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  • 151.
    Girard, Justin
    et al.
    Institute for Aerospace Studies, University of Toronto.
    Emami, Reza
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    A robust approach to robot team learning2016Inngår i: Autonomous Robots, ISSN 0929-5593, E-ISSN 1573-7527, Vol. 40, nr 8, s. 1441-1457Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The paper achieves two outcomes. First, it summarizes previous work on concurrent Markov decision processes (CMDPs) currently demonstrated for use with multi-agent foraging problems. When using CMDPs, each agent models the environment using two Markov decision process (MDP). The two MDPs characterize a multi-agent foraging problem by modeling both a single-agent foraging problem, and multi-agent task allocation problem, for each agent. Second, the paper studies the effects of state uncertainty on a heterogeneous robot team that utilizes the aforementioned CMDP modelling approach. Furthermore, the paper presents a method to maintain performance despite state uncertainty. The resulting robust concurrent individual and social learning (RCISL) mechanism leads to an enhanced team learning behaviour despite state uncertainty. The paper analyzes the performance of the concurrent individual and social learning mechanism with and without a particle filter for a heterogeneous foraging scenario. The RCISL mechanism confers statistically significant performance improvements over the CISL mechanism

  • 152.
    Girard, Justin
    et al.
    Institute for Aerospace Studies, University of Toronto.
    Emami, Reza
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Concurrent Markov decision processes for robot team learning2015Inngår i: Engineering applications of artificial intelligence, ISSN 0952-1976, E-ISSN 1873-6769, Vol. 39, s. 223-234, artikkel-id 12Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Multi-agent learning, in a decision theoretic sense, may run into deficiencies if a single Markov decision process (MDP) is used to model agent behaviour. This paper discusses an approach to overcoming such deficiencies by considering a multi-agent learning problem as a concurrence between individual learning and task allocation MDPs. This approach, called Concurrent MDP (CMDP), is contrasted with other MDP models, including decentralized MDP. The individual MDP problem is solved by a Q-Learning algorithm, guaranteed to settle on a locally optimal reward maximization policy. For the task allocation MDP, several different concurrent individual and social learning solutions are considered. Through a heterogeneous team foraging case study, it is shown that the CMDP-based learning mechanisms reduce both simulation time and total agent learning effort.

  • 153.
    Granvik, Mikael
    et al.
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik. Department of Physics, University of Helsinki.
    Brown, Peter
    Department of Physics and Astronomy, University of Western Ontario.
    Identification of meteorite source regions in the solar system2018Inngår i: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 311, s. 271-287Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Over the past decade there has been a large increase in the number of automated camera networks that monitor the sky for fireballs. One of the goals of these networks is to provide the necessary information for linking meteorites to their pre-impact, heliocentric orbits and ultimately to their source regions in the solar system. We re-compute heliocentric orbits for the 25 meteorite falls published to date from original data sources. Using these orbits, we constrain their most likely escape routes from the main asteroid belt and the cometary region by utilizing a state-of-the-art orbit model of the near-Earth-object population, which includes a size-dependence in delivery efficiency. While we find that our general results for escape routes are comparable to previous work, the role of trajectory measurement uncertainty in escape-route identification is explored for the first time. Moreover, our improved size-dependent delivery model substantially changes likely escape routes for several meteorite falls, most notably Tagish Lake which seems unlikely to have originated in the outer main belt as previously suggested. We find that reducing the uncertainty of fireball velocity measurements below  ∼ 0.1 km/s does not lead to reduced uncertainties in the identification of their escape routes from the asteroid belt and, further, their ultimate source regions. This analysis suggests that camera networks should be optimized for the largest possible number of meteorite recoveries with measured speed precisions of order 0.1 km/s.

  • 154.
    Grosse, Christian
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Development of a Star Camera Algorithm for Calculating thePosition on Earth and Mars (VaMEx DLR)2017Independent thesis Advanced level (degree of Master (Two Years)), 20 poäng / 30 hpOppgave
    Abstract [en]

    Within the scope of the Valles Marineris Explorer mission of the German Aerospace Center(DLR), the position of a ground vehicle on Mars needs to be determined. Due to the fact thatMars has no global magnetic field and no positioning system like Earth, a facility independentsystem is necessary. In this context, a star camera based positioning system shall be developed.The subject of this thesis is the development, implementation and verification of a star camerabased transformation algorithm for calculating the latitudinal and longitudinal position on Mars.In addition, an Earth based transformation algorithm is developed, implemented and evaluated fortesting purposes, that is for testing the hardware for the mission under real conditions on Earth.The thesis gives an insight into the fundamental theory of transformation algorithms commonlyused for the transformation between celestial and planet coordinate systems. A self-developedtransformation algorithm is presented which is able to transform a tilted star vector in TopocentricHorizon Coordinate System into observer longitude and latitude position. Moreover, themodular implementation in MATrix LABoratory (MATLAB) and Realtime Onboard DependableOperating System (RODOS) is described in detail. In the further course of the thesis the results ofdifferent test scenarios are outlined and evaluated. The overall results are eventually discussed regardingtheir implications for the mission and subsequently, suggestions for further improvementof the algorithm are made.

