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  • 1. Abbak, Ramazan A.
    et al.
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Geoinformatics.
    Ellmann, Artu
    Ustun, Aydin
    A precise gravimetric geoid model in a mountainous area with scarce gravity data: a case study in central Turkey2012In: Studia Geophysica et Geodaetica, ISSN 0039-3169, E-ISSN 1573-1626, Vol. 56, no 4, 909-927 p.Article in journal (Refereed)
    Abstract [en]

    In mountainous regions with scarce gravity data, gravimetric geoid determination is a difficult task that needs special attention to obtain reliable results satisfying the demands, e.g., of engineering applications. The present study investigates a procedure for combining a suitable global geopotential model and available terrestrial data in order to obtain a precise regional geoid model for Konya Closed Basin (KCB). The KCB is located in the central part of Turkey, where a very limited amount of terrestrial gravity data is available. Various data sources, such as the Turkish digital elevation model with 3 '' x 3 '' resolution, a recently published satellite-only global geopotential model from the Gravity Recovery and Climate Experiment satellite (GRACE) and the ground gravity observations, are combined in the least-squares sense by the modified Stokes' formula. The new gravimetric geoid model is compared with Global Positioning System (GPS)/levelling at the control points, resulting in the Root Mean Square Error (RMS) differences of +/- 6.4 cm and 1.7 ppm in the absolute and relative senses, respectively. This regional geoid model appears to he more accurate than the Earth Gravitational Model 2008, which is the best global model over the target area, with the RMS differences of +/- 8.6 cm and 1.8 ppm in the absolute and relative senses, respectively. These results show that the accuracy of a regional gravimetric model can be augmented by the combination of a global geopotential model and local terrestrial data in mountainous areas even though the quality and resolution of the primary terrestrial data are not satisfactory to the geoid modelling procedure.

  • 2.
    Abrehdary, Majid
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    Bagherbandi, Mohammad
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning. Univ Gavle, Dept Ind Dev IT & Land Management, SE-80176 Gavle, Sweden.
    Combined Moho parameters determination using CRUST1.0 and Vening Meinesz-Moritz model2015In: Journal of Earth Science, ISSN 1674-487X, E-ISSN 1867-111X, Vol. 26, no 4, 607-616 p.Article in journal (Refereed)
    Abstract [en]

    According to Vening Meinesz-Moritz (VMM) global inverse isostatic problem, either the Moho density contrast (crust-mantle density contrast) or the Moho geometry can be estimated by solving a non-linear Fredholm integral equation of the first kind. Here solutions to the two Moho parameters are presented by combining the global geopotential model (GOCO-03S), topography (DTM2006) and a seismic crust model, the latter being the recent digital global crustal model (CRUST1.0) with a resolution of 1A(0)x1A(0). The numerical results show that the estimated Moho density contrast varies from 21 to 637 kg/m(3), with a global average of 321 kg/m(3), and the estimated Moho depth varies from 6 to 86 km with a global average of 24 km. Comparing the Moho density contrasts estimated using our leastsquares method and those derived by the CRUST1.0, CRUST2.0, and PREM models shows that our estimate agrees fairly well with CRUST1.0 model and rather poor with other models. The estimated Moho depths by our least-squares method and the CRUST1.0 model agree to 4.8 km in RMS and with the GEMMA1.0 based model to 6.3 km.

  • 3.
    Abrehdary, Majid
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    Bagherbandi, Mohammad
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    Modelling Moho depth in ocean areas based on satellite altimetry using Vening Meinesz–Moritz’ method2016In: Acta Geodaetica et Geophysica Hungarica, ISSN 1217-8977, E-ISSN 1587-1037, Vol. 51, no 2, 137-149 p.Article in journal (Refereed)
    Abstract [en]

    An experiment for estimating Moho depth is carried out based on satellite altimetryand topographic information using the Vening Meinesz–Moritz gravimetric isostatichypothesis. In order to investigate the possibility and quality of satellite altimetry in Mohodetermination, the DNSC08GRA global marine gravity field model and the DTM2006 globaltopography model are used to obtain a global Moho depth model over the oceans with aresolution of 1 x 1 degree. The numerical results show that the estimated Bouguer gravity disturbancevaries from 86 to 767 mGal, with a global average of 747 mGal, and the estimatedMoho depth varies from 3 to 39 km with a global average of 19 km. Comparing the Bouguergravity disturbance estimated from satellite altimetry and that derived by the gravimetricsatellite-only model GOGRA04S shows that the two models agree to 13 mGal in root meansquare (RMS). Similarly, the estimated Moho depths from satellite altimetry andGOGRA04S agree to 0.69 km in RMS. It is also concluded that possible mean dynamictopography in the marine gravity model does not significantly affect the Moho determination.

  • 4.
    Abrehdary, Majid
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    Bagherbandi, Mohammad
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    The spherical terrain correction and its effect on the gravimetric-isostatic Moho determination2016In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 204, no 1, 262-273 p.Article in journal (Refereed)
    Abstract [en]

    In this study, the Moho depth is estimated based on the refined spherical Bouguer gravity disturbance and DTM2006 topographic data using the Vening Meinesz-Moritz gravimetric-isostatic hypothesis. In this context, we compute the refined spherical Bouguer gravity disturbances in a set of 1 degrees x 1 degrees blocks. The spherical terrain correction, a residual correction to each Bouguer shell, is computed using rock heights and ice sheet thicknesses from the DTM2006 and Earth2014 models. The study illustrates that the defined simple Bouguer gravity disturbance corrected for the density variations of the oceans, ice sheets and sediment basins and also the non-isostatic effects needs a significant terrain correction to become the refined Bouguer gravity disturbance, and that the isostatic gravity disturbance is significantly better defined by the latter disturbance plus a compensation attraction. Our study shows that despite the fact that the lateral variation of the crustal depth is rather smooth, the terrain affects the result most significantly in many areas. The global numerical results show that the estimated Moho depths by the simple and refined spherical Bouguer gravity disturbances and the seismic CRUST1.0 model agree to 5.6 and 2.7 km in RMS, respectively. Also, the mean value differences are 1.7 and 0.2 km, respectively. Two regional numerical studies show that the RMS differences between the Moho depths estimated based on the simple and refined spherical Bouguer gravity disturbance and that using CRUST1.0 model yield fits of 4.9 and 3.2 km in South America and yield 3.2 and 3.4 km in Fennoscandia, respectively.

  • 5.
    Abrehdary, Majid
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    Bagherbandi, Mohammad
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    Sampietro, Daniele
    Modelling Moho parameters and their uncertainties from the combination of the seismic and satellite gravity dataManuscript (preprint) (Other academic)
    Abstract [en]

    We present a method for estimating a new global Moho model (KTH15C), containing Moho depth and density contrast, from a combination of global models of gravity (GOCO05S), topography (DTM2006) and seismic information (CRUST1.0 and MDN07) to a resolution of 1°×1° based on a solution of Vening Meinesz-Moritz’ inverse problem of isostasy. Particularly, this article has its emphasis on the modelling of the observation standard errors propagated from the Vening Meinesz-Moritz and CRUST1.0 models in estimating the uncertainty of the final Moho model. The numerical results yield Moho depths ranging from 6.5 to 70.1 km, with a global average of 23.4 ± 13 km. The estimated Moho density contrasts range from 21 to 680 kg/m3, with a global average of 345.4 ± 112 kg/m3. Moreover, test computations display that in most areas estimated uncertainties in the parameters are less than 3 km and 50 kg/m3, respectively, but they reach to more significant values under Gulf of Mexico, Chile, Eeastern Mediterranean, Timor sea and parts of polar regions. Comparing the Moho depths estimated by KTH15C and those derived by KTH11C, GEMMA2012C, CRUST1.0, KTH14C, CRUST14 and GEMMA1.0 models shows that KTH15C agree fairly well with CRUST1.0 but rather poor with other models. The Moho density contrasts estimated by KTH15C and those of the KTH11C and KTH14C model agree to 120 and 80 kg/m3 in RMS. The regional numerical studies show that the RMS differences between KTH15C and Moho depths from seismic information yields fits of 2 to 4 km in South and North America, Africa, Europe, Asia, Australia and Antarctica, respectively.    

