06 Fakultät Luft- und Raumfahrttechnik und Geodäsie
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/7
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Item Open Access Setting-up of GPS reference stations and investigating the effects of antenna radome(2003) Ogonda, Godfrey OnyangoWith the shutting down of Selective Availability (S/A) in year 2000 , accuracies as better as 5 to 9 metres in the horizontal and 8 to 9 metres in the vertical have been guaranteed in GPS positioning using code measurements in Single Point Positioning (SPP) mode. Although the accuracies attainable through unassisted SPP are sufficient for most applications such as navigation, GIS and recreation, a whole range of experience has shown that millimetre accuracies can be attained through performance of GPS surveys using enhanced satellite systems, improved equipment and streamlined field procedures. Such concepts include the tendency to use reference stations (i.e. relative Positioning) to generate DGPS corrections and maintenance of reference frames. The precisions and accuracies at which the reference stations are established and monitored are very high. All the possible sources of error to which the antennas and receivers at the site are susceptible to, must be identified and minimised or eliminated. This include Phase Centre Variation (PCV) and multipath. To protect the antennas from bad weather and vandalism, reference station antennas are usually covered. The PCV patterns are further complicated from the fact that addition of antenna covers (radomes) are known to have effects on the positions and the existence of several correction models. In this study, two reference stations were established and an investigation on the effect of conical radome on one of the reference stations was carried out. A baseline of about 5 metres was set-up on top of the building housing the Institute of Navigation on Breitscheid 2. At one end of the baseline was station 1, mounted with a choke ring antenna, and the other end station 2, mounted with a compact L1/L2 antenna. Twenty four hour GPS observations at a data rate of 2 seconds were carried out in six consecutive days. The antenna setting for every two days was the same. Part of the data files collected on day 1 was used to fix the positions of the two reference stations with respect to the SAPOS network. A further analysis was done with the six day data files to determine the effect of the radome and the radome mount plate on station 1. The solutions obtained show that the reference stations were successfully established and that the conical radome has a negligible effect of about 1.5 mm on the height component of station 1.Item Open Access Ein Beitrag zur Stützung eines Software GNSS Empfängers mit MEMS-Inertialsensoren(2016) Gäb, Michael; Kleusberg, Alfred (Prof. Dr.-Ing.)Die genaue Bestimmung von Position und Geschwindigkeit mit einem globalen Navigationssatellitensystem (engl. Global Navigation Satellite System) (GNSS) in Echtzeit bildet eine essentielle Grundlage für viele Anwendungen in der Navigation. Bei mobilen Navigationsanwendungen finden häufig GNSS-Messungen unter ungünstigen Beobachtungsbedingungen statt, so dass Fehler in den Ergebnissen auftreten können. In diesem Fall ist der GNSS-Empfänger in Bewegung, so dass die Empfangsumgebung sich ständig ändert. So können Objekte, wie z.B. Gebäude, Bäume, Tunnel usw. zu Reflektionen, Dämpfung bzw. Abschattung der GNSS-Signale führen. Eine hohe zusätzliche Dynamik bzgl. der beobachteten Dopplerfrequenzverschiebung erfährt das empfangene GNSS-Signal durch die Bewegung des GNSS-Empfängers. Das beeinflusst die Signalnachführung (Tracking) und verursacht Fehler bei der Positions- und Geschwindigkeitsbestimmung oder eine Bestimmung ist sogar unmöglich. Eine Verbesserung der Positions- und Geschwindigkeitsbestimmung wird durch die gleichzeitige Messung mit einem GNSS-Empfänger und einem inertialen Navigationssystem (engl. Inertial Navigation System) (INS) ermöglicht. Kostengünstige INS beinhalten heute Mikro-Elektro-Mechanische Systeme (engl. Micro Electro Mechanical Systems) (MEMS), die als MEMS-Inertialsensoren bezeichnet werden. Die gemeinsame Nutzung der Messdaten von GNSS und INS wird als GNSS/INS-Integration bezeichnet. Diese Arbeit befasst sich mit der