known as the weak underbelly of West Antarctica because of its retrograde bed slope and high potential for raising future sea level.Rbasinscale ice mass change; Amundsen Sea Sector, Antarctica; satellite gravimetryAmerican Geophysical Union
20150220)uuid:755ecbfc742f4737a5a3ddc8f8ababb5Dhttp://resolver.tudelft.nl/uuid:755ecbfc742f4737a5a3ddc8f8ababb5jObserving coseismic gravity change from the Japan TohokuOki 2011 earthquake with GOCE gravity gradiometryNFuchs, M.J.; Bouman, J.; Broerse, D.B.T.; Visser, P.N.A.M.; Vermeersen, L.L.A.The Japan TohokuOki earthquake (9.0 Mw) of 11 March 2011 has left signatures in the Earth's gravity field that are detectable by data of the Gravity field Recovery and Climate Experiment (GRACE) mission. Because the European Space Agency's (ESA) satellite gravity mission Gravity field and steadystate Ocean Circulation Explorer (GOCE) launched in 2009 aims at high spatial resolution, its measurements could complement the GRACE information on coseismic gravity changes, although timevariable gravity was not foreseen as goal of the GOCE mission. We modeled the coseismic earthquake geoid signal and converted this signal to vertical gravity gradients at GOCE satellite altitude. We combined the single gradient observations in a novel way reducing the noise level, required to detect the coseismic gravity change, subtracted a global gravity model, and applied tailored outlier detection to the resulting gradient residuals. Furthermore, the measured gradients were alongtrack filtered using different gradient bandwidths where in the space domain Gaussian smoothing has been applied. Oneyear periods before and after earthquake occurrence have been compared with the modeled gradients. The comparison reveals that the earthquake signal is we< ll above the accuracy of the vertical gravity gradients at orbital height. Moreover, the obtained signal from GOCE shows a 1.3 times higher amplitude compared with the modeled signal. Besides the statistical significance of the obtained signal, it has a high spatial correlation of ~0.7 with the forward modeled signal. We conclude therefore that the coseismic gravity change of the Japan TohokuOki earthquake left a statistically significant signal in the GOCE measured gravity gradients.NGOCE; gravity gradients; Japan TohokuOki earthquake; coseismic gravity change
20140417!Civil Engineering and GeosciencesGeoscience & Remote Sensing)uuid:783d9e8a8aff46169be065b6b8442fa7Dhttp://resolver.tudelft.nl/uuid:783d9e8a8aff46169be065b6b8442fa7YValidation of GOCE gravity field models by means of orbit residuals and geoid comparisons6Gruber, T.; Visser, P.N.A.M.; Ackermann, C.; Hosse, M.Three GOCEbased gravity field solutions have been computed by ESA s highlevel processing facility and were released to the user community. All models are accompanied by variancecovariance information resulting either from the least squares procedure or a MonteCarlo approach. In order to obtain independent external quality parameters and to assess the current performance of these models, a set of independent tests based on satellite orbit determination and geoid comparisons is applied. Both test methods can be regarded as complementary because they either investigate the performance in the long wavelength spectral domain (orbit determination) or in the spatial domain (geoid comparisons). The test procedure was applied to the three GOCE gravity field solutions and to a number of selected prelaunch models for comparison. Orbit determination results suggest, that a pure GOCE gravity field model does not outperform the multiyear GRACE gravity field solutions. This was expected as GOCE is designed to improve the determination of the medium to high frequencies of the Earth gravity field (in the range of degree and order 50 to 200). Nevertheless, in case of an optimal combination of GOCE and GRACE data, orbit determination results should not deteriorate. So this validation procedure can also be used for testing the optimality of the approach adopted for producing combined GOCE and GRACE models. Results from geoid comparisons indicate that with the 2 months of GOCE data a significant improvement in the determination of the spherical harmonic spectrum of the global gravity field between degree 50 and 200 can be reached. Even though the ultimate mission goal has not yet been