  • 155.
    Grotzinger, J.P.
    et al.
    California Institute of Technology, Pasadena, Division of Geological and Planetary Sciences, California Institute of Technology.
    Crisp, J.A.
    Indiana University, Jet Propulsion Laboratory, California Institute of Technology, Pasadena.
    Vasavada, Ashwin
    Jet Propulsion Laboratory, California Institute of Technology, Pasadena.
    Martin-Torres, Javier
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Curiosity's Mission of Exploration at Gale Crater, Mars2015Inngår i: Elements, ISSN 1811-5209, E-ISSN 1811-5217, Vol. 11, nr 1, s. 19-26Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Landed missions to the surface of Mars have long sought to determine the material properties of rocks and soils encountered during the course of surface exploration. Increasingly, emphasis is placed on the study of materials formed or altered in the presence of liquid water. Placed in the context of their geological environment, these materials are then used to help evaluate ancient habitability. The Mars Science Laboratory mission—with its Curiosity rover—seeks to establish the availability of elements that may have fueled microbial metabolism, including carbon, hydrogen, sulfur, nitrogen, phosphorus, and a host of others at the trace element level. These measurements are most valuable when placed in a geological framework of ancient environments as interpreted from mapping, combined with an understanding of the petrogenesis of the igneous rocks and derived sedimentary materials. In turn, the analysis of solid materials and the reconstruction of ancient environments provide the basis to assess past habitability.

  • 156.
    Grulich, Maria
    et al.
    Technische Universität München.
    Conte, Davide
    The Pennsylvania State University.
    Borg, Kyle
    University of Colorado, Boulder.
    Burg, Alexander
    George Washington University.
    Burgoyne, Hayden
    California Institute of Technology.
    Celik, Onur
    School of Aerospace, Transport and Manufacturing, Cranfield University.
    Kraetzig, Benjamin
    Neptec Design Group, Atlas Building (R27), Harwell Campus Didcot, Fermi Avenue, Oxfordshire.
    Meskoob, Behnoosh
    Electrical and Electronics Faculty, Istanbul Technical University, Maslak.
    Tió, Meritxell Viñas
    Thales Alenia Space Cannes, 5 Allée des Gabians, Cannes.
    Yesil, Ali
    Institute of Space Systems, University of Stuttgart.
    Heizmann, Sören
    Universität Stuttgart.
    Scherrmann, Marcel
    Institute of Space Systems, University of Stuttgart.
    Laudan, Katarina
    Technical University Berlin.
    Pérez, Ana Alexandra
    Universidad Simón Bolívar, Sartenejas, Caracas.
    Roos, Tobias
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Weisser, Constantin
    Massachusetts Institute of Technology.
    Next - Exploration universal station (NEXUS)2016Inngår i: Proceedings of the International Astronautical Congress, IAC, 2016Konferansepaper (Fagfellevurdert)
    Abstract [en]

     From July 25 to August 1, 2015 the Space Station Design Workshop (SSDW) was held at the University of Stuttgart in Germany with students coming from around the world. During the SSDW the participants where challenged to develop a concept for a new international platform like the International Space Station (ISS) for future space research. This paper discuss the Preliminary Design Document of the architectural design, deployment strategy and operational phase of NEXUS: the Next EX-ploration Universal Station, an international crewed space platform in cis-lunar space to support the current vision for future deep space exploration. This station is designed to be modular, extensible, sustainable and serves a number of novel applications, including unique research, supporting current and future robotic and human planetary exploration, and providing a platform for international cooperation and commercial development. This space station will be the center of space exploration during its operation and will dramatically enhance the opportunities for every partner to explore the unknown and new locations beyond Low Earth Orbit. The world has successfully collaborated for many years at the ISS. However, the ISS is only currently supported through mid-2020s. The next step is to further the research and exploration done in space and provide an intermediate staging location for missions beyond Earths sphere of influence. NEXUS is located in cis-lunar space, in a halo orbit around the Earth-Moon Libration Point 2 (EML2). While the priorities of potential international partners are extremely varied, NEXUS location enhances and supports the vision of nearly every international space agency and commercial companies. The station offers numerous opportunities for research and technology testing in space and on the Moon. The overall mission architecture is separated into two phases: the construction phase and the operations phase. In order to align with the end of the ISS, the construction will begin in 2024. The construction phase would last 6 years during which the various station modules are sent to EML2 using heavy lift launchers such as the SLS Block 1 and 1B through weak stability boundary trajectories. The station will become fully operational in 2030 and will receive supplies from Earth by using a solar electric tug which would taxi supplies from Earth to NEXUS. NEXUS will be used as an intermediate location for human and robotic missions to explore the Moon, Mars, and other destinations in our Solar System