Tiefen GNSS/INS-Integration, so dass mit der Information eines INSs in die Tracking Loop des GNSS-Empfängers eingegriffen wird, um Verbesserungen beim Tracking des Signals und der daraus bestimmten GNSS-Beobachtungen zu erzielen. Dieser Eingriff wird auch als Stützung (engl. aiding) bezeichnet. Die Stützung erfolgt über die Regelgrößen (Dopplerfrequenz und Codephase) in der Tracking Loop für den jeweiligen Kanal des GNSS-Empfängers. Dazu werden die Regelgrößen zunächst prädiziert, bevor diese beim Tracking des empfangenen GNSS-Signals berücksichtigt werden können. Für die Prädiktion müssen die Position und Geschwindigkeit des Satelliten und des Empfängers vorliegen. Außerdem müssen die Uhrenfehlerrate des Satelliten und des Empfängers bekannt sein. Aus diesen Größen kann die Dopplerfrequenz und daraus die Rate der Codephase berechnet werden. Die zeitliche Integration der Codephasenrate ergibt die aktuelle Codephase. Aus den Ephemeriden des GNSS-Satelliten kann die Position, die Geschwindigkeit und die Uhrenfehlerrate für einen zeitnahen beliebigen Zeitpunkt berechnet werden. Die Uhrenfehlerrate des Empfängers wird hierbei mittels einer Extrapolation vorausbestimmt. Mit den MEMS-Inertialsensoren kann die Position und Geschwindigkeit des Empfängers fortgeführt werden. Diese Fortführung erfolgt über eine Lose GNSS/INS-Integration mittels einer Strapdown-Rechnung und einem Kalman-Filter für 15 Fehlerzustände. Als Stützwerte für den Kalman-Filter dienen hier die Position und Geschwindigkeit des SGEs. Die Stützung sollte bei einem Signalabriss oder für die Stabilisierung des Trackings eingeschaltet werden. Die Stützungsmethode wird mit einem Software GNSS-Empfänger (SGE) und einer Inertiale Messeinheit (engl. inertial measurement unit) (IMU) aus MEMS-Inertialsensoren (MEMS-IMU) in der Landfahrzeugnavigation getestet und analysiert. Der SGE ist für die Signale des globalen Positionierungssystem (engl. Global Positioning System) (GPS) mit dem C/A-Code auf der L1-Frequenz entwickelt und bestimmt die Position und Geschwindigkeit mit einer Messrate von 1 kHz. Die MEMS-IMU beeinhaltet 6 Freiheitsgrade mit jeweils einem 3-achsigen MEMS-Beschleunigungs- und MEMSDrehratensensor. Die Messrate des MEMS-Beschleunigungssensors beträgt 1 kHz und die Messrate des MEMS-Drehratensensors beträgt 800 Hz. Mit diesem Messsystem ist es möglich, die Prädiktion der Regelgrößen für jede Millisekunde durchzuführen, so dass diese bei jedem Tracking-Durchlauf mit dem C/A-Code vorliegen. Aufgrund der hohen Messraten können nur wenige Minuten Messdaten erfasst werden und die Auswertung erfolgt in der Nachbearbeitung (engl. post-processing). Die Stützungsmethode wird durch Fahrten mit einem Messfahrzeug getestet. Dazu werden Messfahrten mit verschiedenen Fahrmanöver ausgeführt, um im empfangenen GPS-Signal unterschiedliche Raten in der Dopplerfrequenz zu erzeugen. Eine Fahrt unter Bäumen ermöglicht den Einfluss des zeitlichen Verlaufs im Signal-Rauschverhältnis (engl. signal-to-noise ratio) (S/N) des empfangenen GPS-Signals auf die Stützungsmethode zu untersuchen. Bei dieser Messfahrt treten auch immer wieder kurzzeitige Signalabrisse auf. Die in der vorliegenden Arbeit dokumentierten Messfahrten und Ergebnisse zeigen, dass die Stützung für einen SGE erfolgreich in der Landfahrzeugnavigation durchgeführt werden kann. Die prädizierten Regelgrößen werden für die verschiedenen Messfahrten zuverlässig berechnet und das Tracking kann damit stabilisiert und sogar bei einem Signalabriss vorgesteuert werden. Außerdem können Ausreißer direkt in den Beobachtungen minimiert werden, so dass Verbesserungen in der Positions- und Geschwindigkeitsbestimmung des SGEs erzielt werden können.Item Open Access Satelliten im Dienst der Geodäsie und Geoinformatik(2003) Kleusberg, AlfredAm 4. Oktober 1957 wurde vom Gebiet der damaligen UdSSR der erste künstliche Satellit der Erde in den Weltraum geschickt. Von den vielen Nachfolgemissionen der vergangenen 46 Jahre waren einige Satelliten speziell für Aufgaben der Geodäsie und Geoinformatik designiert; viele weitere, ursprünglich für andere Aufgaben in Betrieb genommene Satelliten erwiesen sich im Nachhinein als sehr gute Lieferanten von Daten zur Lösung geodätischer Probleme. Um diesen Beitrag der Satellitenmethoden im richtigen Licht zu sehen, ist es aufschlussreich, zuerst die Entwicklung der Geodäsie von ihren Ursprüngen bis zum Jahre 1957 nachzuvollziehen. Dies kann natürlich im Rahmen dieses Beitrags nur in Form von mehr oder weniger subjektiv herausgegriffenen Einzelereignissen der Geschichte erfolgen.Item Open Access Die Nutzung des GPS zur dreidimensionalen Ionosphärenmodellierung(2003) Dettmering, Denise; Kleusberg, Alfred (Prof. Dr.