06 Fakultät Luft- und Raumfahrttechnik und Geodäsie

Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/7

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Open Access
Analysis of coordinate transformation with different polynomial models
(2017) Gao, Yueqing
The main task of geodesy is providing geodetic networks with fixed points in order to create a uniform geographical spatial reference frame as a fundament for the data collection by the official geodesy survey institutes. A german geodesy survey institute called AdV (Arbeitsgemeinschaft der Vermessungsverwaltungen der Länder der Bundesrepublik Deutschland) declared in 1991 that the ETRS89 datum should be introduced in Germany as a reference system. In order to transform the already exciting coordinate informations in the Gauß-Krüger coordinate system into the later introduced UTM coordinate system, different transformation models have been developed and discussed. Besides the most commonly used 7-parameter Helmert transformation and 6-parameter affine transformation models, polynomial transformation models can also be applied. A method for improving the transformation results of a polynomial model will be discussed, with which a significance test (T-test) for each parameter will be done and the polynomial terms with lower significance to the model will be eliminated in order to get the optimal polynomial model. Here different transformation models are reviewed and the transformation results based on these models with the Least Squares estimation method are compared and analysed.
Open Access
Analysis of helicopter-borne gravity gradiometry
(2017) Xia, Zhuge
Nowadays the FALCON partial tensor gravity gradiometer is the only applied AGG instrument to be used in a helicopter to collect high-resolution data for commercial demand. In order to know its principles and make full use of its advantages, in this study thesis, a helicopterborne gravity gradiometry data set offered by USGS is processed and analysed. It is interesting to figure out that whether the gradient tensor invariants are truly "invariant". Also through Fourier transformation, all gradient tensor components can be determined mathematically. Besides, the effects of terrain corrections and different flight line spacings and drape heights are also concerned to obtain high-resolution data in the easiest way.
Open Access
Analysis of river surface slope using ICESat-2 satellite altimetry
(2022) Bao, Jingyi
Open Access
Analyzing the spatio-temporal behavior of Poyang Lake using Google Earth Engine
(2020) Schneider, Nicholas M.
Poyang Lake, China’s largest freshwater lake, undergoes a yearly repeating cycle of drastic inundation and subsequent considerable shrinkage. Essentially, as a result of local precipitation and feedings from its tributaries, as well as a natural water exchange with the Yangtze River, Poyang Lake experiences such dimensional fluctuations on a annual and interannual scale. The ongoing change plays a significant role for the surrounding anthropogenic activity and wildlife. Despite being considered as a hydrological phenomenon, the dynamics of this Chinese water body set up a hurdle for any accurate documentation of its regime and therefore remains insufficiently studied upon to this day. Further impeding the comprehension of Poyang Lake’s behavior is the near inaccessibility and nonexistence of in situ data, such as water level measurements and bathymetric maps. Consequently, this study, driven by its aim to analyze the spatio-temporal behavior of Poyang Lake, focuses solely on satellite observations. Making use of the cloud computing platform, Google Earth Engine, image time series are used from Landsat-8 and Sentinel-1 datasets in order to map Poyang Lake’s spatio-temporal behavior on an annual and interannual scale. Produced from the Landsat-8 dataset, results show that only under circumstances, do techniques, such as the combination of visible and infrared bandsand the calculation of the Normalized Difference Water Index, provide a reasonable approach for the delineation of continental water bodies. For the study on Poyang Lake, a water body subject to humid climate and thus frequent cloud coverage, these techniques do not apply very well. With synthetic aperture radar observations from the Sentinel-1 dataset, dynamic water masks, involving the removal of certain elevated areas and the classification of water from thresholding, could be generated. The resulting binary water masks are then merged with a digital terrain model to create monthly maps of the study area. The results show an evident correlation between this study’s visual and numeric findings. Although the results are nearly impossible to compare with any in situ data, they show a trend that annually occurs in Poyang Lake’s hydrological regime. In particular, they reveal the cycle of drastic inundation in rainy summer months and considerable shrinkage in dry winter months, especially when examining the years prior to 2019. Using SAR imagery for continental water body delineation, particularly in humid climates, proved to be a suitable technique and should be considered for future documentations of the lake.