  • 157.
    Guha, Yash
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Investigation of progressive damage mechanisms in aerospace grade composites2017Independent thesis Advanced level (degree of Master (Two Years)), 20 poäng / 30 hpOppgave
  • 158.
    Guldevall, Peter
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Development of an FPGA based Trigger for Slow Moving Events for the EUSO-TA Telescope2017Independent thesis Advanced level (degree of Master (Two Years)), 20 poäng / 30 hpOppgave
  • 159.
    Gultepe, Ismail
    et al.
    Cloud Physics and Severe Weather Research Section, Environment Canada.
    Kuhn, Thomas
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Pavolonis, Michael J.
    NOAA, NESDIS, Madison, WI.
    Calvert, C.
    CIMSS, University of Wisconsin-Madison, Madison, WI.
    Gurka, James J.
    NOAA, NESDIS, Greenbelt, MD.
    Heymsfield, Andrew J.
    NCAR, Boulder, Colorado.
    Liu, P.S.K.
    Cloud Physics and Severe Weather Research Section, Environment Canada.
    Zhou, B.
    I.M. Systems Group, NOAA/NWS/NCEP, Camp Springs, MD.
    Ware, Randolph H.
    Radiometrics Corporation, CIRES, University of Colorado, Boulder, CO.
    Ferrier, Brad Schoenberg
    I.M. Systems Group, NOAA/NWS/NCEP, Camp Springs, MD.
    Milbrandt, Jason A.
    RPN, CMC, Environment Canada.
    Bernstein, Ben C.
    Leading Edge Atmospherics, Boulder, CO.
    Ice fog in arctic during fram-ice fog project aviation and nowcasting applications2014Inngår i: Bulletin of The American Meteorological Society - (BAMS), ISSN 0003-0007, E-ISSN 1520-0477, Vol. 95, nr 2, s. 211-226Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Increased understanding of ice fog microphysics can improve frost and ice fog prediction using forecast models and remote-sensing retrievals, thereby reducing potential hazards to aviation

  • 160.
    Gultepe, Ismail
    et al.
    Cloud Physics and Severe Weather Research Section, Environment Canada.
    Zhou, B.
    I.M. Systems Group, NOAA/NWS/NCEP, Camp Springs, MD.
    Milbrandt, Jason A.
    RPN, CMC, Environment Canada.
    Bott, A.
    Meteorolooieches lnstitut, University of Bonn.
    Li, Y.
    Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing.
    Heymsfield, Andrew J.
    National Center for Atmospheric Research, Boulder, Colorado.
    Ferrier, Brad Schoenberg
    I.M. Systems Group, NOAA/NWS/NCEP, Camp Springs, MD.
    Ware, Randolph H.
    Radiometrics Corporation, CIRES, University of Colorado, Boulder, CO.
    Pavolonis, Michael J.
    NOAA, NESDIS, Madison, WI.
    Kuhn, Thomas
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Gurka, James J.
    NOAA, NESDIS, Greenbelt, MD.
    Liu, P.
    Cloud Physics and Severe Weather Research Section, Environment Canada.
    Cermak, J.
    Ruhr-Universität Bochum, Department of Geography.
    A review on ice fog measurements and modeling2015Inngår i: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895, Vol. 151, s. 2-19Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The rate of weather-related aviation accident occurrence in the northern latitudes is likely 25 times higher than the national rate of Canada. If only cases where reduced visibility was a factor are considered, the average rate of occurrence in the north is about 31 times higher than the Canadian national rate. Ice fog occurs about 25% of the time in the northern latitudes and is an important contributor to low visibility. This suggests that a better understanding of ice fog prediction and detection is required over the northern latitudes. The objectives of this review are the following: 1) to summarize the current knowledge of ice fog microphysics, as inferred from observations and numerical weather prediction (NWP) models, and 2) to describe the remaining challenges associated with measuring ice fog properties, remote sensing microphysical retrievals, and simulating/predicting ice fog within numerical models. Overall, future challenges related to ice fog microphysics and visibility are summarized and current knowledge is emphasized.