-Ing.)Die freien Elektronen in der Erdatmosphäre beeinflussen die Ausbreitung elektromagnetischer Signale. Für die Nutzung satellitengestützter Navigationssysteme (wie beispielsweise das Global Positioning System GPS), welche mit Frequenzen größer als 30 MHz arbeiten, wird der Einfluss der Ionosphäre auf die Radiosignale gemeinhin als Fehlerquelle für die genaue Positionierung auf der Erde betrachtet. Andererseits kann man den ionosphärischen Refraktionseffekt auch nutzen, um aus GPS-Messungen Modelle über die Verteilung der freien Elektronen in der Erdatmosphäre abzuleiten. In der vorliegenden Arbeit wird ein Algorithmus zur dreidimensionalen globalen Modellierung der freien Elektronen in der Ionosphäre vorgestellt, analysiert und bewertet. Er basiert auf Zweifrequenz-GPS-Messungen global verteilter Bodenstationen sowie von GPS-Empfängern auf erdnahen Satelliten (LEO). Der entwickelte Ansatz modelliert die horizontale Elektronenverteilung durch eine Kugelfunktionsentwicklung, die mit Chapman-Funktionen für die Vertikalprofile verknüpft wird. Um zu optimalen Ergebnissen zu gelangen, erfolgt die Schätzung der unbekannten Funktionskoeffizienten im Rahmen einer Ausgleichung nach kleinsten Quadraten bzw. sequentiell über ein statisches Kalman-Filter. Simulationen zeigen, dass bei optimaler Modellauflösung mit diesem Ansatz Genauigkeiten in den Ionosphärenverzögerungen erreicht werden können, die bis auf Zentimeterniveau den Beobachtungsunsicherheiten der eingesetzten Daten entsprechen. Das bedeutet für die Chapman-Parameter Genauigkeiten von ca. einem halben Kilometer, maximal 0.2 TECU Fehler im VTEC und Unsicherheiten kleiner als 0.05 ns für die Hardwarebias. Wegen fehlender LEO-Satelliten kann dieses Ergebnis allerdings nicht vollständig mit realen Daten verifiziert werden. Zur Zeit ist die Modellierungsgenauigkeit für Bodenstationsdaten deshalb auf ca. 0.2m begrenzt, so dass der Übergang von der zweidimensionalen auf die dreidimensionale Modellierung für Einfrequenz-GPS-Korrekturen in der momentanen Situation nur Vorteile bei hoher Ionosphärenaktivität und in Äquatornähe mit sich bringt.Item Open Access Stochastic modeling with robust Kalman filter for real-time kinematic GPS single-frequency positioning(2023) Wang, Rui; Becker, Doris; Hobiger, ThomasThe centimeter-level positioning accuracy of real-time kinematic (RTK) depends on correctly resolving integer carrier-phase ambiguities. To improve the success rate of ambiguity resolution and obtain reliable positioning results, an enhanced Kalman filtering procedure has been developed. Based on a posteriori residuals of measurements and state predictions, the measurement noise variance-covariance matrix for double-differenced measurements is adaptively estimated, rather than approximated by an empirical function which uses satellite elevation angle as input. Since, in real-world situations, unexpected outliers and carrier-phase outages can degrade the filter performance, a stochastic model based on robust Kalman filtering is proposed, for which the double-differenced measurement noise variance-covariance matrix is computed empirically with a modified version of the IGG (Institute of Geodesy and Geophysics) III method in order to detect and identify outliers. The performance of the proposed method is assessed by two tests, one with simulated data and one with real data. In addition, the performance of F-ratio and W-ratio tests as proxies for the success of ambiguity fixing is investigated. Experimental results reveal that the proposed method can improve the reliability and robustness of relative kinematic positioning for simulation scenarios as well as in a real urban test.Item Open Access Precise orbit determination of agile and non-agile satellites(2024) Gutsche, Kevin; Hobiger, Thomas (Prof. Dr. techn.)Precise orbit determination (POD) is key in fulfilling the scientific objectives of