reached, especially due to the limited time span of used GOCE data (only 2 months), it was found that existing satelliteonly gravity field models, which are based on 7 years of GRACE data, can already be enhanced in terms of spatial resolution. It is expected that with the accumulation of more GOCE data the gravity field model resolution and quality can be further enhanced, and the GOCE mission goal of 1 2 cm geoid accuracy with 100 km spatial resolution can be achieved.GOCE; gravity field; validation; orbit; geoid)uuid:5f11d8f194d54229a05c4963306f3854Dhttp://resolver.tudelft.nl/uuid:5f11d8f194d54229a05c4963306f3854)GPSderived orbits for the GOCE satelliteyBock, H.; Jggi, A.; Meyer, U.; Visser, P.N.A.M.; Van den IJssel, J.A.A.; Van Helleputte, T.; Heinze, M.; Hugentobler, U.1The first ESA (European Space Agency) Earth explorer core mission GOCE (Gravity field and steadystate Ocean Circulation Explorer) was launched on 17 March 2009 into a sunsynchronous dusk dawn orbit with an exceptionally low initial altitude of about 280 km. The onboard 12channel dualfrequency GPS (Global Positioning System) receiver delivers 1 Hz data, which provides the basis for precise orbit determination (POD) for such a very low orbiting satellite. As part of the European GOCE Gravity Consortium the Astronomical Institute of the University of Bern and the Department of Earth Observation and Space Systems are responsible for the orbit determination of the GOCE satellite within the GOCE High< level Processing Facility. Both quicklook (rapid) and very precise orbit solutions are produced with typical latencies of 1 day and 2 weeks, respectively. This article summarizes the special characteristics of the GOCE GPS data, presents POD results for about 2 months of data, and shows that both latency and accuracy requirements are met. Satellite Laser Ranging validation shows that an accuracy of 4 and 7 cm is achieved for the reduceddynamic and kinematic Rapid Science Orbit solutions, respectively. The validation of the reduceddynamic and kinematic Precise Science Orbit solutions is at a level of about 2 cm.6GOCE; GPS; precise orbit determination; SLR validation)uuid:126ced2c046646b5b4a0ac3f17ccbe1bDhttp://resolver.tudelft.nl/uuid:126ced2c046646b5b4a0ac3f17ccbe1bJModelling and observing the 8.8 Chile and 9.0 Japan earthquakes using GOCEYBroerse, D.B.T.; Visser, P.N.A.M.; Bouman, J.; Fuchs, M.; Vermeersen, L.L.A.; Schmidt, M.Large earthquakes do not only heavily deform the crust in the vicinity of the fault, they also change the gravity field of the area affected by the earthquake due to mass redistribution in the upper layers of the Earth. Besides that, for suboceanic earthquakes deformation of the ocean floor causes relative sealevel changes and mass redistribution of water that has again a significant effect on the gravity field. Such a suboceanic earthquake occurred on 27 February 2010 in central Chili with a magnitude of Mw 8.8 and on 11 March 2011 with a magnitude of Mw 9.0 near the east coast of Honshu, Japan. This makes both a potential candidate for detecting the coseismic gravity changes in the GOCE gradiometer data. We will assess the detectability of gravity field changes in the GOCE gravity gradients by modelling these earthquakes using a forward model as well as taking differences of of GOCE data before and after the respective earthquakes.`GOCE; coseismic deformation; Maule 8.8 earthquake; Tohoku 9.0 earthquake; time variable gravityconference paperEuropean Space Agency (ESA)!Astrodynamics & Satellite Systems)uuid:41b0ab7d195048e2b2c1baafd15ec30eDhttp://resolver.tudelft.nl/uuid:41b0ab7d195048e2b2c1baafd15ec30e@GOCE SSTI L2 tracking losses and their impact on POD performanceXVan den IJssel, J.A.A.; Visser, P.N.A.M.; Doornbos, E.N.; Meyer, U.; Bock, H.; Jggi, A.The stateoftheart GOCE SatellitetoSatellite Tracking Instrument (SSTI) delivers highquality GPS data with an almost continuous 1 Hz data rate, which allows for very Precise Orbit Determination (POD). Despite this good performance, the GPS receiver shows occasional unexpected L2 tracking losses, which mainly occur close to the geomagnetic poles and, to a lesser extent, also along the geomagnetic equator. The number of unexpected L2 tracking losses varies in time and shows some correlation with solar activity. Less than 3% of the observation data