  • 161.
    Guzewich, Scott D.
    et al.
    Universities Space Research Association/NASA Goddard Space Flight Center.
    Newman, C.
    Ashima Research Inc.
    De La Torre Juárez, Manuel
    Jet Propulsion Laboratory, California Institute of Technology, Pasadena.
    Mason, E.
    Texas A&M University, College Station, TX.
    Battalio, M.
    Texas A&M University, College Station, TX.
    Zorzano Mier, Maria-Paz
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Moores, John E.
    Earth and Space Science and Engineering , York University.
    Moore, C.A.
    Earth and Space Science and Engineering , York University.
    Kloos, J.L
    Earth and Space Science and Engineering , York University.
    Martinez, M.D.
    Uni-versity of Michigan, Ann Arbor.
    Smith, M.D.
    NASA Goddard Space Flight Center, Greenbelt.
    The Mars Science Laboratory dust storm campaign2017Konferansepaper (Annet vitenskapelig)
  • 162.
    Guzewich, Scott D.
    et al.
    NASA Goddard Spaceflight Center, Greenbelt, MD.
    Newman, C. E.
    Aeolis Research, Pasadena, CA.
    Smith, M. D.
    NASA Goddard Spaceflight Center, Greenbelt, MD.
    Moores, J. E.
    Department of Earth and Space Science and Engineering, York University, Toronto, ON, Canada.
    Smith, C. L.
    Department of Earth and Space Science and Engineering, York University, Toronto, ON, Canada.
    Moore, C.
    Department of Earth and Space Science and Engineering, York University, Toronto, ON, Canada.
    Richardson, M. I.
    Aeolis Research, Pasadena, CA.
    Kass, D.
    Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA.
    Kleinböhl, A.
    Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA.
    Mischna, M.
    Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA.
    Martín-Torres, Javier
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik. Instituto Andaluz de Ciencias de la Tierra (CSIC-UGR), Granada, Spain.
    Zorzano Mier, Maria-Paz
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik. Centro de Astrobiología (INTA-CSIC), Torrejón de Ardoz, Madrid, Spain.
    Battalio, M.
    Department of Atmospheric Sciences, Texas A&M University, College Station, TX.
    The Vertical Dust Profile over Gale Crater, Mars2017Inngår i: Journal of Geophysical Research - Planets, ISSN 2169-9097, E-ISSN 2169-9100, Vol. 122, nr 12, s. 2779-2792Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We create a vertically coarse, but complete, vertical profile of dust mixing ratio from the surface to the upper atmosphere over Gale Crater, Mars, using the frequent joint atmospheric observations of the orbiting Mars Climate Sounder (MCS) and the Mars Science Laboratory (MSL) Curiosity rover. Using these data and an estimate of planetary boundary layer (PBL) depth from the MarsWRF general circulation model, we divide the vertical column into three regions. The first region is the Gale Crater PBL, the second is the MCS-sampled region, and the third is between these first two. We solve for a well-mixed dust mixing ratio within this third (middle) layer of atmosphere to complete the profile.

    We identify a unique seasonal cycle of dust within each atmospheric layer. Within the Gale PBL, dust mixing ratio maximizes near southern hemisphere summer solstice (Ls = 270°) and minimizes near winter solstice (Ls = 90-100°) with a smooth sinusoidal transition between them. However, the layer above Gale Crater and below the MCS-sampled region more closely follows the global opacity cycle and has a maximum in opacity near Ls = 240° and exhibits a local minimum (associated with the “solsticial pause” in dust storm activity) near Ls = 270°. With knowledge of the complete vertical dust profile, we can also assess the frequency of high-altitude dust layers over Gale. We determine that 36% of MCS profiles near Gale Crater contain an “absolute” high-altitude dust layer wherein the dust mixing ratio is the maximum in the entire vertical column.

  • 163.
    Güttler, Johannes
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Field-Site Prototype for HABIT (FSP-HABIT): Characterizing Martian Salts Prior to the ExoMars 2020 Mission2016Independent thesis Advanced level (degree of Master (Two Years)), 20 poäng / 30 hpOppgave
    Abstract [en]

    One of the major remaining question about Mars is its habitability - if the requirements necessary to allow for life are presently fulfilled. One of the most relevant ingredients for life, as we know it, is water. Indirect evidence of transient liquid water on Mars has been retrieved from both rover [Martín-Torres et al., 2015] and orbiter [Ojha et al., 2015].

    [Martín-Torres et al., 2015] inferred the existence of an active water cycle, driven by chlorate and perchlorate salts, which are commonly found on the Martian surface, and absorb atmospheric water to form stable hydrated compounds and liquid solutions. This happens through a process called deliquescence (absorption of moisture from the atmosphere by the salts and dissolving into a liquid solution). One of the goals of HABIT is to confirm the hypothesis about the water cycle on Mars. HABIT will record the behavior of a selection of salts on Mars, and will also record Martian environmental conditions (UVdose, air and ground temperatures).

    The Field-Site Prototype for HABIT (FSP-HABIT) was the first prototype of HABIT deployed during field-site campaigns. Three campaigns took place during summer 2016: First, a short preparatory campaign in Abisko, Sweden, was carried out. The second campaign took place in Iceland, within the EU COST Action TD1308 ORIGINS (Origins and evolution of life on Earth and in the Universe), and the third campaign was conducted within the NASA Spaceward Bound India Program in Ladakh. After providing the corresponding background on the mission framework and the scientific background, this document covers the mechanical, electrical, and software design of the instrument. Afterwards, the steps taken to test the instrument and their results are covered, followed by a rating of the instrument and ideas for future improvements. Instruments like FSP-HABIT will enable the characterization of hygroscopic salts by their conductivity as liquid brines are good conductors, hydrated salts are poor conductors, and dehydrated salts are insulators. During the field-site campaigns, the measurements of FSP-HABIT were used to characterize the near surface environment by its temperature, pressure and relative humidity. Now, these measurements are available for comparison with microbiological studies of the water, ice and soils to characterize the habitability of the explored site. The lessons learned while designing and building FSP-HABIT can be used to inform the development of further prototypes for space missions such as HABIT.