many Earth observation missions. For example, synthetic aperture radars (SARs) onboard low Earth orbit (LEO) satellites provide monitoring capabilities irrespective of lighting and weather conditions but are restricted in their field of view. Agile satellites, which can rapidly rotate around all three spacecraft axes, promise to overcome these limitations. The implications of a satellite’s agility on precise orbit determination have not yet been studied. However, their understanding and handling are crucial for the success of agile SAR missions. This thesis fills this gap by identifying the impact of agility on orbit determination, as well as devising and implementing ways to mitigate these effects. The novel software and its POD concept developed within this thesis are equally applicable to non-agile and agile satellites and are especially suited for extending to other or more numerous measurements in the future. The precise orbits of LEO satellites are most commonly determined from the abundant measurements collected by a global navigation satellite system (GNSS) receiver onboard the satellite. The newly developed C++ software Precise Orbit Determination for Complex and Agile Satellite Technology (PODCAST) pursues the reduced-dynamic orbit determination. This concept processes the GNSS observations combined with rigorous modeling of the satellite’s orbital dynamics. Additional accelerations are estimated to counteract remaining deficiencies in the dynamic models. Based on the measurements and models, the orbits are inferred using a sequential estimator, the extended Kalman filter (EKF). In contrast to common practice, this choice promises lower computational complexity and increased flexibility to incorporate more estimated parameters or measurements. While additional estimated variables can arise from the system’s agility, an increase in the observations is anticipated as the software is extended to new measurement types or multi-frequency multi-GNSS signals. The work offers an in-depth explanation of the employed methods, their rationale, and the new developments required by the EKF approach. A comprehensive study utilizing simulated and in-orbit measurements shows the influence of specific design considerations in POD based on the EKF. Certain shortcomings of this method are entirely circumvented by adhering to specific design criteria. The application to the LEO satellites Sentinel-3A, Sentinel-6A, and GRACE-FO further evidence the viability of using a sequential estimator. The derived orbits are competitive with other solutions derived by the more computationally demanding batch least-squares method. The impact of agility on GNSS-based reduced-dynamic orbit determination is assessed using simulations with realistic mission profiles. The rapid attitude changes of the satellite are shown to lead to a deterioration in the GNSS observables and increase the reliance on the dynamic model. Moreover, an analysis using GRACE-FO as a surrogate agile satellite suggests that the employed POD strategy effectively alleviates these changes, as well as the attitude-related errors in the non-gravitational force models. The results testify to the remarkable resilience of reduced-dynamic POD to measurement gaps caused by attitude maneuvers or jamming. Finally, errors in the lever arm - the vector between the GNSS antenna phase center and satellite center of mass - are shown to cause increased adverse effects for agile spacecraft. Furthermore, established techniques employed for satellites with slowly varying attitudes can only partially compensate for the negative impact on agile satellites. At the same time, using an observability analysis, Monte Carlo simulations, as well as in-orbit data of Sentinel-6A, this work proves the ability to directly estimate the lever arm in the presence of rapid attitude changes. Hence, this property enables dedicated attitude maneuvers to calibrate the lever arm for both agile and non-agile missions, thereby mitigating the negative effects on POD. Ultimately, this work presents an alternative precise orbit determination approach that is effective for both non-agile and agile missions. Compared to existing solutions, the derived orbits for non-agile satellites are consistent by approximately one centimeter. Applying the approach to in-orbit data of future agile satellites, which is not yet available, will enhance its verification and validation. Furthermore, the POD concept is well-suited to cope with upcoming challenges, such as an increasing number of measurements or parameters, and the fusion of various types of observations.Item Open Access Instantaneous estimation of attitude from GNSS(2017) Suhandri, Hendy Fitrian; Kleusberg, Alfred (Prof. Dr.