Open Access
Analyzing the surface height of Nam Co by using CryoTrack
(2018) Liu, Mo
In recent decades, the global climate change is increasingly accelerating and intensifying. The plateau climate is an important part of the global climate. In this thesis, we will take the plateau lake Nam Co as an object to research and analyze the climatic variation in this area. Since Nam Co is located in Tibet Plateau with a high elevation and it doesn’t have too much human disturbance, it is an ideal study object to observe. In this thesis, we will use the data from the satellite CryoSat-2 and the software CryoTrack to obtain the surface height of Nam Co as well as the trend of the water level variation of it. Meanwhile, by using CryoTrack we can also observe the frozen and breakup time of the lake ice.
Open Access
Assessing the performance of different classification methods to detect inland surface water extent
(2015) Walton, Alexander
In recent decades, political as well as environmental conflicts about the Earth’s water resources became a significant issue with constantly growing importance all over the world. These issues include flooding as well as drying shrinkage of seas and rivers. In order to estimate the dimension of these impacts, reliable frequent observation of the surface water over a long period of time is essential. By making use of the special spectral reflectance properties of water, especially in higher spectral ranges, it is possible to distinguish between water and other surface materials and create thematic maps using different classification methods. These methods can be based on supervised algorithms which make use of training data to classify an image. On the other hand they can be automatized computing algorithms which assign pixels to a class without any prior knowledge. The latter are referred as unsupervised methods. Assessing the performance of these various methods will be task of this thesis. Here in this study four satellite images of Landsat 7 are selected for our case study. The images show the region around the Po River in northern Italy. The valley alongside the river is one of the strongest economic and agricultural region in Italy but it also suffers from regular flooding, especially in the delta region around Ferrara close to the Adriatic Sea. The different classification methods are implemented in the software ENVI which is commonly used by remote sensing professionals to process and analyse geospatial imagery. To make a statement about the accuracy of the single classification methods every classifier undergoes a few essential steps: A binary water mask is created in which the river width is measured at two selected gauge spots. At these spots precise in-field measurements were applied regularly in recent years and the results serve as reference values for a comparison in between the classification methods. In general the supervised methods performed better than the unsupervised methods. The best performance is approached by the mahalanobis distance classification which is based on probability statistics with consistent covariances. This way the pixels can be classified by calculating their minimum euclidean distance in spectral space. The results of this study are limited by the low resolution of 30m which does not leave vast room for interpretation. More reliable results could be achieved by measuring effective widths alongside the river and apply an area-wise comparison. However approximate estimates about the performance of the different classification methods and good first impressions of the advantages and disadvantages are given.
Open Access
Comparison between gravity gradients from dense CryoSat-2 altimetry and from shipborne gradiometry
(2017) Buss, Roman
Satellite altimetry is a tool to gather information about Earth’s oceans and ice coverage on a global extent. Altimetric data can be used to monitor ice sheet changes, create bathymetric maps or analyze the sea surface. Extensive comprehension of the vast masses of water on Earth will benefit not only geodetic but also various scientific aspects. This paper will deal with altimetric data collected by ESA’s satellite CryoSat-2. The satellite is designed to perform measurements of both ocean surface and ice sheets. Vertical gravity gradients will be calculated on the basis of sea surface height measurement. The area of interest is located between Faroe Islands and Shetland Islands north of Scotland. The created vertical gravity gradients will be compared with a dataset collected by Bell Geospace via shipborne gradiometry. Satellite altimetry enables a rather convenient acquisition of global data compared to traditional gradiometric methods. The main objective is to find out whether satellite altimetry is a viable alternative to generate gravity gradient data on a global extent.
Open Access
Create a fluid-structure simulation framework for cycloidal rotors
(2021) Schließus, Julian
The deformation of cyclorotor blades caused by aerodynamic forces and its effect on the rotor efficiency is not well understood. Therefore the aim of this thesis is to set up coupled Fluid Structure Interaction (FSI) simulations of cycloidal rotors. Furthermore, the results of this work shall create a base for prospective refinements of current cyclorotor CFD models in using a coupled simulation. For the FSI setup, the Computational Fluid Dynamics (CFD) simulation software OpenFOAM and the Multibody Dynamics (MBD) simulation software MBDyn are coupled using the coupling library preCICE. During this thesis, MBDyn models are set up and the advantages and limits of the currently existing preCICE OpenFOAM- and MBDyn-Adapter are discussed. To extend the coupling possibilities into the territory of rotating bodies, the creation of a custom OpenFOAM class is explained in this work. In addition, the influence of the cell displacement tolerance on mesh deformation is investigated and issues in OpenFOAM such as unwanted scaling or nonphysical motions are solved. Further, 2D and 3D setup cases are run and tested successfully on a High Performance Computing (HPC) cluster. Finally, certain coupling methods are reviewed and evaluated for the considered case.