  • 164.
    Hagen, Christoph
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    A Bluetooth based intra-satellite communication system2017Independent thesis Advanced level (degree of Master (Two Years)), 20 poäng / 30 hpOppgave
    Abstract [en]

    This thesis presents a wireless communication system for intra-satellite communication based on Bluetooth Low Energy technology, which can have many benefits regarding the design and operation of satellites. The proposed design based on the nRF53832 chip from Nordic Semiconductor is described, followed by the results of several tests regarding the most important design criteria for its application in small satellites. The tested aspects include the power consumption of the wireless module in different operation modes, which is sufficiently low for the application even in small satellites. Signal strength measurements for various output power settings and obstacles show that reliable communication is possible in a satellite mockup. No packet error was detected, and latencies of less than 30 ms combined with achievable data rates between 200 and 700 kbps should be sufficient for most CubeSat satellites. Additionally, details are given to successfully integrate the chip with existing satellite subsystems. A code library is provided to simplify the communication between the modules, and a concept of a redundant system is established to increase the reliability for critical satellite subsystems. The overall assessment of the technology suggests that the presented system is suitable for in-orbit deployment with the Aalto-3 satellite (currently being developed at Aalto University), which will provide further validation of the technology. 

  • 165.
    Hagos, Tesfamichael Marikos
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik. Aalto University .
    Estimation of phases for compliant motion: Auto-regressive HMM, multi-class logistic regression, Learning from Demonstration (LfD), Gradient descent optimization,2017Independent thesis Advanced level (degree of Master (Two Years)), 20 poäng / 30 hpOppgave
  • 166.
    Hakima, Houman
    et al.
    Institute for Aerospace Studies, University of Toronto .
    Bazzocchi, Michael C. F.
    Institute for Aerospace Studies, University of Toronto.
    Emami, Reza
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik. Institute for Aerospace Studies, University of Toronto.
    A Deorbiter CubeSat for Active Orbital Debris Removal2018Inngår i: Advances in Space Research, ISSN 0273-1177, E-ISSN 1879-1948, Vol. 61, nr 9, s. 2377-2392Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper introduces a mission concept for active removal of orbital debris based on the utilization of the CubeSat form factor. The CubeSat is deployed from a carrier spacecraft, known as a mothership, and is equipped with orbital and attitude control actuators to attach to the target debris, stabilize its attitude, and subsequently move the debris to a lower orbit where atmospheric drag is high enough for the bodies to burn up. The mass and orbit altitude of debris objects that are within the realms of the CubeSat’s propulsion capabilities are identified. The attitude control schemes for the detumbling and deorbiting phases of the mission are specified. The objective of the deorbiting maneuver is to decrease the semi-major axis of the debris orbit, at the fastest rate, from its initial value to a final value of about 6,471 km (i.e., 100 km above Earth considering a circular orbit) via a continuous low-thrust orbital transfer. Two case studies are investigated to verify the performance of the deorbiter CubeSat during the detumbling and deorbiting phases of the mission. The baseline target debris used in the study are the decommissioned KOMPSAT-1 satellite and the Pegasus rocket body. The results show that the deorbiting times for the target debris are reduced significantly, from several decades to one or two years.

  • 167.
    Hakima, Houman
    et al.
    University of Toronto Institute for Aerospace Studies, 4925 Dufferin Street, Toronto.
    Emami, Reza
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik. Institute for Aerospace Studies, University of Toronto.
    Assessment of active methods for removal of LEO debris2018Inngår i: Acta Astronautica, ISSN 0094-5765, E-ISSN 1879-2030, Vol. 144, s. 225-243Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper investigates the applicability of five active methods for removal of large low Earth orbit debris. The removal methods, namely net, laser, electrodynamic tether, ion beam shepherd, and robotic arm, are selected based on a set of high-level space mission constraints. Mission level criteria are then utilized to assess the performance of each redirection method in light of the results obtained from a Monte Carlo simulation. The simulation provides an insight into the removal time, performance robustness, and propellant mass criteria for the targeted debris range. The remaining attributes are quantified based on the models provided in the literature, which take into account several important parameters pertaining to each removal method. The means of assigning attributes to each assessment criterion is discussed in detail. A systematic comparison is performed using two different assessment schemes: Analytical Hierarchy Process and utility-based approach. A third assessment technique, namely the potential-loss analysis, is utilized to highlight the effect of risks in each removal methods

