Vahab Nafisi

Vahab Nafisi
University of Isfahan · Department of Geomatics Engineering

Ph.D.

About

39
Publications
6,012
Reads
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331
Citations
Additional affiliations
February 2010 - September 2011
TU Wien
Position
  • Researcher
December 1999 - present
University of Isfahan
Position
  • Professor (Assistant)

Publications

Publications (39)
Article
Full-text available
Northwest of Iran, as a tectonically active region, has experienced numerous devastating earthquakes. That is why it is so important to study the earth deformation in this area and to provide more precise insights. So far, most researchers have had the preference of using the invariant component of strain rate tensor for investigating the Earth's s...
Article
Full-text available
Knowledge on the ionospheric total electron content (TEC) and its prediction are of great practical importance and engineering relevance in many scientific disciplines. We investigate regular ionospheric anomalies and TEC prediction by applying the least squares harmonic estimation (LS-HE) technique to a 15 year time series of the vertical TEC (VTE...
Article
Full-text available
Drought is a gradual phenomenon that may intensify with time. In fact, drought is the result of interactions between natural and human activities and usually appears in the following ways: reduction in water resources, accelerating desertification, the vegetation cover change and etc. Monitoring and predicting drought is essential for sustainable w...
Article
Full-text available
In this paper, we employ Empirical Mode Decomposition (EMD) together with Hilbert Transform to analyze precipitation time series over the Caspian Sea catchment. Several studies have shown that EMD can extract nonlinear and non-stationary signals better than Fast Fourier Transform (FFT) and Wavelet Transform. EMD decomposes the time series into a fi...
Article
Full-text available
Fitting a smooth surface on irregular data is a problem in many applications of data analysis. Spline polynomials in different orders have been used for interpolation and approximation in one or two-dimensional space in many researches. These polynomials can be made by different degrees and they have continuous derivative at the boundaries. The adv...
Article
Full-text available
The Urmia Lake is located in northwestern Iran. It is the sixth largest salt lake in the world. In the past 14 years, this lake strongly faced to the water level decrease and surface water reduction. Several space-borne techniques have been employed for detecting the hydrological cycle and its inter-annual changes. The goal of this paper is to inv...
Article
In the last few years, satellite-based positioning techniques, such as the Global Positioning System (GPS), proved their abilities in various geodetic fields. Height determination, as one of such applications, requires a more likely correct geoid model to provide reliable geoid heights for transformation of the ellipsoidal heights to orthometric he...
Article
The ordinary kriging interpolation method is adopted to estimate the 2-D strain tensor elements. The results are then compared with those of the linear and spline interpolation methods. The ordinary kriging method is implemented on 12 GPS permanent stations in South California, collected during 2006 to 2012. By employing these time series, the coor...
Article
In this paper, universal kriging with linear trend is used to interpolate the strain tensor elements over a region along San Andreas Fault in California. The main goal of this paper is to improve the ordinary kriging interpolation results. A 7-year time series (2006–2012) of 12 permanent stations is utilized to obtain the coordinate changes in UTM...
Chapter
This part describes the effects of the troposphere—strictly speaking the neutral atmosphere—on the propagation delay of space geodetic signals. A theoretical description of this tropospheric propagation delay is given as well as strategies for correcting for it in the data analysis of the space geodetic observations. The differences between the tro...
Chapter
We introduce a static a priori gradient model (APG) based on a spherical harmonic expansion up to degree and order nine to describe the azimuthal asymmetry of tropospheric delays. APG is determined from climatology data of the European Centre for Medium-Range Weather Forecasts (ECMWF), and the refined model can be used in the analysis of observatio...
Article
Due to recent improvements of numerical weather models (NWM), ray-tracing has become an interesting and feasible method for different scientific purposes. One application in geosciences is the estimation of tropospheric delays for the correction of space geodetic techniques. Here, we present and discuss the recently developed ray-tracing algorithm...
Article
We develop a ray-tracing package for the calculation of path delays of microwave signals in the troposphere based on numerical weather models which we use for the determination of the delays of geodetic Very Long Baseline Interferometry (VLBI) observations. We show results for a two-week campaign of continuous VLBI sessions in 2008 (CONT08), where...