  • 6.
    Alizadeh Khameneh, Mohammad Amin
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    Eshagh, Mehdi
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning. University West, Division of Surveying Engineering.
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    Optimisation of Lilla Edet Landslide GPS Monitoring Network2015In: Journal of Geodetic Science, ISSN 2081-9919, E-ISSN 2081-9943, Vol. 5, no 1, 57-66 p.Article in journal (Refereed)
    Abstract [en]

    Since the year 2000, some periodic investigations have been performed in the Lilla Edet region to monitor and possibly determine the landslide of the area with the GPS measurements. The responsible consultant has conducted this project by setting up some stable stations for GPS receivers in the risky areas of Lilla Edet and measured the independent baselines amongst the stations according to their observation plan. Here, we optimise the existing surveying network and determine the optimal configuration of the observation plan based on different criteria. We aim to optimise the current network to become sensitive to detect 5 mm possible displacements in each net point. The network quality criteria of precision, reliability and cost are used as object functions to perform single-, bi- and multi-objective optimisation models. It has been shown in the results that the single-objective model of reliability, which is constrained to the precision, provides much higher precision than the defined criterion by preserving almost all of the observations. However, in this study, the multi-objective model can fulfil all the mentioned quality criteria of the network by 17% less measurements than the original observation plan, meaning 17% of saving time, cost and effort in the project.

  • 7.
    Alizadeh Khameneh, Mohammad Amin
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    Eshagh, Mehdi
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning. Department of Engineering Science, University West, Trollhättan, Sweden.
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    The Effect of Instrumental Precision on Optimisation of Displacement Monitoring Networks2016In: Acta Geodaetica et Geophysica, ISSN 2213-5820, Vol. 51, no 4, 761-772 p.Article in journal (Refereed)
    Abstract [en]

    In order to detect the geo-hazards, different deformation monitoring networks are usually established. It is of importance to design an optimal monitoring network to fulfil the requested precision and reliability of the network. Generally, the same observation plan is considered during different time intervals (epochs of observation). Here, we investigate the case that instrumental improvements in sense of precision are used in two successive epochs. As a case study, we perform the optimisation procedure on a GPS monitoring network around the Lilla Edet village in the southwest of Sweden. The network was designed for studying possible displacements caused by landslides. The numerical results show that the optimisation procedure yields an observation plan with significantly fewer baselines in the latter epoch, which leads to saving time and cost in the project. The precision improvement in the second epoch is tested in several steps for the Lilla Edet network. For instance, assuming two times better observation precision in the second epoch decreases the number of baselines from 215 in the first epoch to 143 in the second one.

  • 8.
    Alizadeh Khameneh, Mohammad Amin
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning. WSP Civils, Department of Geographic Information and Asset Management.
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    Jensen, Anna B. O.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    Optimisation of GNSS Networks - Considering Baseline Correlations2017In: Survey review - Directorate of Overseas Surveys, ISSN 0039-6265, E-ISSN 1752-2706, 1-8 p.Article in journal (Refereed)
    Abstract [en]

    By considering GNSS observations one can perform optimisation according to some pre-defined criteria and come up with the best location of receivers and optimum number of baselines. In practice, it is quite common to neglect the effect of correlations between baselines, and instead assume single-baseline adjusted data in the optimisation procedure. However, in each session of observation usually more than two receivers are simultaneously taking data from a number of common GNSS satellites, implying that the single or double difference observations are correlated. Our study designs an optimal observation plan for a GPS network in Skåne in southern Sweden, with the aim of determining possible displacements. Assuming three receivers in each session of observations leads to correlation between the GPS baselines, and consequently a fully populated weight matrix for each session of observation. A bi-objective optimisation model of precision and reliability is chosen to optimise the variance factor of each session, and eventually, design an observation plan. It is shown in this study that observing 6 out of 10 possible sessions are sufficient to enable the network to detect a 5 mm displacement at each station. Assuming that the double difference phase observations are uncorrelated changes the observation plan by retaining 2 more sessions. However, defining the weight matrix based on the double difference observations requires the correlations to be taken into account, and neglecting them leads to incorrect results.

  • 9.
    Bagherbandi, Mohammad
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    Bai, Yongliang
    Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences No.7 Nanhai Rd, Qingdao 266071, China.
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    Tenzer, Robert
    School of Geodesy and Geomatics, Wuhan University, 129 Luoyu Road, Wuhan, China..
    Abrehdary, Majid
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    Miranda, Silvia
    Departamento de Geofísica y Astronomía, FCEFN. Universidad Nacional de San Juan, Meglioli 1160 Sur, 5400. Rivadavia, San Juan, Argentina.
    Sanchez, Juan
    Departamento de Geofísica y Astronomía, FCEFN. Universidad Nacional de San Juan, Meglioli 1160 Sur, 5400. Rivadavia, San Juan, Argentina..
    Effect of the lithospheric thermal state on the Moho geometryManuscript (preprint) (Other academic)
    Abstract [en]

    Gravimetric methods applied for a Moho recovery in areas with sparse and irregular distribution of seismic data often assume only a constant crustal density. Results of the latest studies, however, indicate that corrections for the crustal density heterogeneities could improve the gravimetric result especially in regions with a complex geologic/tectonic structure. Moreover, the isostatic mass balance reflects also the density structure within the mantle. The gravimetric methods should therefore incorporate an additional correction for the sub-crustal density heterogeneities. Following this principle, we solve the Vening Meinesz-Moritz (VMM) inverse problem of isostasy constrained on seismic data to determine the Moho depth of the South American tectonic plate including surrounding oceans, while taking into consideration the crustal and mantle density heterogeneities. Our numerical result confirms that the contribution of sediments significantly modifies the Moho geometry especially along the continental margins with large sediment deposits. To account for the mantle density heterogeneities we develop and apply a method of correcting the Moho geometry for the contribution of the lithospheric thermal state (i.e., the lithospheric thermal-pressure correction). In addition, the misfit between the isostatic and seismic Moho models, attributed mainly to deep mantle density heterogeneities and other geophysical phenomena, is corrected for by applying the non-isostatic correction. The results reveal that the application of the lithospheric thermal-pressure correction improves the RMS fit of the VMM gravimetric Moho solution to the CRUST1.0 seismic model and the point-wise seismic data in South America about 40% and 7% respectively.  

  • 10.
    Bagherbandi, Mohammad
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Geoinformatics.
    Eshagh, Mehdi
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Geoinformatics.
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Geoinformatics.
    Multi-objective versus single-objective models in geodetic network optimization2009In: Nordic Journal of Surveying and Real Estate Research, ISSN 1459-5877, E-ISSN 2341-6599, Vol. 6, no 1, 7-20 p.Article in journal (Refereed)
  • 11.
    Bagherbandi, Mohammad
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geoinformatik och Geodesi.
    Sjoberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geoinformatik och Geodesi.
    Comparison of crustal thickness from two gravimetric-isostatic models and CRUST2.02011In: Studia Geophysica et Geodaetica, ISSN 0039-3169, E-ISSN 1573-1626, Vol. 55, no 4, 641-666 p.Article in journal (Refereed)
    Abstract [en]

    The MohoroviiA double dagger discontinuity is the boundary between the Earth's crust and mantle. Several isostatic hypotheses exist for estimating the crustal thickness and density variation of the Earth's crust from gravity anomalies. The goal of this article is to compare the Airy-Heiskanen and Vening Meinesz-Moritz (VMM) gravimetric models for determining Moho depth, with the seismic Moho (CRUST2.0 or SM) model. Numerical comparisons are performed globally as well as for some geophysically interesting areas, such as Fennoscandia, Persia, Tibet, Canada and Chile. These areas are most complicated areas in view of rough topography (Tibet, Persia and Peru and Chile), post-glacial rebound (Fennoscandia and Canada) and tectonic activities (Persia). The mean Moho depth provided by CRUST2.0 is 22.9 +/- 0.1 km. Using a constant Moho density contrast of 0.6 g/cm(3), the corresponding mean values for Airy-Heiskanen and VVM isostatic models become 25.0 +/- 0.04 km and 21.6 +/- 0.08 km, respectively. By assuming density contrasts of 0.5 g/cm(2) and 0.35 g/cm(3) for continental and oceanic regions, respectively, the VMM model yields the mean Moho depth 22.6 +/- 0.1 km. For this model the global rms difference to CRUST2.0 is 7.2 km, while the corresponding difference between Airy-Heiskanen model and CRUST2.0 is 11 km. Also for regional studies, Moho depths were estimated by selecting different density contrasts. Therefore, one conclusion from the study is that the global compensation by the VMM method significantly improves the agreement with the CRUST2.0 vs. the local compensation model of Airy-Heiskanen. Also, the last model cannot be correct in regions with ocean depth larger than 9 km (e.g., outside Chile), as it may yield negative Moho depths. This problem does not occur with the VMM model. A second conclusion is that a realistic variation of density contrast between continental and oceanic areas yields a better fit of the VMM model to CRUST2.0. The study suggests that the VMM model can primarily be used to densify the CRUST2.0 Moho model in many regions based on separate data by taking advantage of dense gravity data. Finally we have found also that the gravimetric terrain correction affects the determination of the Moho depth by less than 2 km in mean values for test regions, approximately. Hence, for most practical applications of the VMM model the simple Bouguer gravity anomaly is sufficient.

