81 research outputs found
Precise Relative Positioning of Formation Flying Spacecraft using GPS
No full textSpacecraft formation flying is considered as a key technology for advanced space missions. Compared to large individual spacecraft, the distribution of sensor systems amongst multiple platforms offers improved flexibility, shorter times to mission, and the prospect of being more cost effective. Besides these advantages, satellite formations in low Earth orbit provide advanced science opportunities, such as measuring small scale variations in the Earth's gravity field or higher resolution imagery and interferometry. One of the fundamental aspects of spacecraft formation flying missions is the precise determination of the relative state (position and velocity) between the satellite vehicles within the formation. GPS receivers are often considered as the primary instruments for this task in future spacecraft formation flying missions. As is commonly known, precise relative positioning between GPS receivers in geodetic networks is exercised on a routine basis. Furthermore, GPS receivers are already frequently used onboard satellites to perform all kinds of navigational tasks. Moreover they are suitable for real-time applications and provide measurements with a 3-dimensional nature. This dissertation presents a thorough overview of various GPS based strategies for precise relative spacecraft positioning, which have all been tested using real-world GPS data from the GRACE satellite mission. In addition, a substantial part of this work is also dedicated to quality aspects of the GPS observation data used as well as precise GPS based orbit determination strategies of single spacecraft. This dissertation is therefore recommended to all readers interested in GPS for spacecraft positioning applications.Aerospace Engineerin
Thermospheric density and wind determination from satellite dynamics
No full textAstrodynamics and Satellite MissionsAerospace Engineerin
Crustal rheology and postseismic deformation: Modeling and application to the Apinnenes
No full textAerospace Engineerin
Identification and Modeling of Sea Level Change Contributors: On GRACE satellite gravity data and their applications to climate monitoring
No full textSince early 2002, the Gravity Recovery and Climate Experiment (GRACE) twin satellite mission provide the scientific community with accurate monthly maps of the Earth's gravity field. On short time-scale, the observed variations are mainly related to a redistribution of water on the Earth's surface. This new data set has lead to a leap forward in our understanding of the various components of the Earth's water cycle, and their mutual interaction. The GRACE data provided by the science teams are contaminated by noise, hampering the interpretation of the observations. The first part of this thesis describes a statistical filtering method which removes the majority of the noise and allows utilization of the GRACE data at their full potential. Using the filtered observations, variations in the water budget of the climate system have been studied. A method was developed to obtain a picture of the mass balance of the Greenland ice sheet at a regional scale. From the research in this dissertations, it shows that Greenland lost about 200 cubic kilometers of ice each year on average between 2003 and 2008, causing a global mean rise of sea level by 0.5 mm/yr. An acceleration of the thinning of the ice sheet is observed, with a contribution of 0.75 mm/yr to global mean sea level rise in the last two years. Furthermore, the GRACE observations have been used to identify the seasonal exchange of roughly three thousand gigatons of water between land and ocean, and, in combination with sea level measurements from altimeter satellites, to constrain the cycle in heat content of the ocean. Finally, a method was developed to model the passive adjustment of the sea level to changes in the gravity field of the earth, induced by mass redistribution on the continents. Including this 'selfgravitation effect' into numerical ocean model would result in a better agreement between modeled and observational ocean data in several regions.Department of Space EngineeringAerospace Engineerin
Megathrust Earthquakes: Study of Fault Slip and Stress Relaxation Using Satellite Gravity Observations
No full textDuring earthquakes large scale mass displacements take place when slip on a fault deforms the earth’s crust. Besides, in the days to decades after the main shock ongoing deformation is usually observed that is related to relaxation of stresses caused by the earthquake. In this thesis I relate gravity changes as observed by the GRACE satellite mission to solid earth deformation caused by earthquakes. Special attention is given to modeling the contribution of ocean mass redistribution to gravity changes, and its relation to changes in bathymetry. I show that common modeling practices are usually not properly taking into account the effect of ocean water redistribution when computing seismic gravity changes. Using a combination of GRACE data and GPS observations I interpret ongoing gravity changes and crustal motions after the 2004 Sumatra-Andaman earthquake as dominantly caused by viscoelastic mantle flow. Contrasts in relaxation styles from both observation types are related to lateral variations in mantle rheology below the subduction zone. The combined analysis of GPS and GRACE data strongly suggest that the mantle at the continental side of the subduction zone is weaker than the oceanic side.Astrodynamics and Satellite MissionsAerospace Engineerin