Open Access
Create an automated structured mesh generation method for rotor blades using exclusively open source software
(2023) Heider, Michael Andreas
The generation of high-quality CFD meshed wings for wind energy turbines currently requires either a very laborious process of building each wing by hand or requires the purchase of very expensive software. With university budgets and the advantages of open-source in mind, an open-source solution is desirable. The following thesis tackles this problem via a grid generation code in Python with Gmsh and extrusion via Pyhyp. However, designing software that has all of the features of commercial software with open source software leads to situations in which the intended way of usage is not the best way for the given task. An example of another open-source approach is the MACH-AERO framework which is an aerodynamic shape optimization tool that creates a grid along the way and extrudes it. However, this does not cover the intended usage of openblademesh for purely structured grid generation for predefined wings. Here we show one way to achieve an open-source solution with Gmsh and Pyhyp. All the necessary software, the versions needed, and installation instructions are described. Also how the wing generation process is achieved with special concentration on the wing-tip is part of the following thesis. The thesis also describes how to use the presented software Openblademesh and how to achieve the best results. The tool uses various automatisms to take the workload off of the user and helps to achieve the best mesh. Finally, the advantages and limitations of the tool are assessed and future extensions are described.
Open Access
Cross-over analysis of altimetry over ocean and investigating the orbital error’s effect on inter-mission/track bias in inland altimetry
(2024) Kappich, Oliver
The largest part of Earth’s surface (approximately 71%) is covered with water. Given the constantly changing environment, particularly amidst accelerated climate change, it is crucial to continuously measure the water levels of oceans and lakes. Therefore, satellite altimetry becomes essential. The orbits of the altimetry satellites are selected in a way that allows satellites to pass over the same locations after a specific interval. These orbits are termed as repeat orbits, facilitating the creation of time series measurements. Over the past 40 years, numerous altimetry satellite missions have been launched. When multi-mission monitoring of water bodies is targeted, each satellite altimeter possesses its own biases, which should be removed for comparability among different missions. This ensures the creation of long-term data records by combining data from various missions. Over open oceans, this is typically achieved through a cross-calibration method. However, these methods prove effective for ocean data but not for inland altimetry. In this thesis, I investigated the reasons for the bias among water values measured by different satellites. Additionally, I explored potential solutions to merge the data. The main focus lays on the tandem phases of Jason 1 and 2, as well as Jason 2 and 3. The study area focused on Lake Erie, situated in the Great Lakes region in the northwest of the US. To reduce the bias, I employed a cross-calibration method to estimate and reduce radial error components. As this approach does not resolve the entire bias problem, I investigated the retracking algorithms by comparing their results. Differences between height measurements of Jason 2 and Jason 3, both using MLE4, were identified. It could be determined that MLE4 in Jason 3 finds systematically lower values compared to Jason 2. Over the whole tandem phase, Jason 3 finds the retracking point approximately 20%, in respect to the leading edge, lower than Jason 2. The influence of this systematic difference on the SSH/LLH remains unclear, as no further investigations are done. To get a better understanding if the bias can be reduced when the mid-height point is used, two simple threshold retracking algorithms are employed. The outcome is, that the difference between Jason 2 and Jason 3 increased to 18.3 cm on average. Lastly, I examined the corrections that need to be added to the range measurement of the satellite. This includes the geoid undulation, tidal height variations, the ocean surface response caused by atmospheric pressure and propagation delay due to the atmosphere. I found differences of 5 to 8 cm over Lake Erie in the atmosphere corrections. Employing the same corrections for two satellites yielded the most effective bias reduction. However, employing this technique, necessitates satellites passing the same location within a few minutes of each other. Consequently, the Jason satellites were selected during their tandem phases. On average the bias could be reduced from 7 cm to 2.4 cm. The study delved into understanding and reducing biases in satellite altimetry measurements, particularly focusing on the tandem phases of Jason satellites, revealing challenges and promising methods to significantly reduce biases.