  • 12.
    Bagherbandi, Mohammad
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Geoinformatics.
    Sjöberg, Lars E.
    University of Gävle, Sweden.
    A synthetic Earth gravity model based on a topographic-isostatic model2012In: Studia Geophysica et Geodaetica, ISSN 0039-3169, E-ISSN 1573-1626, Vol. 56, no 4, 935-955 p.Article in journal (Refereed)
    Abstract [en]

    The Earth's gravity field is related to the topographic potential in medium and higher degrees, which is isostatically compensated. Hence, the topographic-isostatic (TI) data are indispensable for extending an available Earth Gravitational Model (EGM) to higher degrees. Here we use TI harmonic coefficients to construct a Synthetic Earth Gravitational Model (SEGM) to extend the EGMs to higher degrees. To achieve a high-quality SEGM, a global geopotential model (EGM96) is used to describe the low degrees, whereas the medium and high degrees are obtained from the TI or topographic potential. This study differes from others in that it uses a new gravimetric-isostatic model for determining the TI potential. We test different alternatives based on TI or only topographic data to determine the SEGM. Although the topography is isostatically compensated only to about degree 40-60, our study shows that using a compensation model improves the SEGM in comparison with using only topographic data for higher degree harmonics. This is because the TI data better adjust the applied Butterworth filter, which bridges the known EGM and the new high-degree potential field than the topographic data alone.

  • 13.
    Bagherbandi, Mohammad
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geoinformatik och Geodesi.
    Sjöberg, Lars E
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geoinformatik och Geodesi.
    Determination of crustal thickness by Vening Meinesz-Moritz hypothesis and its geodetic applications2010Conference paper (Other (popular science, discussion, etc.))
  • 14.
    Bagherbandi, Mohammad
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Geoinformatics.
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Geoinformatics. University of Gävle, Sweden .
    Improving gravimetric-isostatic models of crustal depth by correcting for non-isostatic effects and using CRUST2.02013In: Earth-Science Reviews, ISSN 0012-8252, E-ISSN 1872-6828, Vol. 117, 29-39 p.Article in journal (Refereed)
    Abstract [en]

    The principle of isostasy is important in different fields of geosciences. Using an isostatic hypothesis for estimating the crustal thickness suffers from the more or less incomplete isostatic model and that the observed gravity anomaly is not only generated by the topographic/isostatic signal but also by non-isostatic effects (NIEs). In most applications of isostatic models the NIEs are disregarded. In this paper, we study how some isostatic models related with Vening Meinez's isostatic hypothesis can be improved by considering the NIE. The isostatic gravity anomaly needs a correction for the NIEs, which varies from as much as 494 mGal to -308 mGal. The result shows that by adding this correction the global crustal thickness estimate improves about 50% with respect to the global model CRUST2.0, i.e. the root mean square differences of the crustal thickness of the best Vening Meinesz type and CRUST2.0 models are 6.9 and 3.2 km before and after improvement, respectively. As a result, a new global model of crustal thickness using Vening Meinesz and CRUST2.0 models is generated. A comparison with an independent African crustal depth model shows an improvement of the new model by 6.8 km vs. CRUST2.0 (i.e. rms differences of 3.0 and 9.8 km, respectively). A comparison between oceanic lithosphere age and the NIEs is discussed in this study, too. One application of this study can be to improve crustal depth in areas where CRUST2.0 data are sparse and bad and to densify the resolution vs. the CRUST2.0 model. Other applications can be used to infer the viscosity of the mantle from the NIEs signal to study various locations around the Earth for understanding complete, over- and under-compensations of the topography.

  • 15.
    Bagherbandi, Mohammad
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geoinformatik och Geodesi.
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geoinformatik och Geodesi.
    Non-isostatic effects on crustal thickness: A study using CRUST2.0 in Fennoscandia2012In: Physics of the Earth and Planetary Interiors, ISSN 0031-9201, E-ISSN 1872-7395, Vol. 200, 37-44 p.Article in journal (Refereed)
    Abstract [en]

    The crustal thickness can be determined based on gravimetric-isostatic and seismic models. Modelling crustal thickness by a gravimetric-isostatic model suffers from some problems. The isostatic assumption for compensating the topographic potential is incomplete, as there are other effects which should be considered. Using the isostatic hypothesis for determining the depth of crust causes some disturbing signals, non-isostatic effects, which influence the crustal thickness determination. Isostatic and non-isostatic compensations are the main issues in this paper. We present three methods to overcome the problem due to the disturbing signals, namely the approach by truncating the spherical harmonic approach, determination of non-isostatic correction using a seismic crustal thickness model (e.g., CRUST2.0) and combination of isostatic and seismic models by applying a least-squares adjustment method. The estimated results of the non-isostatic effects varies between 65.2 and 391.8 mGal in Fennoscandia. The root mean squares difference of the crustal thickness obtained from the gravimetric-isostatic model and CRUST2.0 is improved up to six times (from 6.15 to 0.97 km) when the non-isostatic effects are considered.

  • 16.
    Bagherbandi, Mohammad
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning. University of Gävle, Sweden .
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    Tenzer, Robert
    Abrehdary, Majid
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    A new Fennoscandian crustal thickness model based on CRUST1. 0 and a gravimetric–isostatic approach2015In: Earth-Science Reviews, ISSN 0012-8252, E-ISSN 1872-6828, Vol. 145, 132-145 p.Article, review/survey (Refereed)
    Abstract [en]

    In this paper a new gravimetric–isostatic crustal thickness model (VMM14_FEN) is estimated for Fennoscandia. The main motivation is to investigate the relations between geological and geophysical properties, the Moho depth and crust–mantle density contrast at the crust–mantle discontinuity. For this purpose the Bouguer gravity disturbance data is corrected in two main ways namely for the gravitational contributions of mass density variation due to the different layers of the Earth's crust such as ice and sediments, as well as for the gravitational contribution from deeper masses below the crust. This second correction (for non-isostatic effects) is necessary because in general the crust is not in complete isostatic equilibrium and the observed gravity data are not only generated by the topographic/isostatic masses but also from those in the deep Earth interior. The correction for non-isostatic effects is mainly attributed to unmodeled mantle and core boundary density heterogeneities. These corrections are determined using the recent seismic crustal thickness model CRUST1.0. We compare our modeling results with previous studies in the area and test the fitness. The comparison with the external Moho model EuCRUST-07 shows a 3.3 km RMS agreement for the Moho depth in Fennoscandia. We also illustrate how the above corrections improve the Moho depth estimation. Finally, the signatures of geological structures and isostatic equilibrium are studied using VMM14_FEN, showing how main geological unit structures attribute in isostatic balance by affecting the Moho geometry. The main geological features are also discussed in the context of the complete and incomplete isostatic equilibrium.

  • 17.
    Bagherbandi, Mohammad
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Geoinformatics.
    Tenzer, Robert
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Geoinformatics.
    Moho depth uncertainties in the Vening-Meinesz Moritz inverse problem of isostasy2014In: Studia Geophysica et Geodaetica, ISSN 0039-3169, E-ISSN 1573-1626, Vol. 58, no 2, 227-248 p.Article in journal (Refereed)
    Abstract [en]

    We formulate an error propagation model based on solving the Vening Meinesz-Moritz (VMM) inverse problem of isostasy. The system of observation equations in the VMM model defines the relation between the isostatic gravity data and the Moho depth by means of a second-order Fredholm integral equation of the first kind. The corresponding error model (derived in a spectral domain) functionally relates the Moho depth errors with the commission errors of used gravity and topographic/bathymetric models. The error model also incorporates the non-isostatic bias which describes the disagreement, mainly of systematic nature, between the isostatic and seismic models. The error analysis is conducted at the study area of the Tibetan Plateau and Himalayas with the world largest crustal thickness. The Moho depth uncertainties due to errors of the currently available global gravity and topographic models are estimated to be typically up to 1-2 km, provided that the GOCE gravity gradient observables improved the medium-wavelength gravity spectra. The errors due to disregarding sedimentary basins can locally exceed similar to 2 km. The largest errors (which cause a systematic bias between isostatic and seismic models) are attributed to unmodeled mantle heterogeneities (including the core-mantle boundary) and other geophysical processes. These errors are mostly less than 2 km under significant orogens (Himalayas, Ural), but can reach up to similar to 10 km under the oceanic crust.