is affected by these losses. Therefore, the effect on the POD remains limited. However, systematic effects might be present, as the quality of the GOCE orbits is slightly reduced over the polar regions. The striking correlation between the global distribution of ionospheric irregularities and L2 losses suggests scintillation effects might be present. Analysis of the time derivative of the geometryfree combination of GPS phase observations shows that unexpected L2 losses occur during times of rapid ionospheric fluctuations. GPS satellites in crosstrack direction are most affected by L2 losses.6GOCE; GPS; tracking losses; ionosphere; scintillations)uuid:68108b2ef488469588ae2d521c31d490Dhttp://resolver.tudelft.nl/uuid:68108b2ef488469588ae2d521c31d490hEvaluating GOCE data near a midocean ridge and possible application to crustal structure in ScandinaviaKVan der Wal, W.; Wang, L.; Visser, P.N.A.M.; Sneeuw, N.; Vermeersen, L.L.A.GOCE gravity fields are assessed in an area around Reykjanes Ridge. Ship gravity measurements were found to be to inaccurate to determine possible improvement of GOCE gravity field models compared to the best available GRACE gravity field model. Dif< ferences between the GOCE gravity field models and EGM2008 does not appear to contain a component of the midocean ridge signal. However the differences follow the Greenland coastline, which could indicate small errors in EGM 2008 there as a result of piecing together different gravity field observations. A Butterworth bandpass filter was applied to gradiometer observations at orbit height. After filtering, differences between repeat tracks with a magnitude of tens of mE are present, which can not be explained by position or attitude of the satellite. In order to reach the repeatability that can be expected from GOCE measurements, filtering methods need to improve. It was found that differences between global GRACE and GOCE gravity field models are small compared to uncertainty in crustal and upper mantle structure. Thus, geophysical inversion studies should focus on the gravity gradient observations in the instrument reference frame and at orbit height.)uuid:96cd39c81c5b4e449ee013358795f40dDhttp://resolver.tudelft.nl/uuid:96cd39c81c5b4e449ee013358795f40dGOCE level 2 gravity gradientsqBouman, J.; Fiorot, S.; Fuchs, M.; Gruber, T.; Schrama, E.J.O.; Tscherning, C.C.; Veicherts, M.; Visser, P.N.A.M.Two of the GOCE Level 2 products are the gravity gradients (GGs) in the Gradiometer Reference Frame (GRF) and the GGs in the Local NorthOriented Frame (LNOF). The GRF is an instrument frame and the GGs are derived from the L1b GGs. The L1b to L2 GG processing involves corrections for temporal gravity variations, outlier detection and data gap interpolation, as well as the external calibration of the GGs using independent gravity field information. Because of the gradiometer configuration, four out of the six GGs  VXX, VYY, VZZ and VXZ  will have a small error in the Measurement Band (MB), whereas the other two  VXY and VYZ  will have low accuracy. The GRF GGs are rotated to the LNOF that is directly related to the Earth.)uuid:f24a9cd197f44178bfad5d4ada4a64beDhttp://resolver.tudelft.nl/uuid:f24a9cd197f44178bfad5d4ada4a64be,GOCE gravitational gradients along the orbitoBouman, J.; Fiorot, S.; Fuchs, M.; Gruber, T.; Schrama, E.J.O.; Tscherning, C.; Veicherts, M.; Visser, P.N.A.M. GOCE is ESA s gravity field mission and the first satellite ever that measures gravitational gradients in space, that is, the second spatial derivatives of the Earth s gravitational potential. The goal is to determine the Earth s mean gravitational field with unprecedented accuracy at spatial resolutions down to 100 km. GOCE carries a gravity gradiometer that allows deriving the gravitational gradients with very high precision to achieve this goal. There are two types of GOCE Level 2 gravitational gradients (GGs) along the orbit: the gravitational gradients in the gradiometer reference frame (GRF) and the gravitational gradients in the local north oriented frame (LNOF) derived from the GGs in the GRF by pointwise rotation. Because the V XX , V YY , V ZZ and V XZ are much more accurate than V XY and V YZ , and because the error of the accurate GGs increases for low frequencies, the rotation requires that part of the measured GG signal is replaced by