Open Access
CryoSat-2 for hydrological purposes : data processing, visualization and analysis
(2013) Mayer, Volker
CryoSat-2, a remote sensing satellite of ESA, which is originally designed for the monitoring of sea and land ice surfaces, provides global radar altimetry data, that can also be used for other areas of application. This thesis will analyze its capability for hydrological studies of rivers, more precisely of their water level, extent and slope. For this purpose a GUI based on MATLAB, called CryoTrack, has been developed, which allows the operator to comprehend the tracks of the CryoSat-2 satellite on a global grid and to access their measurements for selected area and date. This data was used to determine water extent and slope of rivers, by combining the information of several radar altimetry quantities. Analyzing several intersections between the satellite tracks and the Niger River, algorithms for the designation of river width and slope were derived and tested for their capabilities and limitations. The transitions between water and land surfaces can be detected for wide rivers and their distance allows an estimation the river width. The moderate accuracy of this water extent calculation stands in contrast to the very good results of the slope computation between two or more intersections in the second part of the analysis.
Open Access
Dealing with challenges of altimetry-based surface water height derivation over boreal catchments : case study of Mackenzie river
(2022) Liu, Jiaxin
The Earth is a watery place, which fills the oceans, rivers, and lakes. Approximately 71% of the Earth’s surface is water-covered. Measurements of surface water level in oceans, lakes, rivers and coastal waters are important for a variety of reasons. In the short term, this can, for example, help to alert to dangerous water level so that actions can be taken in advance, while in the long term, monitoring water levels can provide even greater insight into patterns of water dispersal in the area, and measure, for example, the effects of global warming. Satellite altimetry, which was originally designed for oceanography in the 1970s, has revolutionized our knowledge of the marine gravity field, of the dynamics of the oceans and even ofland hydrology. It is a space measurement technique that uses artificial satellites to measure the altitude from the satellite to the Earth’s surface. Due to its high resolution, global coverage and short revisit time, it is playing an increasingly important role in measurements of water level. For some years, this technology has also been used to retrieve water levels from rivers, lakes, and any inland water body as well. However, compared with the wide seas, measurements of inland water bodies involve many challenges. In this paper, we will take the Mackenzie River in northern Canada as a research object and process the data through the Matlab-based program Atlbundle+ to study the potential of inland altimetry, the problems it faces and the possible ways to find the solution. In general, there are two perspectives that will be considered: Firstly, how we can accurately remove outliers in the measurements, and secondly, how to improve the altimetry-driven water level time series by improving the retracking methods. Finally, based on the results and the procedure, a systematic analysis of the inland altimetry can be carried out.
Open Access
Digitalisierung und Bereitstellung von historischen Luftbildern des Landesamts für Geoinformation und Landentwicklung Baden-Württemberg (LGL)
(2017) Wein, Isabel
Die Digitalisierung und Bereitstellung von historischen Luftbildern des Landesamts für Geoin-formation und Landentwicklung Baden-Württemberg (LGL) soll im Hinblick auf die dauerhafte Sicherung der wertvollen Luftbilder und die bessere Verfügbarkeit für die unterschiedlichen Kundengruppen vorangetrieben werden. Hierzu ist ein Konzept zu entwickeln. Im Rahmen des Digitalisierungsprozesses werden neben dem eigentlichen Scannen und Georeferenzieren in einer Vorbereitungsphase die Originalbilder aufbereitet. In einer Nachbereitungsphase werden Qualitätssicherungsmaßnahmen geprüft. Schließlich wird der Umfang des Digitalisierungsprojektes hinsichtlich Personal-, Sach- und Zeitaufwand eingeschätzt. Neben der nachhaltigen Sicherung dieser Luftbilder durch Digitalisierung soll durch eine standardisierte Bereitstellung der digitalen Luftbilder über das Internet, z. B. Geodatendienste, dem wachsenden Bedarf der Nutzer Rechnung getragen werden. Hierfür werden Speicherbedarf, Metadaten und Nutzungsrechte untersucht. Die vorhandenen Luftbildinformationssysteme in den Bundesländern Thüringen, Bayern, Hessen, Rheinland-Pfalz und Niedersachsen sowie in den Nachbarländern Schweiz und Österreich werden verglichen. Auffällig dabei ist der unterschiedliche Stand der Digitalisierung.