  • 18.
    Bagherbandi, Mohammad
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Geoinformatics. University of Gävle, Sweden.
    Tenzer, Robert
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Geoinformatics.
    Abrehdary, Majid
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Geoinformatics.
    On the residual isostatic topography effect in the gravimetric Moho determination2015In: Journal of Geodynamics, ISSN 0264-3707, E-ISSN 1879-1670, Vol. 83, 28-36 p.Article in journal (Refereed)
    Abstract [en]

    In classical isostatic models, a uniform crustal density is typically assumed, while disregarding the crustal density heterogeneities. This assumption, however, yields large errors in the Moho geometry determined from gravity data, because the actual topography is not fully isostatically compensated. Moreover, the sub-crustal density structures and additional geodynamic processes contribute to the overall isostatic balance. In this study we investigate the effects of unmodelled density structures and geodynamic processes on the gravity anomaly and the Moho geometry. For this purpose, we define the residual isostatic topography as the difference between actual topography and isostatic topography, which is computed based on utilizing the Vening Meinesz-Moritz isostatic theory. We show that the isostatic gravity bias due to disagreement between the actual and isostatically compensated topography varies between 382 and 596 mGal. This gravity bias corresponds to the Moho correction term of 16 to 25 km. Numerical results reveal that the application of this Moho correction to the gravimetrically determined Moho depths significantly improves the RMS fit of our result with some published global seismic and gravimetric Moho models. We also demonstrate that the isostatic equilibrium at long-to-medium wavelengths (up to degree of about 40) is mainly controlled by a variable Moho depth, while the topographic mass balance at a higher-frequency spectrum is mainly attained by a variable crustal density.

  • 19.
    Bagherbandi, Mohammad
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Geoinformatics.
    Tenzer, Robert
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Geoinformatics.
    Novak, Pavel
    Improved global crustal thickness modeling based on the VMM isostatic model and non-isostatic gravity correction2013In: Journal of Geodynamics, ISSN 0264-3707, E-ISSN 1879-1670, Vol. 66, 25-37 p.Article in journal (Refereed)
    Abstract [en]

    In classical isostatic models for a gravimetric recovery of the Moho parameters (i.e., Moho depths and density contrast) the isostatic gravity anomalies are usually defined based on the assumption that the topographic mass surplus and the ocean mass deficiency are compensated within the Earth's crust. As acquired in this study, this assumption yields large disagreements between isostatic and seismic Moho models. To assess the effects not accounted for in classical isostatic models, we conduct a number of numerical experiments using available global gravity and crustal structure models. First, we compute the gravitational contributions of mass density contrasts due to ice and sediments, and subsequently evaluate respective changes in the Moho geometry. Residual differences between the gravimetric and seismic Moho models are then used to predict a remaining non-isostatic gravity signal, which is mainly attributed to unmodeled density structures and other geophysical phenomena. We utilize three recently developed computational schemes in our numerical studies. The apparatus of spherical harmonic analysis and synthesis is applied in forward modeling of the isostatic gravity disturbances. The Moho depths are estimated globally on a 1 arc-deg equiangular grid by solving the Vening-Meinesz Moritz inverse problem of isostasy. The same estimation model is applied to evaluate the differences between the isostatic and seismic models. We demonstrate that the application of the ice and sediment density contrasts stripping gravity corrections is essential for a more accurate determination of the Moho geometry. We also show that the application of the additional non-isostatic correction further improves the agreement between the Moho models derived based on gravity and seismic data. Our conclusions are based on comparing the gravimetric results with the CRUST2.0 global crustal model compiled using results of seismic surveys.

  • 20.
    Bagherlbandi, Mohammad
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geoinformatik och Geodesi.
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geoinformatik och Geodesi.
    Modelling the density contrast and depth of the Moho discontinuity by seismic and gravimetric-isostatic methods with an application to Africa2012In: Journal of African Earth Sciences, ISSN 1464-343X, Vol. 68, 111-120 p.Article in journal (Refereed)
    Abstract [en]

    The crustal thickness (Moho depth) is of interest in several geosciences applications, such as geography, geophysics and geodesy. Usually the crustal depth and density variations are estimated by seismic survey data. As such data collection is very time-consuming and expensive an attractive option could be to use a gravimetric/isostatic model. In this case, realistic estimates for the crustal density and Moho density contrast (MDC) are important. In this study, we first use the seismic crustal thickness of CRUST2.0 model as a known parameter in combination with gravimetric data in estimating the crust-mantle density contrast by the isostatic model of Vening Meinesz-Moritz. We present different models to estimate the MDC and its impact on the modelling of the gravimetric-isostatic Moho depth. The theory is applied to estimate the Moho depth of the African continental crust by using different models for the MDC: (a) constant value (0.6 g/cm(3)), (b) Pratt-Hayford's model, (c) CRUST2.0 as input to three gravimetric/isostatic models based on Vening Meinesz-Moritz theory. The isostatic models agree by 5.8-7.1 km in the rms with the regional seismic model at a resolution of 2 degrees x 2 degrees, and the smallest rms difference at a resolution of 1 degrees x 1 degrees is of 7.2 km. For comparison, the rms differences of CRUST2.0 and the regional seismic model are 8.8 and 9.1 km at the resolutions of 2 degrees (interpolated) and 1 degrees, respectively. The result suggests that the gravimetric/isostatic Moho model can be used in densification of the CRUST2.0 Moho geometry, and to improve it in areas with poor data.

  • 21. Charleton, C.
    et al.
    Dardengo, C.Macnab, R.Shipman, S.Sjöberg, Lars ErikKTH, School of Architecture and the Built Environment (ABE), Transport and Economics (closed 20110301).Tani, S.
    Manual on Technical Aspects of the UN Law of the Sea2006Collection (editor) (Other academic)
  • 22.
    Danila, Uliana
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    A gravimetric quasigeoid model over Moldova2013Conference paper (Refereed)
  • 23.
    Eshagh, Mehdi
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geoinformatik och Geodesi.
    Bagherbandi, Mohammad
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geoinformatik och Geodesi.
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geoinformatik och Geodesi.
    A COMBINED GLOBAL MOHO MODEL BASED ON SEISMIC AND GRAVIMETRIC DATA2011In: ACTA GEOD GEOPHYS HUNG, ISSN 1217-8977, Vol. 46, no 1, 25-38 p.Article in journal (Refereed)
    Abstract [en]

    The Moho depth can be determined using seismic and/or gravimetric methods. These methods will not yield the same result as they are based on different hypotheses as well as different types, qualities and distributions of data. Here we present a new global model for the Moho computed based on a stochastic combination of seismic and gravimetric Moho models. This method employs condition equations in the spectral domain for the seismic and gravimetric models as well as degree-order variance component estimation to optimally weight the corresponding harmonics in the combination. The preliminary data for the modelling are the seismic model CRUST2.0 and a new gravimetric Moho model based on the inverse solution of the Vening Meinez-Moritz isostatic hypothesis and the global Earth Gravitational Model EGM08. Numerical results show that this method of stochastic combination agrees better with the seismic Moho model (3.6 km rms difference) than the gravimetric one. The model should be a candidate for dandifying the frequently sparsely data CRUST2.0. We expect that this way of combining seismic and gravimetric data would be even more fruitful in a regional study.

  • 24.
    Eshagh, Mehdi
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Geoinformatics.
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Geoinformatics.
    Satellite Gravity Gradiometry: An approch to high resolution gravity field modelling from space2009Book (Other academic)
  • 25.
    Eshagh, Mehdi
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Transport and Economics, Geodesy.
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Transport and Economics, Geodesy.
    The Modified Best Quadratic Unbiased Non-Negative Estimator (MBQUNE) of Variance Components2008In: Studia Geophysica et Geodaetica, ISSN 0039-3169, E-ISSN 1573-1626, Vol. 52, no 3, 305-320 p.Article in journal (Refereed)
    Abstract [en]

    Estimated variance components may come out as negative numbers without physical meaning. One way out of this problem is to use non-negative methods. Different approaches have been presented for the solution. Sjöberg presented a method of Best Quadratic Unbiased Non-Negative Estimator (BQUNE) in the Gauss-Helmert model. This estimator does not exist in the general case. Here we present the Modified BQUNE (MBQUNE) obtained by a simple transformation from the misclosures used in the BQUE to residuals. In the Gauss-Markov adjustment model the BQUNE and MBQUNE are identical, and they differ in condition and Gauss-Helmert models only by a simple transformation. If the observations are composed of independent/disjunctive groups the MBQUNE exists in any adjustment model and it carries all the properties of the BQUNE (when it exists). The presented variance component models are tested numerically in some simple examples. It is shown that the MBQUNE works well for disjunctive groups of observations.

