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Bosch W.,Deutsches Geodatisches Forschungsinstitut DGFI | Savcenko R.,Deutsches Geodatisches Forschungsinstitut DGFI
International Association of Geodesy Symposia | Year: 2010

Since the essential improvements of GRACE gravity field models reliable signatures of the dynamic ocean topography (DOT) can be obtained by subtracting geoid heights from the sea surface. The differences are usually performed after an initial data gridding of sea surface heights implying already an undesirable loss of signal. On the other hand, even the latest gravity field solutions from GRACE exhibit a meridional striping in the geoid and require a smoothing. In order to preserve the high along-track resolution of altimetry the present paper investigates a profile approach which (i) performs a spectral smoothing of the GRACE gravity field (ii) merges mean-tide geoid profiles to the along-track sea level measurements of satellite altimetry and (iii) performs a common low pass filtering of along-track differences in order to make filtered sea level and geoid heights spectrally consistent. The approach is performed with the latest GRACE-only gravity field models and the sea surface height profiles of TOPEX and Jason-1 and produces time varying profiles of the DOT. Globally, the profiles exhibit the expected topographic features which are compared with independent estimates of the DOT. © Springer-Verlag Berlin Heidelberg 2010.

Dettmering D.,Deutsches Geodatisches Forschungsinstitut DGFI | Bosch W.,Deutsches Geodatisches Forschungsinstitut DGFI
Marine Geodesy | Year: 2010

Multi-mission altimeter observations are successfully used for global cross calibration of altimeters. The approach utilizes a least squares adjustment minimizing single and dual satellite crossover differences as well as consecutive differences of the radial component of single satellites. The method is applied to obtain a characterization of the radial errors of the first year of OSTM/Jason-2 data. A mean relative range bias of about 7.5 ± 0.2 cm with respect to Jason-1 was computed. The radial errors show increased auto-correlation at the orbit revolution period, which is related to geographically correlated error pattern with up to about 2 cm amplitude. © 2010, Taylor & Francis Group, LLC.

Schmidt M.,Deutsches Geodatisches Forschungsinstitut DGFI
International Association of Geodesy Symposia | Year: 2012

For analyzing and representing a one-dimensional signal wavelet methods are used for a long time. The basic feature of wavelet analysis is the localization property, i.e. it is - depending on the chosen wavelet - possible to study a signal just in a finite interval. Nowadays a large number of satellite missions allows to monitor various geophysical phenomena. Since often regional phenomena have to be studied, multi-dimensional wavelet methods come into question. In this paper the basic principles of a multi-scale representation of multi-dimensional signals using B-spline wavelets are presented. Finally the procedure is applied to an example of ionosphere research. © Springer-Verlag Berlin Heidelberg 2012.

Bosch W.,Deutsches Geodatisches Forschungsinstitut DGFI | Dettmering D.,Deutsches Geodatisches Forschungsinstitut DGFI | Schwatke C.,Deutsches Geodatisches Forschungsinstitut DGFI
Remote Sensing | Year: 2014

Climate studies require long data records extending the lifetime of a single remote sensing satellite mission. Precise satellite altimetry exploring global and regional evolution of the sea level has now completed a two decade data record. A consistent long-term data record has to be constructed from a sequence of different, partly overlapping altimeter systems which have to be carefully cross-calibrated. This cross-calibration is realized globally by adjusting an extremely large set of single-and dual-satellite crossover differences performed between all contemporaneous altimeter systems. The total set of crossover differences creates a highly redundant network and enables a robust estimate of radial errors with a dense and rather complete sampling for all altimeter systems analyzed. An iterative variance component estimation is applied to obtain an objective relative weighting between altimeter systems with different performance. The final time series of radial errors is taken to estimate (for each of the altimeter systems) an empirical auto-covariance function. Moreover, the radial errors capture relative range biases and indicate systematic variations in the geo-centering of altimeter satellite orbits. The procedure has the potential to estimate for all altimeter systems the geographically correlated mean errors which is not at all visible in single-satellite crossover differences but maps directly to estimates of the mean sea surface. © 2014 by the authors; licensee MDPI, Basel, Switzerland.