Conference Paper
Full-text available
VLBI Intensive sessions are used for the estimation of UT1–UTC (∆UT1) which is needed in near-real-time for the accurate prediction of Universal Time (UT1) as well as for navigation purposes. These 1-hour IVS sessions (INT1 and INT2) are carried out every day to provide this quantity on a regular basis. Due to the small number of observables per se...
Article
A comparison campaign to evaluate and compare troposphere delays from different ray-tracing software was carried out under the umbrella of the International Association of Geodesy Working Group 4.3.3 in the first half of 2010 with five institutions participating: the GFZ German Research Centre for Geosciences (GFZ), the Groupe de Recherche de Geode...
Conference Paper
The Vienna VLBI Software (VieVS) is a new geodetic Very Long Baseline Interferometry (VLBI) data analysis software which has been developed at the Institute of Geodesy and Geophysics, Vienna University of Technology, since 2008. This paper presents the software, its latest developments, and the planned future developments. A few results obtained wi...
Poster
Tropospheric propagation is a serious source of error in the analysis of space geodetic observations at radio wavelengths such as VLBI, GNSS, and DORIS. In recent years direct ray-tracing methods based on numerical weather models have been developed by different researchers in order to determine the true trajectory of a specific ray and its path de...
Conference Paper
Full-text available
Abstract: Geoid as an equipotential surface has the important role in surveying engineering e.g. leveling, height datum unification, lay out of civil projects, etc. For many years the determination of Geoid has been hot topic in the world, of course, nowadays some scientists are yet working on the subject. One of methods for Geoid determination is...
Conference Paper
Full-text available
Satellite altimetry has proven its Potentiality in the observations of sea level variations in the global sense. The sea level variations observed by the satellite altimetry missions can be processed in order to derive the tidal constituents and the static sea level known as Mean Sea Level (MSL) in geodesy. In this paper, satellite altimetry der...
Conference Paper
Full-text available
Nowadays satellite altimetry observations have proven their value in the global oceanographic studies, and have become standard tool for global ocean tide modeling. One of the outputs of ocean tide models is the zero frequency tide constituent, which is also known in marine geodesy as Mean Sea Level (MSL). Mean Sea Level is not an equipotential...
Conference Paper
Regression models play an important role in many applied settings, providing prediction and classification rules, and data analytic tools for understanding the interactively behavior of different variables. Although attractively simple, the traditional linear model often fails in these situations: in real life effects are generally nonlinear wit...
Conference Paper
Wavelets have proven to be powerful bases for use in numerical analysis and signal processing. Their power lies in the fact that they only require a small number of co-efficients to represent general functions and large data sets accurately. This allows compression and efficient computations. In this paper we tried to have the spherical wavelet rec...
Conference Paper
In this paper we will illustrate the result of our attempt to construct a set of orthogonal functions over the Persian Gulf and Oman Sea (24 • ≤ φ ≤ 27 • , 51 • ≤ λ ≤ 62 •) and compute Sea Surface Topography (SST) for this area. For this reason three methods are used: Spherical Helmholtz equation, Shwarz-Christoffel Conformal Mapping and Gram-Schmi...
Article
One of the major error sources in the global positioning system (GPS) is ionospheric delay (refraction). This delay is frequency dependent and therefore can be removed by dual frequency receivers. Using eight coefficients transmitted as part of the navigation message a correction can be provided for single frequency users called the Klobuchar model...
Article
It is possible to use single frequency GPS receivers to estimate the Total Electron Content (TEC). In this research, we improved an algorithm presented by Giffard [2], that is based on a least squares solution. We investigated the effect of the use of different weights (elevation of satellites, signal to noise ratio, combination of elevation and si...
Article
Variance components estimation is a method that compute suitable weights for observations. The types of observations can be different (distance, direction, angle). Investigations show that estimations have a meaningful relation with systematic errors or blunders. In other words, if systematic errors exceed some values -that these values can be calc...

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