-Ing.)The use of the Global Navigation Satellite System (GNSS) is widely spread from position determination to attitude determination of a platform in space. This system offers time invariant estimation position. Another thing that can be an advantage is that the flexibility to operate the GNSS receiver variants, from the low-cost until the high-performance GNSS receivers. In terms of attitude determination application at least three receivers are required to determine three spatial axes, where the cost-effective GNSS attitude determination systems can be constructed with today’s receiver technology. At the moment, however, algorithms are lacking which are fast and efficient enough to estimate the position angles without delay. For this reason, the present work deals with the development of algorithms for the attitude determination in space of a platform under the help of the "GNSS" Global Positioning System (GPS). The investigation through this work is classified into three sequential parts: The first part is the estimation of the optimal configuration of baseline array as well as the estimation of the integer ambiguity of carrier phase differences. The estimated integer ambiguity is then used to estimate the high precision baseline coordinates. The second part is to estimate the attitude of the platform in space by means of quaternion using batch process, and the last part is to improve the algorithm using a recursive algorithm for the kinematic application purpose. The precise attitude determination about three spatial axes is possible if at least three GNSS receivers with fixed baselines are used in particular array configurations. Assuming that the basic lengths of the baselines are known apriori, the attitude angles can be calculated via the combination of carrier phase and pseudorange observations. Since the carrier of the GPS signal is propagated in short-wave form, the measured phase differences are ambiguous. The multiples of the GPS signal phases together with the baseline lengths are therefore estimated and improved in a first step with the aid of the a priori baseline lengths information. The multiple-baseline float solution estimation method is used. However, the approach does not provide optimal results. Therefore, an alternative algorithm for the float solution is presented, which estimates the float solution by using the so-called the gradient based iterative method of the least-squares. It shows that method is able to give convergent estimate parameter. It is also shown here that the proposed method outperforms the conventional iterative least-squares in terms of iteration number and computational time. For instantaneous applications, the Least-squares AMBiguity Decorrelation Adjustment (LAMBDA) method is not optimal for fixing the integer multiples of the carrier phase differences for several baseline lengths. In addition, this method requires a high computational effort as soon as a larger number of baseline lines enter into the calculation. An improvement in this work is utilising the partial LAMBDA method, which only uses a subset of the integer multiples to be determined. This algorithm improves the determination of integer multiples and precise calculation of the baseline lengths. The advantages of this algorithm are discussed, and it is empirically demonstrated that the ambiguities are better resolved. Furthermore, the estimation of the attitude angles with the aid of quaternions is theoretically improved and analysed. Two processing strategies are investigated: the least-squares method and the Kalman Filter (KF) method. For the static case, the least-squares is applied and tested. Simulations show that the developed gradient based iterative method of the least-squares provides better estimates than the conventional adjustment methods. It is also shown that the number of