Open Access
Estimation of atmospheric signals from daily gravity field solution
(2017) Mattes, Dennis Frederic
In this thesis the question will be solved, if it is possible to extract gravity signals from the atmosphere in the measured gravity field of the earth. At the beginning, this will be done by comparing the Daily Wiese Solutions, which are simulated measured gravity fields of the earth and the atmosphere, to have an idea in which way the data is created and how the basic data have already similarities. In the following analysis, two possibilities are tested to estimate the atmosphere. The analysis is based on the idea that the estimation of the atmosphere proves the atmospheric signals in the gravity field of the earth. One possibility is to use masks to extract the desired signals, while the other way is to add the differences of two daily solutions to a start field. There it can be seen, that the first method offers the best correlation with the given atmosphere. The latter method has a lower correlation but has the advantage to require a lower number of atmospheric models. Also the requirement of low pass filters in further atmospheric estimations is shown. Two anomalies which are detected are intensity peaks in the estimated atmosphere, which could be related to extreme weather at that time and also the big variation between points in the upper part of the field and in the lower part.
Open Access
Estimation of significant wave height using Sentinel-3 data
(2020) Gou, Junyang
Coastal area is one of the most important area for us. More than 600 million people (around 10% of the word’s population) live in coastal areas that are less than 10m above sea level. Nearly 2.4 billion people (about 40% of the world’s population) live within 100km of the coast. Therefore, monitoring of coastal waters is extremely important. Due to the limitation of the number and location, the tide gauge stations around the world cannot provide a sufficient amount of in-situ data. Therefore, satellite altimetry plays an increasingly important role, especially when the SAR altimeter is put into use. However, due to the complexity of the coastal water surfaces, the performance of the satellite altimeter over the coastal area is far worse than over ocean. This thesis is dedicated to developing a method to determine one of the essential characters of the water surfaces - the significant wave height (SWH), using the Sentinel-3 data in the coastal area. The three primary steps of the method are extracting the thermal noise and the leading edge, fitting this part of waveform and determining the relationship between the new retracker and the physical model. In the first step, an algorithm is developed to avoid the interferences of the noise on the trailing edge. Therefore, the peak of the leading edge could be determined more accurately. The condition for the start point of the leading edge of the PLRM waveforms is Dwf > 0.01, inherited from ALES, whereas a more appropriate threshold for the SAR waveforms has been found as Dwf > 0.03. In the second step, the limitation of the Gauss-Markov model for the waveform adjustment has been discussed. Thus, the Levenberg-Marquardt method has been chosen to adjust the waveform. In the third step, the relationship between the raising time and the beta4 has been found. Then, we could estimate the SWH directly from the raising time which makes it possible to estimate the SWH from some complicated waveforms in coastal areas. We have employed the developed methodology to determine the significant wave height in the coastal area near the Cuxhaven. The quality of the results has been proved by comparing with the in-situ data from the Elbe measuring station provided by the Federal Maritime and Hydrographic Agency of Germany. The validation showed that the proposed method can determine reliable SWH from approximately 1km offshore, which is an improvement of earlier results.
Open Access
Experimental validation of a cycloidal rotor URANS CFD model and geometric parameter optimization
(2022) Donners, Leonard
Cycloidal rotors offer many properties that are beneficial to VTOL rotor concepts. Two examples are high manoeuvrability and low aerodynamic noise emission. These rotors use cyclically pitched axial blades to generate thrust and operate in highly unsteady flow regimes, including curvilinear flow and dynamic stall phenomena. The current work develops an unsteady, incompressible 3D URANS CFD simulation setup that models a given small-scale experimental rotor. Measurements and simulation are conducted at a relatively low and slim Reynolds number range of 12x10^3 ≤ Re ≤ 76x10^3. By comparison of simulation results and experimental data, the CFD setup is validated. The validated setup is used to investigate flow phenomena on the initial rotor configuration and further conduct multiple separate parametric studies to optimise the rotor's energy efficiency. The investigated parameters are the blades' pitch profile, the number of blades, blade span and the addition of endplates at the blade tips. The highest efficiency increase of 14% compared to the experimental rotor is hereby found when adding endplates. The most efficient pitch profile results with a pitch rod length of l_opt=87 mm. The optimal blade span aspect ratio is 6 and a four-blade rotor is most efficient. Further research may aim at conducting this optimisation in a wider Reynolds number range or finding an optimum of multiple variable parameters combined.