  • 26.
    Eshagh, Mehdi
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Transport and Economics (closed 20110301), Geodesy (closed 20110301).
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Transport and Economics (closed 20110301), Geodesy (closed 20110301).
    Topographic and atmospheric effects on goce gradiometric data in a local north-oriented frame: A case study in Fennoscandia and Iran2009In: Studia Geophysica et Geodaetica, ISSN 0039-3169, E-ISSN 1573-1626, Vol. 53, no 1, 61-80 p.Article in journal (Refereed)
    Abstract [en]

    Satellite gradiometry is an observation technique providing data that allow for evaluation of Stokes' (geopotential) coefficients. This technique is capable of determining higher degrees/orders of the geopotential coefficients than can be achieved by traditional dynamic satellite geodesy. The satellite gradiometry data include topographic and atmospheric effects. By removing those effects, the satellite data becomes smoother and harmonic outside sea level and therefore more suitable for downward continuation to the Earth's surface. For example, in this way one may determine a set of spherical harmonics of the gravity field that is harmonic in the exterior to sea level. This article deals with the above effects on the satellite gravity gradients in the local north-oriented frame. The conventional expressions of the gradients in this frame have a rather complicated form, depending on the first-and second-order derivatives of the associated Legendre functions, which contain singular factors when approaching the poles. On the contrary, we express the harmonic series of atmospheric and topographic effects as non-singular expressions. The theory is applied to the regions of Fennoscandia and Iran, where maps of such effects and their statistics are presented and discussed.

  • 27.
    Eshagh, Mehdi
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Transport and Economics, Geodesy.
    Sjöberg, Lars Erik
    KTH, School of Architecture and the Built Environment (ABE), Transport and Economics, Geodesy.
    Atmospheric effects on satellite gravity gradiometry data2009In: Journal of Geodynamics, ISSN 0264-3707, E-ISSN 1879-1670, Vol. 47, no 1, 9-19 p.Article in journal (Refereed)
    Abstract [en]

    Atmospheric masses play an important role in precise downward continuation and validation of satellite gravity gradiometry data. In this paper we present two alternative ways to formulate the atmospheric potential. Two density models for the atmosphere are proposed and used to formulate the external and internal atmospheric potentials in spherical harmonics. Based on the derived harmonic coefficients, the direct atmospheric effects oil the satellite gravity gradiometry data are investigated and presented ill the orbital frame over Fennoscandia. The formulas of the indirect atmospheric effects oil gravity anomaly and geoid (downward continued quantities) are also derived using the proposed density models. The numerical results show that the atmospheric effect can only be significant for precise validation or inversion of the GOCE gradiometric data at the mE level.

  • 28.
    Eshagh, Mehdi
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geoinformatik och Geodesi.
    Sjöberg, Lars Erik
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geoinformatik och Geodesi.
    Determination of gravity anomaly at sea level from inversion of satellite gravity gradiometric data2011In: Journal of Geodynamics, ISSN 0264-3707, E-ISSN 1879-1670, Vol. 51, no 5, 366-377 p.Article in journal (Refereed)
    Abstract [en]

    Gravity gradients can be used to determine the local gravity field of the Earth. This paper investigates downward continuation of all elements of the disturbing gravitational tensor at satellite level using the second-order partial derivatives of the extended Stokes formula in the local-north oriented frame to determine the gravity anomaly at sea level. It considers the inversion of each gradient separately as well as their joint inversion. Numerical studies show that the gradients T-zz, T-xx, T-yy and T-xz have similar capability of being continued downward to sea level in the presence of white noise, while the gradient T-yz is considerably worse than the others. The bias-corrected joint inversion process shows the possibility of recovering the gravity anomaly with 1 mGal accuracy. Variance component estimation is also tested to update the observation weights in the joint inversion.

  • 29.
    Eshagh, Mehdi
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Transport and Economics (closed 20110301), Geodesy (closed 20110301).
    Sjöberg, Lars Erik
    KTH, School of Architecture and the Built Environment (ABE), Transport and Economics (closed 20110301), Geodesy (closed 20110301).
    Impact of Topography and Atmosphere over Iran on Validation and Inversion of GOCE Gradiometric Data2008In: Journal of the Earth and Space Physics, ISSN 0378-1046, Vol. 34, no 3, 15-30 p.Article in journal (Refereed)
  • 30.
    Eshagh, Mehdi
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Transport and Economics, Geodesy.
    Sjöberg, Lars
    KTH, School of Architecture and the Built Environment (ABE), Transport and Economics, Geodesy.
    Kiamehr, Ramin
    Evaluating of robust techniques in suppressing the impact of outliers in a deformation monitoring network: A case study on the Teheran Milad tower network2007In: Acta Geodaetica et Geophysica Hungaria, ISSN 1217-8977, Vol. 42, no 4, 449-463 p.Article in journal (Refereed)
    Abstract [en]

    The problem of handling outliers in a deformation monitoring network is of special importance, because the existence of outliers may lead to false deformation parameters. One of the approaches to detect the outliers is to use robust estimators. In this case the network points are computed by such a robust method, implying that the adjustment result is resisting systematic observation errors, and, in particular, it is insensitive to gross errors and even blunders. Since there are different approaches to robust estimation, the resulting estimated networks may differ. In this article, different robust estimation methods, such as the M-estimation of Huber, the "Danish", and the L-1-norm estimation methods, are reviewed and compared with the standard least squares method to view their potentials to detect outliers in the Tehran Milad tower deformation network. The numerical studies show that the L-1-norm is able to detect and down-weight the outliers best, so it is selected as the favourable approach, but there is a lack of uniqueness. For comparison, Baarda's method "data snooping" can achieve similar results when the outlier magnitude of an outlier is large enough to be detected; but robust methods are faster than the sequential data snooping process.

  • 31. Fujita, S.
    et al.
    Holmlund, P.
    Andersson, I.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Geoinformatics.
    Brown, I.
    Enomoto, H.
    Fujii, Y.
    Fujita, K.
    Fukui, K.
    Furukawa, T.
    Hansson, M.
    Hara, K.
    Hoshina, Y.
    Igarashi, M.
    Iizuka, Y.
    Imura, S.
    Ingvander, S.
    Karlin, T.
    Motoyama, H.
    Nakazawa, F.
    Oerter, H.
    Sjöberg, Lars
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Geoinformatics.
    Sugiyama, S.
    Surdyk, S.
    Strom, J.
    Uemura, R.
    Wilhelms, F.
    Spatial and temporal variability of snow accumulation rate on the East Antarctic ice divide between Dome Fuji and EPICA DML2011In: The Cryosphere, ISSN 1994-0416, E-ISSN 1994-0424, Vol. 5, no 4, 1057-1081 p.Article in journal (Refereed)
    Abstract [en]

    To better understand the spatio-temporal variability of the glaciological environment in Dronning Maud Land (DML), East Antarctica, a 2800-km-long Japanese-Swedish traverse was carried out. The route includes ice divides between two ice-coring sites at Dome Fuji and EPICA DML. We determined the surface mass balance (SMB) averaged over various time scales in the late Holocene based on studies of snow pits and firn cores, in addition to radar data. We find that the large-scale distribution of the SMB depends on the surface elevation and continentality, and that the SMB differs between the windward and leeward sides of ice divides for strong-wind events. We suggest that the SMB is highly influenced by interactions between the large-scale surface topography of ice divides and the wind field of strong-wind events that are often associated with high-precipitation events. Local variations in the SMB are governed by the local surface topography, which is influenced by the bedrock topography. In the eastern part of DML, the accumulation rate in the second half of the 20th century is found to be higher by similar to 15% than averages over longer periods of 722 a or 7.9 ka before AD 2008. A similar increasing trend has been reported for many inland plateau sites in Antarctica with the exception of several sites on the leeward side of the ice divides.

  • 32.
    Horemuž, Milan
    et al.
    KTH, Superseded Departments, Geodesy and Photogrammetry.
    Sjöberg, Lars Erik
    KTH, Superseded Departments, Infrastructure.
    Rapid GPS ambiguity resolution for short and long baselines2002In: Journal of Geodesy, ISSN 0949-7714, E-ISSN 1432-1394, Vol. 76, no 07-jun, 381-391 p.Article in journal (Refereed)
    Abstract [en]

    A method of quick initial carrier cycle ambiguity resolution is described. The method applies to high-quality dual-band global positioning system observations. Code measurements on both frequencies must be available. The rapidity of the method is achieved through smoothing pseudoranges by phase observables and forming linear combinations between the phase observables. Two cases are investigated. Case 1: ionospheric bias is neglected (short distances); and case 2: the bias is taken into account (longer distances, more than, say, 10 km). The method was tested on six baselines, from 1 to 31 km long. In most cases, single-epoch ambiguity resolution was achieved.