model signal. However, the actual quality of the gradients in GRF and LNOF needs to be assessed. We analysed the outliers in the GGs, validated the GGs in the GRF using independent gravity field information and compared their assessed error with the requirements. In addition, we compared the GGs in the LNOF with stateoftheart global gravity field models and determined the model contribution to the rotated GGs. We found that the percentage of detected outliers is below 0.1% for all GGs, and external gravity data confirm that the GG scale factors do not differ from one down to the 10?3 level. Furthermore, we found that the error of V XX and V YY is approximately at the level of the requirement on the gravitational gradient trace, whereas the V ZZ error is a factor of 2 3 above the requirement for higher frequencies. We show that the model contribution in the rotated GGs is 2 35% dependent on the gra< vitational gradient. Finally, we found that GOCE gravitational gradients and gradients derived from EIGEN5C and EGM2008 are consistent over the oceans, but that over the continents the consistency may be less, especially in areas with poor terrestrial gravity data. All in all, our analyses show that the quality of the GOCE gravitational gradients is good and that with this type of data valuable new gravity field information is obtained.DGOCE; gravitational gradients; external calibration; tensor rotation)uuid:3e3433c8e5eb4a0fada17c0d693f0020Dhttp://resolver.tudelft.nl/uuid:3e3433c8e5eb4a0fada17c0d693f0020_Dependency of resolvable gravitational spatial resolution on spaceborne observation techniques:Visser, P.N.A.M.; Schrama, E.J.O.; Sneeuw, N.; Weigelt, M.The socalled ColomboNyquist (Colombo, The global mapping of gravity with two satellites, 1984) rule in satellite geodesy has been revisited. This rule predicts that for a gravimetric satellite flying in a (near)polar circular repeat orbit, the maximum resolvable geopotential spherical harmonic degree (lmax) is equal to half the number of orbital revolutions (nr) the satellite completes in one repeat period. This rule has been tested for different observation types, including geoid values at sea level along the satellite ground track, orbit perturbations (radial,alongtrack, crosstrack), lowlow satellitetosatellite tracking, and satellite gravity gradiometry observations (all three diagonal components). Results show that the Colombo Nyquist must be reformulated. Simulations indicate that the maximum resolvable degree is in fact equal to knr + 1, where k can be equal to 1, 2, or even 3 depending on the combination of observation types. However, the original rule is correct to some extent, considering that the quality of recovered gravity field models is homogeneous as a function of geographical longitude as long as l max < nr/2.)uuid:7acff12c4ee94c4b96fc870948e12224Dhttp://resolver.tudelft.nl/uuid:7acff12c4ee94c4b96fc870948e12224GOCE gradiometer: Estimation of biases and scale factors of all six individual accelerometers by precise orbit determination@A method has been implemented and tested for estimating bias and scale factor parameters for all six individual accelerometers that will fly onboard of GOCE and together form the socalled gradiometer. The method is based on inclusion of the individual accelerometer observations in precise orbit determinations, opposed to the baseline method where socalled commonmode accelerometer observations are used. The method was tested using simulated data from a detailed GOCE system simulator. It was found that the observations taken by individual accelerometers need to be corrected for (1) local satellite gravity gradient (SGG), and (2) rotational terms caused by centrifugal and angular accelerations, due to the fact that they are not located in the satellite s center of mass. For these corrections, use is made of a reference gravity field model. In addition, the rotational terms are derived from onboard star tracker observations. With a perfect a priori gravity field model and with the estimation of not only accelerometer biases but also accelerometer drifts, scale factors can be determined with an accuracy and stability better than 0.01 for two of the three axes of each accelerometer, the exception being the axis pointing along the long axis of the satellite (more or less coinciding with the flight