The integration of spaceborne accelerometry in the precise orbit determination of low-flying satellites
No full textThis dissertation describes the integration of accelerometer measurements of LEO-satellites in GPS-based orbit determination.Astrodynamics and Space MissionsAerospace Engineerin
Autonomous Formation Flying in Low Earth Orbit
No full textFormation flying is commonly identified as the collective usage of two or more cooperative spacecraft to exercise the function of a single monolithic virtual instrument. The distribution of tasks and payloads among fleets of coordinated smaller satellites offers the possibility to overcome the classical limitations of traditional single-satellite systems. The science return is enhanced through observations made with larger, configurable baselines and an improved degree of redundancy can be achieved in the event of failures. Different classes of formation flying missions are currently under discussion within the engineering and science community: technology demonstration missions, synthetic aperture interferometers and gravimeters for Earth observation, multi-spacecraft interferometers in the infrared and visible wavelength regions as a key to new astrophysics discoveries and to the direct search for terrestrial exoplanets. These missions are characterized by different levels of complexity, mainly dictated by the payload metrology and actuation needs, and require a high level of on-board autonomy to satisfy the continuously increasing demand of relative navigation and control accuracy.
This dissertation presents the first realistic demonstration of a complete guidance, navigation and control (GNC) system for formation flying spacecraft in low Earth orbit. Numerous technical contributions have been made during the course of this research in the areas of formation flying guidance, GPS-based relative navigation, and impulsive relative orbit control, but the primary contribution of this thesis does not lie in one or more of these disciplines. The innovation and originality of this work stems from the design and implementation of a comprehensive formation flying system through the successful integration of various techniques. This research has led to the full development, testing and validation of the GNC flight code to be embedded in the on-board computer of the active spacecraft of the PRISMA technology demonstration. Furthermore key guidance and control algorithms presented here are going to be demonstrated for the first time in the TanDEM-X formation flying mission. Overall this thesis focuses on realistic application cases closely related to upcoming missions. The intention is to realize a practical and reliable way to formation flying: a technology that is discussed and studied since decades but is still confined in research laboratories. Hardware-in-the-loop real-time simulations including a representative flight computer and the GPS hardware architecture show that simple techniques, which exploit the natural orbit motion to full extent, can meet the demanding requirements of long-term close formation-flying
Assisted-Launch Performance Analysis: Using Trajectory and Vehicle Optimization
No full text55 years have passed since the launch of the first man-made space vehicle, Sputnik 1. While spaceflight has changed significantly in many ways, further improvement in rocket technology is still being pursued. Launch-assist systems, which give the launch vehicle a combination of initial velocity and initial altitude, could create such an improvement. There are two factors that come into play when determining if this might be true. Firstly, the cost of building and operating both the assist platform and the launch vehicle and secondly, the total system performance. This thesis focuses on the performance of the entire system. The definition of the performance of a system utilizing the assisted launch technique was chosen to be the payload mass-to-initial mass ratio of the launcher. This was maximized by optimizing various parameters of the launch vehicle and the launch trajectory using a differential evolution algorithm. Optimization was performed for launchers using either kerolox propellant or hydrolox propellant using a variety of initial altitude and initial velocity combinations. To show continuous trends of the vehicle design parameters through the whole range of simulated launchers, all launchers consist of only one stage. The end result of the thesis provides insight of the relationships between the performance, the launch vehicle design, the trajectory profile and the magnitude and type of the assist. It also provides a comparison between the performance of launchers using low specific impulse and high density propellant, such as kerolox, and high specific impulse and low density propellant, such as hydrolox. The results are intended to be used as a tool to base design decisions on during future concept studies.Astrodynamics & Space MissionsSpace EngineeringAerospace Engineerin
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