  • 33. Jacoby, Wolfgang R.
    et al.
    Hartmann, Oliver
    Wallner, Herbert
    Smilde, Peter L.
    Buerger, Stefan
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Transport and Economics, Geodesy.
    Erlingsson, Sigurdur
    Wolf, Detlef
    Klemann, Volker
    Sasgen, Ingo
    Temporal Gravity Variations near Shrinking Vatnajokull Ice Cap, Iceland2009In: Pure and Applied Geophysics, ISSN 0033-4553, E-ISSN 1420-9136, Vol. 166, no 8-9, 1283-1302 p.Article in journal (Refereed)
    Abstract [en]

    Repeated gravity measurements were carried out from 1991 until 1999 at sites SE of Vatnajokull, Iceland, to estimate the mass flow and deformation accompanying the shrinking of the ice cap. Published GPS data show an uplift of about 13 +/- A 5 mm/a near the ice margin. A gravity decrease of -2 +/- A 1 mu Gal/a relative to the Hofn base station, was observed for the same sites. Control measurements at the Hofn station showed a gravity decrease of -2 +/- A 0.5 A mu Gal/a relative to the station RVIK 5473 at Reykjavik (about 250 km from Hofn). This is compatible, as a Bouguer effect, with a 10 +/- A 3 mm/a uplift rate of the IGS point at Hofn and an uplift rate of similar to 20 mm/a near the ice margin. Although the derived gravity change rates at individual sites have large uncertainties, the ensemble of the rates varies systematically and significantly with distance from the ice. The relationship between gravity and elevation changes and the shrinking ice mass is modelled as response to the loading history. The GPS data can be explained by 1-D modelling (i.e., an earth model with a 15-km thick elastic lithosphere and a 7 center dot 10(17) Pa center dot s asthenosphere viscosity), but not the gravity data. Based on 2-D modelling, the gravity data favour a low-viscosity plume in the form of a cylinder of 80 km radius and 10(17) to 10(18) Pa center dot s viscosity below a 6 km-thick elastic lid, embedded in a layered PREM-type earth, although the elevation data are less well explained by this model. Strain-porosity-hydrology effects are likely to enhance the magnitude of the gravity changes, but need verification by drilling. More accurate data may resolve the discrepancies or suggest improved models.

  • 34.
    Joud, Mehdi S. Shafiei
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    Bagherbandi, Mohammad
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning. University of Gävle, Sweden.
    Use of GRACE data to detect the present land uplift rate in Fennoscandia2017In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 209, no 2, 909-922 p.Article in journal (Refereed)
    Abstract [en]

    After more than 13 yr of GRACE monthly data, the determined secular trend of gravity field variation can be used to study the regions of glacial isostatic adjustment (GIA). Here we focus on Fennoscandia where long-term terrestrial and high-quality GPS data are available, and we study the monthly GRACE data from three analysis centres. We present a new approximate formula to convert the secular trend of the GRACE gravity change to the land uplift rate without making assumptions of the ice load history. The question is whether the GRACEderived land uplift rate by our method is related to GIA. A suitable post-processing method for the GRACE data is selected based on weighted RMS differences with the GPS data. The study reveals that none of the assumed periodic changes of the GRACE gravity field is significant in the estimation of the secular trend, and they can, therefore, be neglected. Finally, the GRACEderived land uplift rates are obtained using the selected post-processing method, and they are compared with GPS land uplift rate data. The GPS stations with significant differences were marked using a statistical significance test. The smallest rms difference (1.0 mm a-1) was obtained by using GRACE data from the University of Texas.

  • 35. Kiamehr, R.
    et al.
    Sjöberg, Lars Erik
    KTH, School of Architecture and the Built Environment (ABE), Transport and Economics, Geodesy.
    An optimum way to determine a precise gravimetric geoid model based on the least-squares modification of Stokes' formula: A case study of Sweden2010In: Acta Geodaetica et Geophysica Hungarica, ISSN 1217-8977, Vol. 45, no 2, 148-164 p.Article in journal (Refereed)
    Abstract [en]

    The modification of Stokes' formula allows the user to compensate the lack of a global coverage of gravity data by a combination of terrestrial gravity and a global geopotential model. The minimization of the errors of truncation gravity data and potential coefficients could be treated in a least-squares sense as is the basic ingredient in the Royal Institute of Technology (KTH) approach as proposed by Sjoberg in 1984. This article presents the results from a joint project between KTH and the National Land Survey of Sweden, whose main purpose is to evaluate the KTH approach numerically and to compute a gravimetric geoid model for Sweden. The new geoid model (KTH06) was computed based on the least-squares modification of Stokes' formula, the GRACE global geopotential model, a high-resolution digital terrain model and the NKG gravity anomaly database. The KTH06 was fitted to 1162 GPS/levelling points by a 7-parameter transformation, yielding an all-over fit of 19 mm and 0.17 ppm. The fit is even smaller than the estimated internal accuracy for the geoid model (28 mm). If we assume that the accuracy of the GPS and levelling heights are 10 mm and 5 mm, respectively, it follows that the accuracy of the expected gravimetric geoid heights are of the order of 11 mm. Also, we found a significant expected difference between the KTH06 and NKG2004 models in rough topographic areas (up to 36 cm). As the major ground data and global geopotential model were almost same in the two models, we believe that there are different reasons that come into play for interpreting the discrepancies between them, as the method for eliminating outliers from the gravity database, the interpolated denser gravity observations using the high-resolution digital elevation model before Stokes' integration, the potential of the LSM kernel, which matches the errors of the terrestrial gravity data, GGM and the truncation error in an optimum way, and the effect of applying more precise correction terms in the KTH approach compared to the remove-compute-restore method. It is concluded that the least-squares modification method with additive corrections is a very promising alternative for geoid computation.

  • 36. Kiamehr, Ramin
    et al.
    Eshagh, Mehdi
    KTH, School of Architecture and the Built Environment (ABE), Transport and Economics, Geodesy.
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Transport and Economics, Geodesy.
    Interpretation of general geophysical patterns in Iran based on GRACE gradient component analysis2008In: Acta Geophysica, ISSN 1895-6572, Vol. 56, no 2, 440-454 p.Article in journal (Refereed)
    Abstract [en]

    Only with satellites it is possible to cover the entire Earth densely with gravity field related measurements of uniform quality within a short period of time. However, due to the altitude of the satellite orbits, the signals of individual local masses are strongly damped. Based on the approach of Petrovskaya and Vershkov we determine the gravity gradient tensor directly from the spherical harmonic coefficients of the recent EIGEN-GL04C combined model of the GRACE satellite mission. Satellite gradiometry can be used as a complementary tool to gravity and geoid information in interpreting the general geophysical and geodynamical features of the Earth. Due to the high altitude of the satellite, the effects of the topography and the internal masses of the Earth are strongly damped. However, the gradiometer data, which are nothing else than the second order spatial derivatives of the gravity potential, efficiently counteract signal attenuation at the low and medium frequencies.

    In this article we review the procedure for estimating the gravity gradient components directly from spherical harmonics coefficients. Then we apply this method as a case study for the interpretation of possible geophysical or geodynamical patterns in Iran. We found strong correlations between the cross-components of the gravity gradient tensor and the components of the deflection of vertical, and we show that this result agrees with theory. Also, strong correlations of the gravity anomaly, geoid model and a digital elevation model were found with the diagonal elements of the gradient tensor.

  • 37.
    Kiamehr, Ramin
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Transport and Economics (closed 20110301).
    Sjöberg, Lars
    KTH, School of Architecture and the Built Environment (ABE), Transport and Economics (closed 20110301), Geodesy (closed 20110301).
    Analysis of Surface Deformation Patterns Analysis Using 3D Finite Elements Method: A case study in Skåne area, Sweden2005In: J. Geodynamics, ISSN 0264-3707, Vol. 39, no 4, 403-412 p.Article in journal (Refereed)
    Abstract [en]

    The application of geodetic techniques to study crustal deformations associated with the geodynamic activity of a region is considered as a fundamental tool in seismic risk mitigation and in earthquake prediction research. In principle, the crustal deformation analysis is a purely geodetic problem as it concerns alteration of the Earth shape, so that deformations of the crust are directly connected with geodetic observables. The Tornquist zone across Skane in southern Sweden is a classical fault zone that separates the Precambrium gneisses of the Baltic shield in the north from Phanerozoic Europe to the south. In this region, a Global Positioning Network (GPS) was established to study possible crustal motions. The aim of this article is to improve on previous study in to estimate the possible crustal strains and dilation parameters by a finite element analysis. Results show that the areas with maximum shear strain and dilation are located exactly in the active fault zones and their intersections. However, further observations in a dense network as well as integration with geological and geophysical data are needed to fully explore the recent crustal motions over the Tornquist zone.

  • 38.
    Kiamehr, Ramin
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Transport and Economics.
    Sjöberg, Lars
    KTH, School of Architecture and the Built Environment (ABE), Transport and Economics, Geodesy.
    Comparison of the qualities of recent global and local gravimetric geoid models in Iran2005In: Studia Geophysica et Geodaetica, ISSN 0039-3169, Vol. 49, no 3, 289–304- p.Article in journal (Refereed)
    Abstract [en]

    A number of regional gravimetric geoid models have recently been determined for the Iran area, and a common problem is to select the best model, e.g. for engineering applications. A related problem is that in order to improve the local geoid models, the selection of the best Global Geopotential Model (GGM) model for the region is essential, to be used in a combined solution from GGM and local gravimetric data. We discuss these problems by taking advantage of 260 GPS/levelling points as an external tool for validation of different global and local geoid models in the absolute and relative senses. By using relative comparisons of the height differences between precise levelling and GPS/geoid models we avoid possible unknown systematic effects between the different types of observables.