Open Access
Investigation of the Reynolds Number on the performance of a cycloidal rotor
(2021) Zimmer, Florian
The Reynolds number has great influence on the provided lift and requested power of a cycloidal rotor. The aim of the present paper is to determine lift, power and efficiencyof a three bladed cycloidal rotor in relation to the Reynolds number. The investigated range of Reynolds numbers reaches from Re=10x10^3 to Re=600x10^3. For the parameter study, a cycloidal rotor under stationary hovering conditions, using a instationary, incompressible 2D URANS CFD simulation has been investigated. To interpret the rotation averaged data, field solutions are provided. Two different flow regimes can be distinguished: For the first regime (10x10^3≤Re≤100x10^3), two dynamic flow detachments occur, the first one at a maximum pitch angle of Ψ=270° and the second shortly afterwards. The second flow regime (200x10^3≤Re≤600x10^3) avoids the first detachment due to the increased Reynolds number. Both detachments are perceptible in the flow field solution as well as in the azimuthal lift distribution. The rotation averaged lift and power follow the proportionalities obtained by momentum theory.
Unknown
Konstellationsentwürfe und Bewertungen einer VLEO Schwerefeldmission
(2024) Edelmann, Christian
Unknown
Least-squares prediction of runoff
(2013) Thor, Robin
When modelling hydrological cycles, the runoff of a drainage basin is an important variable, being an output from hydrological models and an input to many hydrological interactions as a quantity used for validation and calibration. In this context, the decrease of the availability of in situ runoff measurements that has been observed over the last years poses a challenge which this study aims to tackle with the methods of least-squares prediction. This research uses the spatial correlations between in situ runoff measurements generated in a training period to predict values for a validation period during which one of the catchments is assumed ungauged. Different methods include the usage of covariance matrices, which are formed 1. on the signal level, or 2. separately for each of the 12 months of the year, or 3. after the reduction of the monthly mean, or 4. after the reduction of the long-term mean for prediction purposes. For validation, the Nash-Sutcliffe model efficiency coefficient, correlation, and RMSE are computed. The impacts of variations in the length of the training period and of the choice of catchments whose observed measurements are used in the prediction process are analysed. The errors then undergo a spectral analysis to test, which prediction methods are able to capture cyclostationary behaviour best. Most of the methods provide viable results, although the prediction based on covariance matrices generated out of residuals is slightly better than the other methods in a vast majority of configurations. After a training period of 20 years of simultaneous data and with a selection of three catchments used in each prediction process, this method can reach a Nash-Sutcliffe coefficient of over 0.4 for about 90% and of over 0.75 for about 50% of the 25 analysed catchments, although viable results can already be achieved with much shorter training periods of one to three years, depending on the predicted catchment.
Open Access
Noise characterization of GPS time series from the second IGS reprocessing campaign
(2015) Wang, Ganyi
By late 2013, the Analysis Centers (ACs) of the International Global Navigation Satellite System Service (IGS) begun the second reanalysis of the full history of Global Positioning System (GPS) data collected by the IGS global network since 1994 in a fully consistent way using the latest models and methodology. In this thesis, GPS time series of selected sites from different analysis centers were analysed with the software CATS. The purpose of this thesis is to discuss noise content using maximum likelihood estimation (MLE) and determine a noise model that can best describe the noise process in GPS time series of 20 years or more. The noise was assumed to be a combination of white noise and power-law noise. In the first part, we used fixed noise models with integer spectral indices for the MLE. The three noise models are: white noise only, white noise plus flicker noise (ƙ = -1) and white noise plus random walk noise (ƙ = -2). The estimated MLE values of different noise models were compared to determine the preferred noise model. Another noise parameter, amplitude, was also estimated. Latitude dependence of the noise amplitudes and the influence of data cleaning on the noise analysis were discussed. In the second part, the noise model was again assumed to be white noise plus power-law noise, but the spectral index of the power-law noise component was not fixed and was also estimated. The estimated spectral indices were compared with the results of the first part, in order to acquire a more precise description of the realistic noise components. At last, a comparison between the noise features of GPS time series from different ACs was made. Besides the results of noise analysis, the importance and practical value of noise analysis was also discussed.