  • 168.
    Hakima, Houman
    et al.
    University of Toronto Institute for Aerospace Studies, 4925 Dufferin Street, Toronto.
    Emami, Reza
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Debris detumbler: An alternative approach to active debris removal2016Inngår i: Proceedings of the International Astronautical Congress, IAC, 2016Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Since the launch of the first artificial satellite, i.e., Sputnik in 1957, several thousand man-made objects have been launched into the Earth orbits, great majority of which remained in their orbit despite the termination of their mission. A number of viable solutions have been suggested in the research community for actively removing the orbital debris, some of which require the capturing of the debris, while others are contactless methods. A challenging aspect of capturing an orbital debris using any method is due to the tumbling motion of the debris. A majority of the suggested methods require zero or very low rates of debris attitude to perform successfully, or need to follow a synchronization phase with the debris before the capturing and removal operation. This is technically costly and challenging, if not infeasible. This paper proposes an alternative approach to orbital debris removal, which can make various state-of-the-art methods of active debris removal perform at lower costs and risks. The approach utilizes one or more detumbler platforms, in the form of miniaturized cubesats, which are de-signed to be lunched from the remover spacecraft, attach to the target debris, and reduce its attitude rate to zero using the onboard attitude determination sensors and control actuators. The paper outlines the operation of debris detumblers. The conceptual design of such platforms is also presented, based on the one- to three-unit cubesat bus and commercial off-the-shelf technologies. Orbital insertion, rendezvous and attachment maneuvers are also discussed. Finally, through simulations the performance of the proposed approach is compared with that of some well-studied methods in various scenarios using several catalogued debris, based on performance metrics such as delta-v, operation time, trajectory simplicity, total thrust, etc

  • 169.
    Hammarsten, Michael
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    A statistical study of incoherent scatter plasma line enhancements during the International Polar Year ’07-’08 in Svalbard2016Independent thesis Advanced level (degree of Master (Two Years)), 20 poäng / 30 hpOppgave
    Abstract [en]

    There was a large radar campaign during 2007 and 2008, the International Polar Year (IPY),and at that time the EISCAT Svalbard Radar was operated and measured the ionosphere continuouslyat most times. This report presents statistical results from an electron enhancementpoint of view. Until now there has been some research into the field and results based on theions in the ionosphere, and the enhancements we refer to as Naturally enhanced ion acousticlines (NEIALs). Plasma line data from May 2007 to February 2008 has been analysed inorder to find and classify enhancements as NEIALs have been classified but with respect tothe electron distribution instead of the ion distribution. A method of detection was developedin order to differentiate the enhancements from the background with a relation between theminimum and maximum power of each measured dump. Results show that there is a largedifference between the downshifted plasma lines and the upshifted plasma lines, both has arange distribution peak at 180 km and the upshifted plasma line has another peak at 230 kmwhich the downshifted plasma line does not. The occurrence rate of the enhancements was1.64 % for the downshifted plasma line and 4.69 % for the upshifted plasma line. Threedifferent types of enhancements are classified using the variance distribution for the peakfrequency of that detected dump, Single, Profile, and Diffuse. The Single enhancements havea bit different spectral, range, and time of day distributions than of the Profile and Diffusedistributions. The Diffuse classifications are mostly wrong classifications and aliasing and itis very similar to Profile enhancements as seen by its distribution.

  • 170.
    Hirschmüller, Korbinian
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Development and Evaluation of a 3D Point Cloud Based Attitude Determination System2017Independent thesis Advanced level (degree of Master (Two Years)), 20 poäng / 30 hpOppgave
  • 171.
    Hladký, Maroš
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik. Julius-Maximilians-University Würzburg, Informatics VII : Robotics and Telematics .
    Vision Based Attitude Control2018Independent thesis Advanced level (degree of Master (Two Years)), 20 poäng / 30 hpOppgave
    Abstract [en]

    The problematics of precise pointing and more specifically an attitude control is present sincethe first days of flight and Aerospace engineering. The precise attitude control is a matter ofnecessity for a great variety of applications. In the air, planes or unmanned aerial vehicles needto be able to orient precisely. In Space, a telescope or a satellite relies on the attitude control toreach the stars or survey the Earth. The attitude control can be based on various principles, pre-calculated variables, and measurements. It is common to use the gyroscope, Sun/Star/horizonsensors for attitude determination. While those technologies are well established in the indus-try, the rise in a computational power and efficiency in recent years enabled processing of aninfinitely more rich source of information - the vision. In this Thesis, a visual system is used forthe attitude determination and is blended together with a control algorithm to form a VisionBased Attitude Control system.A demonstrator is designed, build and programmed for the purpose of Vision Based AttitudeControl. It is based on the principle of Visual servoing, a method that links image measure-ments to the attitude control, in a form of a set of joint velocities. The intermittent steps arethe image acquisition and processing, feature detection, feature tracking and the computationof joint velocities in a closed loop control scheme. The system is then evaluated in a barrage ofpartial experiments.The results show, that the used detection algorithms, Shi&Tomasi and Harris, performequally well in feature detection and are able to provide a high amount of features for tracking.The pyramidal implementation of the Lucas&Kanade tracking algorithm proves to be a capablemethod for a reliable feature tracking, invariant to rotation and scale change. To further evaluatethe Visual servoing a complete demonstrator is tested. The demonstrator shows the capabilityof Visual Servoing for the purpose of Vision Based Attitude Control. An improvement in thehardware and implementation is recommended and planned to push the system beyond thedemonstrator stage into an applicable system.