    The study shows that the combination of the newly released GRACE model (GGM02C) with EGM96 geoid model fits the GPS/levelling data in Iran with the best absolute and relative accuracy among the GGMs. Among the local geoid models, the newly gravimetric geoid model IRG04 agrees considerably better with GPS/levelling than any of the other recent local geoid models. Its rms fit with GPS/levelling is 55 cm. Hence, we strongly recommend the use of this new model in any surveying engineering or GPS/levelling projects in the area.

  • 39.
    Kiamehr, Ramin
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Transport and Economics.
    Sjöberg, Lars
    KTH, School of Architecture and the Built Environment (ABE), Transport and Economics, Geodesy.
    Effect of the SRTM global DEM on the determination of a high-resolution geoid model: a case study in Iran2005In: Journal of Geodesy, ISSN 0949-7714, E-ISSN 1432-1394, J. Geodesy, Vol. 79, no 9, 540-551 p.Article in journal (Refereed)
    Abstract [en]

    Any errors in digital elevation models (DEMs) will introduce errors directly in gravity anomalies and geoid models when used in interpolating Bouguer gravity anomalies. Errors are also propagated into the geoid model by the topographic and downward continuation (DWC) corrections in the application of Stokes's formula. The effects of these errors are assessed by the evaluation of the absolute accuracy of nine independent DEMs for the Iran region. It is shown that the improvement in using the high-resolution Shuttle Radar Topography Mission (SRTM) data versus previously available DEMs in gridding of gravity anomalies, terrain corrections and DWC effects for the geoid model are significant. Based on the Iranian GPS/levelling network data, we estimate the absolute vertical accuracy of the SRTM in Iran to be 6.5 m, which is much better than the estimated global accuracy of the SRTM (say 16 m). Hence, this DEM has a comparable accuracy to a current photogrammetric high-resolution DEM of Iran under development. We also found very large differences between the GLOBE and SRTM models on the range of -750 to 550 m. This difference causes an error in the range of -160 to 140 mGal in interpolating surface gravity anomalies and -60 to 60 mGal in simple Bouguer anomaly correction terms. In the view of geoid heights, we found large differences between the use of GLOBE and SRTM DEMs, in the range of -1.1 to 1 m for the study area. The terrain correction of the geoid model at selected GPS/levelling points only differs by 3 cm for these two DEMs.

  • 40.
    Kiamehr, Ramin
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Transport and Economics.
    Sjöberg, Lars
    KTH, School of Architecture and the Built Environment (ABE), Transport and Economics, Geodesy.
    Impact of the precise geoid model in studying tectonic structures- a case study in Iran2006In: Journal of Geodynamics, ISSN 0264-3707, E-ISSN 1879-1670, J. Geodynamics, Vol. 42, no 1-3, 1-11 p.Article in journal (Refereed)
    Abstract [en]

    Iran is one of the most active regions in the world with respect,to earthquakes and tectonic motions in the lithosphere. In order to study the impact of the geoid model in detecting plate tectonic boundaries and in the establishment of an accurate height datum for future geodynamic observations, a new combined gravimetric geoid model for Iran (IRG04C) was computed by the method of least squares modification of Stokes formula based on the most recent gravity anomaly database, SRTM high resolution Digital Elevation Model (DEM), GRACE GGM02 Global Geopotential Model and GPS/levelling data. The RMS fit of the new geoid model versus GPS/levelling data is 9 cm, which is a 10 times better fit compared to the most recent published gravimetric geoid model in the area. An integrated approach, combining gravity, geoid and seismology data as well as a digital elevation model, was used to find possible correlations between these parameters. Our investigation indicates that all earthquakes with magnitude over 6.0 in the Richter scale are located in areas with a geoid slope exceeding 7.5%. The study shows a significant correlation between the horizontal gradient of the geoid and plate tectonic activities.

  • 41. Nahavandchi, H.
    et al.
    Sjöberg, Lars Erik
    KTH, Superseded Departments, Infrastructure.
    Precise geoid determination over Sweden using the Stokes-Helmert method and improved topographic corrections2001In: Journal of Geodesy, ISSN 0949-7714, E-ISSN 1432-1394, Vol. 75, no 3-Feb, 74-88 p.Article in journal (Refereed)
    Abstract [en]

    Four different implementations of Stokes formula are employed for the estimation of geoid heights over Sweden: the Vincent and Marsh (1974) model with the high-degree reference gravity field but no kernel modifications: modified Wong and Gore (1969) and Molodenskii et al. (1962) models, which use a high-degree reference gravity field and modification of Stokes kernel: and a least-squares (LS) spectral weighting proposed by Sjoberg (1991). Classical topographic correction formulae are improved to consider long-wavelength contributions. The effect of a Bouguer shell is also included in the formulae, which is neglected in classical formulae due to planar approximation. The gravimetric geoid is compared with global positioning system (GPS)-levelling-derived geoid heights at 23 Swedish Permanent GPS Network SWEPOS stations distributed over Sweden. The LS method is in best agreement, with a 10.1-cm mean and +/-5.5-cm standard deviation in the differences between gravimetric and GPS geoid heights. The gravimetric geoid was also fitted to the GPS-levelling-derived geoid using a four-parameter transformation model. The results after fitting also show the best consistency for the LS method, with the standard deviation of differences reduced to +/-1.1 cm. For comparison, the NKG96 geoid yields a 17-cm mean and +/-8-cm standard deviation of agreement with the same SWEPOS stations. After four-parameter fitting to the GPS stations, the standard deviation reduces to +/-6.1 cm for the NKG96 geoid. It is concluded that the new corrections in this study improve the accuracy of the geoid. The final geoid heights range from 17.22 to 43.62 m with a mean value of 29.01 m. The standard errors of the computed geoid heights, through a simple error propagation of standard errors of mean anomalies, are also computed. They range from +/-7.02 to +/- 13.05 cm. The global root-mean-square error of the LS model is the other estimation of the accuracy of the final geoid, and is computed to be +/- 28.6 cm.

  • 42. Nahavandchi, H.
    et al.
    Sjöberg, Lars Erik
    KTH, Superseded Departments, Infrastructure.
    Two different views of topographical and downward-continuation corrections in the Stokes-Helmert approach to geoid computation2001In: Journal of Geodesy, ISSN 0949-7714, E-ISSN 1432-1394, Vol. 74, no 12-Nov, 816-822 p.Article in journal (Refereed)
    Abstract [en]

    Two different methods to handle the topographical and downward-continuation corrections in Stokes' formula are investigated. The first approach is to correct observed gravity anomalies for the the effect of topographic masses, then downward-continue the corrected gravity anomaly to the geoid using Poisson's integral. After Stokes' formula has been applied, the gravitational effects of the masses on the geoid are restored using the indirect primary effect. The second approach is to add all topographic effects as a total correction to the original Stokes formula, which includes a new method to estimate the effect of downward continuation. These two methods are compared at 23 global positioning system (GPS)-levelling stations in Sweden. The results of this comparison show that both methods work well, with the first method having better agreement with the GPS-levelling geoid. The standard deviation of fit in the first method is computed to be +/-1.1 cm, while it is +/-2.1 rm for the second method after a four-parameter fit.

  • 43. Pan, M.
    et al.
    Sjöberg, Lars Erik
    KTH, Superseded Departments, Infrastructure.
    Asfaw, L. M.
    Asenjo, E.
    Alemu, A.
    Hunegnaw, A.
    An analysis of the Ethiopian Rift Valley GPS campaigns in 1994 and 19992002In: Journal of Geodynamics, ISSN 0264-3707, E-ISSN 1879-1670, Vol. 33, no 3, 333-343 p.Article in journal (Refereed)
    Abstract [en]

    In cooperation with the Geophysical Observatory, Addis Ababa University, the Department of Geodesy and Photogrammetry of the Royal Institute of Technology carried out GPS measurements at three deformation networks in the Ethiopian Rift Valley in 1994 and 1999. For these campaigns the session-to-session repeatabilities were about 5 and 4 mm in the horizontal components and 50 and 10 mm in the vertical components using the IGS precise orbits, respectively. The results show, that the stations of the E3 network in the Rift Valley moved with a magnitude of 2.5 +/- 1.3 mm/a to the SE, and a magnitude of 21.3 +/- 1.4 mm/a to the ENE in the northeast (close to Assab of the Red Sea) relative to the station Addis Ababa on the African plate. The direction of movement is consistent with the local spreading vector of the Red Sea Rift with the N84degreesE slip direction estimated from seismic data. The sites of the E1 network moved by a rate of about 1-2 mm/a to the SE relative also to the station Addis Ababa. Further to the south there is an indication from our GPS data that the stations on the Somalian plate of the E2 network move by a rate of about 2.5 mm/a to the E or SE relative to the African plate. This motion can also be confirmed by geological and geodynamic data.