direction) for which the scale factor estimates are unreliable. This axis coincides with the axis of dragfree control, which results in a small variance of the signal to be calibrated and thus an inaccurate determination of its scale factor in the presence of relatively large (colored) accelerometer observation errors. In the presence of gravity field model errors, it was found that still an accuracy and stability of about 0.015 can be obtained for the accelerometer scale factors by simultaneously estimating empirical accelerations.sAccelerometer; Accelerometer drift; Bias; Calibration; GOCE; Gradiometer; Precise or< bit determination; Scale factor)uuid:db473be37bca4e55aacdf46954d62c31Dhttp://resolver.tudelft.nl/uuid:db473be37bca4e55aacdf46954d62c31kExploring the possibilities for startracker assisted calibration of the six individual GOCE accelerometers=A method has been developed and tested for estimating calibration parameters for the six accelerometers on board the Gravity field and steadystate Ocean Circulation Explorer (GOCE) from star tracker observations. These six accelerometers are part of the gradiometer, which is the prime instrument on board GOCE. It will be shown that by taking appropriate combinations of observations collected by the accelerometers, by modeling acceleration terms caused by gravity gradients from an a priori lowdegree spherical harmonic expansion, and by modeling rotational acceleration terms derived from startracker observations, scale factors of each of the accelerometers can be estimated for each axis. Simulated observations from a socalled endtoend simulator were used to test the method. This endtoend simulator includes a detailed model of the GOCE satellite, its instruments and instrument errors, and its environment. Results of the tests indicate that scale factors of all six accelerometers can be determined with an accuracy of around 0.01 for all components on a daily basis.\GOCE; Gradiometer; Accelerometers; Startracker; Calibration; Bias; Bias drift; Scale factor#Earth Observation and Space Systems)uuid:7fc89d78a9e14f5f9e2692daa98e4f91Dhttp://resolver.tudelft.nl/uuid:7fc89d78a9e14f5f9e2692daa98e4f91GAccuracy assessment of the monthly GRACE geoids based upon a simulation!Schrama, E.J.O.; Visser, P.N.A.M.STemporal gravity; Hydrology; Ocean bottom pressure; Tides; Air pressure; GRACE; GPS)uuid:81201407467e4a8f96e41e3889170d36Dhttp://resolver.tudelft.nl/uuid:81201407467e4a8f96e41e3889170d36}Zonal winds in the equatorial upper thermosphere: Decomposing the solar flux, geomagnetic activity, and seasonal dependenciesILiu, H.; Lhr, H.; Watanabe, S.; Khler, W.; Henize, V.; Visser, P.N.A.M.
Using 3 years (2002 2004), over 16,400 orbits of measurements from the accelerometer on board the CHAMP satellite, we have studied the climatology of the equatorial zonal wind in the upper thermosphere. Several main features are noticed. The most prominent one is that the solar flux significantly influences both the daytime and nighttime winds. It overrides the geomagnetic activity effect, which is found to be rather limited to the nightside. An elevation of the solar flux level from F10.7 ? 100 10?22 W m?2 Hz?1 to F10.7 ? 190 10?22 W m?2 Hz?1 produces an eastward disturbance wind up to ?110 m s?1. This consequently enhances the nighttime eastward wind but suppresses the daytime westward wind. A seasonal variation with weaker wind (by over 50 m s?1 at night) around June solstice than in other seasons has been observed regardless of solar flux and geomagnetic activity levels. The zonal wind is eastward throughout the night except around June solstice, where it ebbs to almost zero or turns even westward in the postmidnight sector at low solar flux level. The daytime wind is found to be generally more stable than the nighttime wind, particularly unresponsive to geomagnetic activities. Predictions from the Horizontal Wind Model find good agreement with the CHAMP?observed wind at high solar flux levels during nighttime. At low solar flux levels, however, the model strongly underestimates the westward wind during morning hours by 50 120 m s?1 depending on season. The major difference between the HWM?predicted and the CHAMP?observed wind is seen in the phase of its diurnal variation. The CHAMP?observed wind turns eastward around 1200 1300 MLT instead of 1600 1700 MLT