Blossfeld M.,Deutsches Geodatisches Forschungsinstitut DGFI | Seitz M.,Deutsches Geodatisches Forschungsinstitut DGFI | Angermann D.,Deutsches Geodatisches Forschungsinstitut DGFI
Journal of Geodesy | Year: 2014

In the conventions of the International Earth Rotation and Reference Systems Service (e.g. IERS Conventions 2010), it is recommended that the instantaneous station position, which is fixed to the Earth's crust, is described by a regularized station position and conventional correction models. Current realizations of the International Terrestrial Reference Frame use a station position at a reference epoch and a constant velocity to describe the motion of the regularized station position in time. An advantage of this parameterization is the possibility to provide station coordinates of high accuracy over a long time span. Various publications have shown that residual non-linear station motions can reach a magnitude of a few centimeters due to not considered loading effects. Consistently estimated parameters like the Earth Orientation Parameters (EOP) may be affected if these non-linear station motions are neglected. In this paper, we investigate a new approach, which is based on a frequent (e.g. weekly) estimation of station positions and EOP from a combination of epoch normal equations of the space geodetic techniques Global Positioning System (GPS), Satellite Laser Ranging (SLR) and Very Long Baseline Interferometry (VLBI). The resulting time series of epoch reference frames are studied in detail and are compared with the conventional secular approach. It is shown that both approaches have specific advantages and disadvantages, which are discussed in the paper. A major advantage of the frequently estimated epoch reference frames is that the non-linear station motions are implicitly taken into account, which is a major limiting factor for the accuracy of the secular frames. Various test computations and comparisons between the epoch and secular approach are performed. The authors found that the consistently estimated EOP are systematically affected by the two different combination approaches. The differences between the epoch and secular frames reach magnitudes of 23.6 μas (0.73 mm) and 39.8 μas (1.23 mm) for the x-pole and y-pole, respectively, in case of the combined solutions. For the SLR-only solutions, significant differences with amplitudes of 77.3 μas (2.39 mm) can be found. © 2013 Springer-Verlag Berlin Heidelberg.

Dettmering D.,Deutsches Geodatisches Forschungsinstitut DGFI | Limberger M.,Deutsches Geodatisches Forschungsinstitut DGFI | Schmidt M.,Deutsches Geodatisches Forschungsinstitut DGFI
Journal of Geodesy | Year: 2014

The Doppler orbitography and radiopositioning integrated by satellite (DORIS) system was originally developed for precise orbit determination of low Earth orbiting (LEO) satellites. Beyond that, it is highly qualified for modeling the distribution of electrons within the Earth’s ionosphere. It measures with two frequencies in L-band with a relative frequency ratio close to 5. Since the terrestrial ground beacons are distributed quite homogeneously and several LEOs are equipped with modern receivers, a good applicability for global vertical total electron content (VTEC) modeling can be expected. This paper investigates the capability of DORIS dual-frequency phase observations for deriving VTEC and the contribution of these data to global VTEC modeling. The DORIS preprocessing is performed similar to commonly used global navigation satellite systems (GNSS) preprocessing. However, the absolute DORIS VTEC level is taken from global ionospheric maps (GIM) provided by the International GNSS Service (IGS) as the DORIS data contain no absolute information. DORIS-derived VTEC values show good consistency with IGS GIMs with a RMS between 2 and 3 total electron content units (TECU) depending on solar activity which can be reduced to less than 2 TECU when using only observations with elevation angles higher than 50(Formula presented.). The combination of DORIS VTEC with data from other space-geodetic measurement techniques improves the accuracy of global VTEC models significantly. If DORIS VTEC data is used to update IGS GIMs, an improvement of up to 12  % can be achieved. The accuracy directly beneath the DORIS satellites’ ground-tracks ranges between 1.5 and 3.5 TECU assuming a precision of 2.5 TECU for altimeter-derived VTEC values which have been used for validation purposes. © 2014, Springer-Verlag Berlin Heidelberg.

Fuchs M.J.,Deutsches Geodatisches Forschungsinstitut DGFI | Bouman J.,Deutsches Geodatisches Forschungsinstitut DGFI
Geophysical Journal International | Year: 2011

ESA's GOCE mission aims at improved global and regional gravity field information with high spatial resolution by measuring gravity gradients. A local analysis of the GOCE gravity gradient tensor may benefit from a rotation from the gradiometer reference frame to local reference frames such as the local north oriented reference frame. As the GOCE gravity gradients include accurate and less accurate measured gradients, the point-wise tensor rotation of the GOCE-only measurements may suffer from the projection of the errors of the less accurate gravity gradients onto the accurate gravity gradients. In addition, the GOCE gravity gradients have high accuracy in the measurement bandwidth but low accuracy below, and tensor rotation may cause leakage of the large error below the measurement bandwidth to the measurement bandwidth. Degradation of the rotated gravity gradients is circumvented by replacing the less accurate tensor components, as well as the signal below the measurement bandwidth of the accurate gravity gradients, with model signal. The combination of GOCE and model gravity gradients is performed by determining the effective measurement bandwidth (EMB), that is, the bandwidth in which the integrated signal-to-noise ratio of the GOCE gravity gradients is maximized. We find that the determination of the EMB is relatively independent of the reference gravity field model that is used. The lower bound of the EMB is well below the pre-mission specifications for the four accurate gravity gradients. In addition, we assess how much GOCE contributes to the gravity gradient signal in local frames and how much the model. For the radial gravity gradient the relative GOCE contribution is 98 per cent on average, whereas this is 65-97 per cent for the other gravity gradients. These numbers strongly depend on the local frame under consideration and on the geographical position. © 2011 The Authors Geophysical Journal International © 2011 RAS.