  • 172.
    Holl, Gerrit
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Microwave and infrared remote sensing of ice clouds: measurements and radiative transfer simulations2011Licentiatavhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    This licentiate thesis considers the combination of multiple instruments for remote sensing of the Earth atmosphere from space. The primary focus is on remote sensing of atmospheric ice. Ice clouds are important for the Earth’s radiation budget, but their properties are difficult to measure and therefore poorly known. A better quantification of ice clouds is needed to improve global climate models. This thesis introduces the reader to the subject and describes how to combine measurements and radiative transfer simulations in an attempt to improve our understanding. A major part of this work is the development of a toolkit to find co-incident measurements, or collocations, between any pair of down-looking satellite sensors. Firstly, this toolkit is used to collocate passive microwave and thermal infrared sensors on meteorological satellites with the Cloud Profiling Radar on CloudSat. With the resulting collocated dataset, the Ice Water Path (IWP) signal in passive thermal radiation is studied and an improved IWP retrieval is presented. The toolkit is also used to better characterise the bias between different copies of passive microwave radiometers on-board polar-orbiting operational satellites. For the Atmospheric Radiative Transfer Simulator (ARTS), version 2, an optimised frequency grid for infrared broadband simulations is shown to be applicable for cloudy simulations. This frequency grid can and will be used to study the IWP signal in thermal infrared radiances. An outlook on a comparison between collocations and simulations is presented in the thesis.

  • 173.
    Holl, Gerrit
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Remote sensing of ice clouds: synergistic measurements and radiative transfer simulations2013Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    This thesis primarily considers the spaceborne remote sensing of ice clouds and frozen precipitation. Ice clouds are important for hydrology and for the Earth’s radiation budget, but many properties are difficult to measure, in particular using spaceborne instruments. A better quantification of ice clouds is needed to improve global climate models. This thesis presents steps toward such an improvement.The first part of the thesis introduces topics related to the research presented in the second part, but presents no new scientific results. It gives a brief introduction to the history of atmospheric remote sensing and describes how the different parts of the electromagnetic spectrum can be used actively or passively. Then, it describes why ice clouds are important and what microphysical, optical, and macrophysical properties are used to describe atmospheric ice. Next, it briefly introduces the relevant topics in atmospheric radiative transfer. The first part concludes with a description of various approaches to retrievals, with a particular focus on those applied in this thesis.The second part of the thesis describes new results. The bulk of the new results is described in five peer-reviewed publications, that are appended verbatim.A major part of the work builds on the development of a toolkit to easily find co-incident measurements, or collocations, between any pair of satellite sensors. Four appended articles rely on this toolkit.The first appended article uses the toolkit to obtain collocations between passive microwave and infrared on operational meteorological satellites with the Cloud Profiling Radar on CloudSat. It presents three examples. Firstly, from the collocated dataset and a dataset of synthetic profiles, the article compares the statistical relations between an official CloudSat Ice Water Path (IWP) product and microwave radiances. Secondly, it shows a point-by-point comparison between the same CloudSat IWP product, and an IWP product based on passive microwave. A more sophisticated set of systematic comparisons, including more satellites and sensors, is presented in a dedicated paper. Finally, the first paper provides a first preview of how the collocations can be used to train a new IWP retrieval from passive operational measurements. This too is the topic of a dedicated paper, where solar, terrestrial infrared, and microwave radiances are combined to obtain an improved IWP product from passive operational sensors, by training with an active combined radar-lidar product from CloudSat-CALIPSO.The second appended article also relies on the collocations toolkit. Here, collocations between different copies of identical or very similar microwave sounders are used to assess how the inter-satellite bias depends on radiance and latitude.The remaining two studies described in the thesis do not use existing measurements, but are based on radiative transfer modelling. One attached paper verifies that optimised frequency grids obtained in clear-sky simulations for terrestrial infrared instrument studies, can be applied directly for cloudy simulations. This result is relevant for future studies. Finally, the thesis includes a short study with retrieval simulations for a new sub-millimetre instrument concept.