predicted by the model. Comparisons with ground FPI observations and the NCAR Thermosphere?Ionosphere?Electrodynamics General Circulation Model (TIEGCM) predictions show that the solar flux effect obtained from CHAMP is consistent with that modeled by TIEGCM. The solar flux dependence of zonal wind found here together with that of< the zonal ion drift found in previous studies reflect the relative importance of the E? and F?region wind dynamo in the thermosphere?ionosphere coupling process. Furthermore, these wind measurements indicate that the Earth's atmosphere superrotates. The average superrotation speed amounts to about 22 m s?1 for a solar flux level of F10.7 ? 100 10?22 W m?2 Hz?1 but increases to 63 m s?1 for F10.7 ? 190 10?22 W m?2 Hz?1. Finally, the wind behavior presented in this study is longitudinally averaged and may differ from wind measurements at a certain longitude.Bneutral wind; thermosphereionosphere coupling; upper thermosphere)uuid:70d808278f8948bc9e61f382fc316181Dhttp://resolver.tudelft.nl/uuid:70d808278f8948bc9e61f382fc316181WEnergy integral method for gravity field determination from satellite orbit coordinates)Visser, P.N.A.M.; Sneeuw, N.; Gerlach, C.:A fast iterative method for gravity field determination from low Earth satellite orbit coordinates has been developed and implemented successfully. The method is based on energy conservation and avoids problems related to orbit dynamics and initial state. In addition, the particular geometry of a repeat orbit is exploited by using a very efficient iterative estimation scheme, in which a set of normal equations is approximated by a sparse blockdiagonal equivalent. Recovery experiments for spherical harmonic gravity field models up to degree and order 80 and 120 were conducted based on a 29day simulated data set of orbit coordinates. The method was found to be very flexible and could be easily adapted to include observations of nonconservative accelerations, such as (to be) provided by satellites like CHAMP, GRACE, and GOCE. A serious drawback of the method is its large sensitivity to satellite velocity errors. Existing orbit determination strategies need to be altered or augmented to include algorithms that focus on optimizing the accuracy of estimated velocities.energy integral; gravity field determination; accelerometer observations; blockdiagonal matrix; orbit errors; orbit coordinates)uuid:88e166c8d1c64c1eb5a43932b55fe13dDhttp://resolver.tudelft.nl/uuid:88e166c8d1c64c1eb5a43932b55fe13dhAiming at a 1cm orbit for low earth orbiters: Reduceddynamic and kinematic precise orbit determination$Visser, P.N.A.M.; Van den IJssel, J.WThe computation of highaccuracy orbits is a prerequisite for the success of Low Earth Orbiter (LEO) missions such as CHAMP, GRACE and GOCE. The mission objectives of these satellites cannot be reached without computing orbits with an accuracy at the few cm level. Such a level of accuracy might be achieved with the techniques of reduceddynamic and kinematic precise orbit determination (POD) assuming continuous SatellitetoSatellite Tracking (SST) by the Global Positioning System CGPS). Both techniques have reached a high level of maturity and have been successfully applied to missions in the past, for example to TOPEX/POSEIDON (TIP), leading to Csub)decimeter orbit accuracy. New LEO gravity missions are (to be) equipped with advanced GPS receivers promising to provide very high quality SST observations thereby opening the possibility for computing cmlevel accuracy orbits. The computation of orbits at this accuracy level does not only require highquality GPS receivers, but also advanced and demanding observation preprocessing and correction algorithms. Moreover, sophisticated parameter estimation schemes need to be adapted and extended to allow the computation of such orbits. Finally, reliable methods need to be employed for assessing the orbit quality and providing feedback to the ditferent processing steps in the orbit computation process.Aprecise orbit determination; reduceddynamic; kinematic; GPS; LEO)uuid:957b041a8e2c4ad7be73214c0727df01Dhttp://resolver.tudelft.nl/uuid:957b041a8e2c4ad7be73214c0727df01IThe use of satellites in gravity field determination and model adjustmentWakker, K.F. (promotor)doctoral thesisDelft University Press
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