Bouman J.,Deutsches Geodatisches Forschungsinstitut DGFI | Fuchs M.J.,Deutsches Geodatisches Forschungsinstitut DGFI
Geophysical Journal International | Year: 2012

The GOCE mission, a part of ESA's Living Planet Programme, aims at improved gravity field modelling at high spatial resolution. On-board GOCE a gradiometer, in combination with a scientific on-board GPS receiver, measures the earth's gravity field with unprecedented accuracy. These measurements have been used to compute GOCE gravity field solutions and combined GOCE/GRACE solutions. The main difference between the solutions is how they incorporate the required a priori information, which consists either of existing gravity field models or Kaula's rule for the signal variances of the gravity field. We assessed four series of models by comparing the gravity gradients they predict with the measured GOCE gradients. The analysis of the gravity gradients fits may reveal differences between the different solutions that can be attributed to the solution strategy, assuming that the measurement errors are homogeneous. We compared the GOCE gradients with existing state-of-the-art global gravity field models and conclude that the gradient errors are indeed globally homogeneous, with the exception of the cross-track gradient especially south of Australia. Furthermore, we find that the use of existing global gravity field models as a priori information should be avoided because this may increase the gradient residuals in spatial and spectral domain. Finally, we may conclude that the GOCE and GRACE data are compatible and complementary. © 2012 The Authors Geophysical Journal International © 2012 RAS.

Dettmering D.,Deutsches Geodatisches Forschungsinstitut DGFI | Heinkelmann R.,Deutsches Geodatisches Forschungsinstitut DGFI | Schmidt M.,Deutsches Geodatisches Forschungsinstitut DGFI
Journal of Geodesy | Year: 2011

The ionosphere is a dispersive medium for microwaves, and most space-geodetic techniques using two or more signal frequencies can be applied to extract information on ionospheric parameters, including terrestrial as well as satellite-based GNSS, DORIS, altimetry, and VLBI. Because of their different sensitivity regarding ionization, their different spatial and temporal data distribution, and their different signal paths, a joint analysis of all observation types seems reasonable and promises the best results for ionosphere modeling. However, it has turned out that there exist offsets between ionospheric observations of the diverse techniques mainly caused by calibration uncertainties or model errors. Direct comparisons of the information from different data types are difficult because of the inhomogeneous measurement epochs and locations. In the approach presented here, all measurements are combined into one ionosphere model of vertical total electron content (VTEC). A variance component estimation is applied to take into account the different accuracy levels of the observations. In order to consider systematic offsets, a constant bias term is allowed for each observation group. The investigations have been performed for the time interval of the CONT08 campaign (2 weeks in August 2008) in a region around the Hawaiian Islands. Almost all analyzed observation techniques show good data sensitivity and are suitable for VTEC modeling in case the systematic offsets which can reach up to 5 TECU are taken into account. Only the Envisat DORIS data cannot provide reliable results. © 2011 Springer-Verlag.

Bouman J.,Deutsches Geodatisches Forschungsinstitut DGFI | Ebbing J.,Geological Survey of Norway | Fuchs M.,Deutsches Geodatisches Forschungsinstitut DGFI
Journal of Geophysical Research: Solid Earth | Year: 2013

The Gravity field and steady state Ocean Circulation Explorer (GOCE) is the European Space Agency's mission that combines GPS tracking and gravity gradiometry to determine the Earth's mean gravity field with unprecedented, global accuracy with a spatial resolution down to 80 km. This resolution makes GOCE gravity gradient data in particular useful for lithospheric scale modeling. However, the relation between coordinates in a model frame and at satellite altitude is not straightforward, and most geophysical modeling programs require a planar approximation, which may not be appropriate for satellite data. We derive the exact relation between the model reference frame, in which gradients from lithospheric modeling are given, and the local north-oriented frame in which GOCE gradients at 255 km altitude are given. We generated gradients from a GOCE gravity field model and assessed whether the orientation differences between local north-oriented frame and model reference frame are relevant. In addition, we assessed the same for airborne gradiometry at an altitude of 5 km because these data are complementary to GOCE. We find that if the regional area has a longitude extension of 5°, the errors stay below 10%. For larger areas the standard deviation of the systematic errors may be 40% of the signal standard deviation. Comparing topographic mass reduction in planar and spherical approximation, one sees significant long wavelength differences in terms of gravity gradients or gradient-tensor invariants. The maximum error is up to 1 E at satellite altitude compared with maximum signal amplitude of 3 E. Planar approximation is therefore not accurate enough for topographic mass reduction of GOCE gravity gradients. ©2012. American Geophysical Union. All Rights Reserved.

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