  • 44. Pan, M.
    et al.
    Sjöberg, Lars Erik
    KTH, Superseded Departments, Infrastructure.
    Talbot, C. J.
    Crustal movements in Skane, Sweden, between 1992 and 1998 as observed by GPS2001In: Journal of Geodynamics, ISSN 0264-3707, E-ISSN 1879-1670, Vol. 31, no 3, 311-322 p.Article in journal (Refereed)
    Abstract [en]

    We estimate a network of crustal deformations in Skane, southern Sweden, using observations of the Global Positioning System (GPS) from 1992, 1996 and 1998. The network straddles the Tornquist zone, potentially one of the most active fault zones in Sweden. In addition to two stations of the Swedish permanent GPS network (Onsala and Hassleholm), it includes seven GPS sites spaced approximately 80 km apart. The precisions of the relative horizontal components for these stations are described by repeatabilities with approximately 3 mm in the north-south direction and about 2 mm in the east-west direction in the campaigns in the reference frame ITRF96. About 70% of the GPS integer carrier phase ambiguities were resolved for the three campaigns. Three stations south of the Tornquist zone moved with a rate of 5+/-0.2 mm/a toward the SW-SE, and two other stations further to the east with a rate of 2+/-0.2 mm/a toward the SE, relative to station Onsala. The displacements mean that a lateral strike-slip transtensional fault exists within the Tornquist zone, and the relative motion between the two sides along the zone was about 2 mm/a. The old fault is still active, which is consistent with geological results. The station Stavershult closest to Onsala moved NE with about 1.5+/-0.8 mm/a relative to the station Onsala. This may imply a transtensional fault between Stavershult and Onsala. If we assume that Stavershult represents the Baltic shield, Onsala has moved 1.5 mm/a toward the southwest with respect to the Baltic shield, which was confirmed by data of Very Long Baseline Interferometry (VLBI) and Satellite Laser Ranging (SLR) from 1976 to 1997 (Argus, D.F., Peltier, W.R., Watkins, M.M. Glacial isostatic adjustment observed by Very Long Baseline Interferometry and Satellite Laser Ranging geodesy. JGR 1999;104(B12);29077-93).

  • 45.
    Reshetyuk, Yuriy
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Transport and Economics (closed 20110301), Geodesy (closed 20110301).
    Horemuž, Milan
    KTH, School of Architecture and the Built Environment (ABE), Transport and Economics (closed 20110301), Geodesy (closed 20110301).
    Sjöberg, Lars Erik
    KTH, School of Architecture and the Built Environment (ABE), Transport and Economics (closed 20110301), Geodesy (closed 20110301).
    Determination of the optimal diameter for spherical targets used in 3D laser scanning2005In: Survey review - Directorate of Overseas Surveys, ISSN 0039-6265, E-ISSN 1752-2706, Vol. 38, no 297, 243-253 p.Article in journal (Refereed)
    Abstract [en]

    An efficient use of 3D laser scanning requires the development of standardized calibration procedures available to the users. This is part of the research recently started at the Royal Institute of Technology; (KTH) in Stockholm and the establishment of a calibration field is planned, which may he realized by means of spherical targets. An approach for the rigorous theoretical determination of the optimal diameter for those targets is presented. It is based on the least-squares adjustment according to the mixed model when observations are combined with the unknown parameters. The weight function was chosen based on the Lambertion standard reflectance model. The computations were performed for the scanners Leica HDS 3000 and Imager 5003 (Zoller+Frohlich) for two cases - with fixed and free (to be estimated in the adjustment) target diameter. The value of 14 cm has been obtained for the optimal diameter, which is independent on the range to the scanner.

  • 46. Ruelke, A.
    et al.
    Dietrich, R.
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Geoinformatics.
    The Antarctic regional GPS network densification- status and results2013Conference paper (Refereed)
  • 47. Rülke, A.
    et al.
    Dietrich, R.
    Capra, A.
    Cisak, J.
    Dongchen, E.
    Eiken, T.
    Fox, A.
    Hothem, L. D.
    Johnston, G.
    Malaimani, E. C.
    Matveev, A. J.
    Milinevsky, G.
    Schenke, H. -W
    Shibuya, K.
    Sjöberg, L. E.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    Zakrajsek, A.
    Fritsche, M.
    Groh, A.
    Knöfel, C.
    Scheinert, M.
    The antarctic regional GPS network densification: Status and results2016In: IAG 150 Years proceedings of the 2013 IAG Scientific Assembly, Postdam, Germany, 1-6 September, 2013, Springer, 2016, 133-139 p.Conference paper (Refereed)
    Abstract [en]

    We report on the activities related to the IAG Subcommission 1.3f “Regional Reference Frame for Antarctica”. Campaign-style GPS observations have been carried out since 1995. Based on the Bernese GNSS Software the latest analysis yields results for about 30 stations aligned to the terrestrial reference frame solution IGS08. The obtained station motions are discussed in the context of plate kinematics and glacial-isostatic adjustment. It is demonstrated that the activities are a valuable contribution both to the ITRF densification in Antarctica and to geodynamic research.

  • 48.
    Shirazian, Masoud
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geoinformatik och Geodesi.
    Sjöberg, Lars E.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geoinformatik och Geodesi.
    Horemuz, Milan
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geoinformatik och Geodesi.
    A remark on the GNSS differenced phase ambiguity parameters2011In: Acta Geodaetica et Geophysica Hungarica, ISSN 1217-8977, E-ISSN 1587-1037, Vol. 46, no 4, 431-440 p.Article in journal (Refereed)
    Abstract [en]

    In the global navigation satellite system (GNSS) carrier phase data processing, cycle slips are limiting factors and affect the quality of the estimators in general. When differencing phase observations, a problem in phase ambiguity parameterization may arise, namely linear relations between some of the parameters. These linear relations must be considered as additional constraints in the system of observation equations. Neglecting these constraints, results in poorer estimators. This becomes significant when ambiguity resolution is in demand. As a clue to detect the problem in GNSS processing, we focused on the equivalence of using undifferenced and differenced observation equations. With differenced observables this equivalence is preserved only if we add certain constraints, which formulate the linear relations between some of the ambiguity parameters, to the differenced observation equations. To show the necessity of the additional constraints, an example is made using real data of a permanent station from the network of the international GNSS service (IGS). The achieved results are notable to the GNSS software developers.

  • 49.
    Sjoberg, Lars E.
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geoinformatik och Geodesi.
    Bagherbandi, Mohammad
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geoinformatik och Geodesi.
    Quasigeoid-to-geoid determination by EGM082012In: Earth Science Informatics, ISSN 1865-0473, Vol. 5, no 2, 87-91 p.Article in journal (Refereed)
    Abstract [en]

    We present a method to estimate the difference between quasigeoid and geoid heights globally from the Earth Gravitational Model EGM08 and a related topographic model. The numerical computations with the standard topographic density of 2.67 g/cm(3) show that the maximum and minimum of the separations are estimated to 5.47 m and -0.11 m on the Tibet plateau and in the Indian Ocean, respectively. These estimates do not consider possible topographic density variations, which result in topographic bias changes proportional to the topographic elevation squared. Assuming such density changes of 10% from the standard value, the separation may change up to 5 dm.

  • 50.
    Sjoberg, Lars E.
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    Grafarend, E. W.
    Joud, Seyed Mehdi Shafiei
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geodesy and Satellite Positioning.
    The zero gravity curve and surface and radii for geostationary and geosynchronous satellite orbits2017In: Journal of Geodetic Science, ISSN 2081-9919, E-ISSN 2081-9943, Vol. 7, no 1, 43-50 p.Article in journal (Refereed)
    Abstract [en]

    A geosynchronous satellite orbits the Earth along a constant longitude. A special case is the geostationary satellite that is located at a constant position above the equator. The ideal position of a geostationary satellite is at the level of zero gravity, i.e. at the geocentric radius where the gravitational force of the Earth equals the centrifugal force. These forces must be compensated for several perturbing forces, in particular for the lunisolar tides. Considering that the gravity field of the Earth varies not only radially but also laterally, this study focuses on the variations of zero gravity not only on the equator (for geostationary satellites) but also for various latitudes. It is found that the radius of a geostationary satellite deviates from its mean value of 42164.2 km only within +/- 2 m, mainly due to the spherical harmonic coefficient J(22), which is related with the equatorial flattening of the Earth. Away from the equator the zero gravity surface deviates from the ideal radius of a geosynchronous satellite, and more so for higher latitudes. While the radius of the former surface increases towards infinity towards the poles, the latter decreases about 520 m from the equator to the pole. Tidal effects vary these radii within +/- 2.3 km.

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