iterations required is less and the computational time is reduced. This algorithm is not useful for kinematic applications where a fast sequence of results is required. A modified Extended Kalman Filter (EKF)-Like algorithm is used for kinematic applications. Experiments show that with this algorithm more stable quaternions can be calculated with fewer outliers than when they are determined by the least-squares method. All newly developed algorithms are theoretically analysed and subjected to extensive simulations and experimental kinematic tests in the field.Item Open Access Improving GNSS meteorology by fusing measurements of several colocated receivers on the observation level(2024) Wang, Rui; Marut, Grzegorz; Hadas, Tomasz; Hobiger, ThomasItem Open Access Feasibility of using low-cost dual-frequency GNSS receivers for land surveying(2021) Wielgocka, Natalia; Hadas, Tomasz; Kaczmarek, Adrian; Marut, GrzegorzGlobal Navigation Satellite Systems (GNSS) have revolutionized land surveying, by determining position coordinates with centimeter-level accuracy in real-time or up to sub-millimeter accuracy in post-processing solutions. Although low-cost single-frequency receivers do not meet the accuracy requirements of many surveying applications, multi-frequency hardware is expected to overcome the major issues. Therefore, this paper is aimed at investigating the performance of a u-blox ZED-F9P receiver, connected to a u-blox ANN-MB-00-00 antenna, during multiple field experiments. Satisfactory signal acquisition was noticed but it resulted as >7 dB Hz weaker than with a geodetic-grade receiver, especially for low-elevation mask signals. In the static mode, the ambiguity fixing rate reaches 80%, and a horizontal accuracy of few centimeters was achieved during an hour-long session. Similar accuracy was achieved with the Precise Point Positioning (PPP) if a session is extended to at least 2.5 h. Real-Time Kinematic (RTK) and Network RTK measurements achieved a horizontal accuracy better than 5 cm and a sub-decimeter vertical accuracy. If a base station constituted by a low-cost receiver is used, the horizontal accuracy degrades by a factor of two and such a setup may lead to an inaccurate height determination under dynamic surveying conditions, e.g., rotating antenna of the mobile receiver.Item Open Access Considering different recent advancements in GNSS on real-time zenith troposphere estimates(2020) Hadas, Tomasz; Hobiger, Thomas; Hordyniec, PawelGlobal navigation satellite system (GNSS) remote sensing of the troposphere, called GNSS meteorology, is already a well-established tool in post-processing applications. Real-time GNSS meteorology has been possible since 2013, when the International GNSS Service (IGS) established its real-time service. The reported accuracy of the real-time zenith total delay (ZTD) has not improved significantly over time and usually remains at the level of 5-18 mm, depending on the station and test period studied. Millimeter-level improvements are noticed due to GPS ambiguity resolution, gradient estimation, or multi-GNSS processing. However, neither are these achievements combined in a single processing strategy, nor is the impact of other processing parameters on ZTD accuracy analyzed. Therefore, we discuss these shortcomings in detail and present a comprehensive analysis of the sensitivity of real-time ZTD on processing parameters. First, we identify a so-called common strategy, which combines processing parameters that are identified to be the most popular among published papers on the topic. We question the popular elevation-dependent weighting function and introduce an alternative one. We investigate the impact of selected processing parameters, i.e., PPP functional model, GNSS selection and combination, inter-system weighting, elevation-dependent weighting function, and gradient estimation. We define an advanced strategy dedicated to real-time GNSS meteorology, which is superior to the common one. The a posteriori error of estimated ZTD is reduced by 41%. The accuracy of ZTD estimates with the proposed strategy is improved by 17% with respect to the IGS final products and varies over stations from 5.4 to 10.1 mm. Finally, we confirm the latitude dependency of ZTD accuracy, but also detect its seasonality.