  • 174.
    Holl, Gerrit
    et al.
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Mendrok, Jana
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Kottayil, Ajil
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Optimised frequency grids for infrared radiative transfer simulations in cloudy conditions2012Inngår i: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 113, nr 16, s. 2124-2134Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper shows that radiometer channel radiances for cloudy atmospheric conditions can be simulated with an optimised frequency grid derived under clear-sky conditions. A new clear-sky optimised grid is derived for AVHRR channel . For HIRS channel 11 and AVHRR channel 5, radiative transfer simulations using an optimised frequency grid are compared with simulations using a reference grid, where the optimised grid has roughly 100–1000 times less frequencies than the full grid. The root mean square error between the optimised and the reference simulation is found to be less than 0.3 K for both comparisons, with the magnitude of the bias less than 0.03 K. The simulations have been carried out with the radiative transfer model Atmospheric Radiative Transfer Simulator (ARTS), version 2, using a backward Monte Carlo module for the treatment of clouds. With this module, the optimised simulations are more than 10 times faster than the reference simulations. Although the number of photons is the same, the smaller number of frequencies reduces the overhead for preparing the optical properties for each frequency. With deterministic scattering solvers, the relative decrease in runtime would be even more. The results allow for new radiative transfer applications, such as the development of new retrievals, because it becomes much quicker to carry out a large number of simulations. The conclusions are applicable to any downlooking infrared radiometer.

  • 175. Holl, Gerrit
    et al.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Rydberg, B.
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    Jiménez, C.
    Laboratoire d'Etude du Rayonnement et de la Matière en Astrophysique, Centre National de la Recherche Scientifique, Observatoire de Paris.
    Collocating satellite-based radar and radiometer measurements: methodology and usage examples2010Inngår i: Atmospheric Measurement Techniques, ISSN 1867-1381, E-ISSN 1867-8548, Vol. 3, nr 3, s. 693-708Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Collocations between two satellite sensors are occasions where both sensors observe the same place at roughly the same time. We study collocations between the Microwave Humidity Sounder (MHS) on-board NOAA-18 and the Cloud Profiling Radar (CPR) on-board CloudSat. First, a simple method is presented to obtain those collocations and this method is compared with a more complicated approach found in literature. We present the statistical properties of the collocations, with particular attention to the effects of the differences in footprint size. For 2007, we find approximately two and a half million MHS measurements with CPR pixels close to their centrepoints. Most of those collocations contain at least ten CloudSat pixels and image relatively homogeneous scenes. In the second part, we present three possible applications for the collocations. Firstly, we use the collocations to validate an operational Ice Water Path (IWP) product from MHS measurements, produced by the National Environment Satellite, Data and Information System (NESDIS) in the Microwave Surface and Precipitation Products System (MSPPS). IWP values from the CloudSat CPR are found to be significantly larger than those from the MSPPS. Secondly, we compare the relation between IWP and MHS channel 5 (190.311 GHz) brightness temperature for two datasets: the collocated dataset, and an artificial dataset. We find a larger variability in the collocated dataset. Finally, we use the collocations to train an Artificial Neural Network and describe how we can use it to develop a new MHS-based IWP product. We also study the effect of adding measurements from the High Resolution Infrared Radiation Sounder (HIRS), channels 8 (11.11 mu m) and 11 (8.33 mu m). This shows a small improvement in the retrieval quality. The collocations described in the article are available for public use

  • 176.
    Holl, Gerrit
    et al.
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Rydberg, Bengt
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    Jiménez, Carlos
    LERMA, CNRS, Observatoire de Paris, Paris, France.
    Collocating satellite-based radar and radiometer measurements –: methodology and usage examples2010Konferansepaper (Annet vitenskapelig)
  • 177.
    Holl, Gerrit
    et al.
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Rydberg, Bengt
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    Jiménez, Carlos
    Laboratoire d'Etude du Rayonnement et de la Matière en Astrophysique, Centre National de la Recherche Scientifique, Observatoire de Paris.
    Collocating satellite-based radar and radiometer measurements: methodology and usage examples2011Inngår i: Proceedings of the ESA Living Planet Symposium, European Space Agency, ESA , 2011Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    Collocations between two satellite sensors are occasions where both sensors observe the same place at roughly the same time. We study collocations between the Microwave Humidity Sounder (MHS) on-board NOAA-18 and the Cloud Profiling Radar (CPR) on-board CloudSat. We present some statistical properties of the collocations. For 2007, we find approximately two and a half million MHS measurements with CPR pixels close to their centrepoints. We present some possible applications. We use the collocations to validate an operational Ice Water Path (IWP) product from MHS measurements, produced by the National Environment Satellite, Data and Information System (NESDIS) in the Microwave Surface and Precipitation Products System (MSPPS). IWP values from the CloudSat CPR are found to be significantly larger than those from the MSPPS. Finally, we use the collocations to train an Artificial Neural Network and describe how we can use it to develop a new MHS-based IWP product. The collocations described in the article are available for public use.

  • 178.
    Holl, Gerrit
    et al.
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Rydberg, Bengt
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    Jiménez, Carlos
    LERMA, CNRS, Observatoire de Paris, Paris, France.
    Collocating satellite-based radar and radiometer measurements: methodology and usage examples2010Konferansepaper (Annet vitenskapelig)
  • 179.
    Holl, Gerrit
    et al.
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Rydberg, Bengt
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    Jiménez, Carlos
    LERMA, CNRS, Observatoire de Paris, Paris, France.
    Hunting ice clouds: quantifying ice from passive infrared and microwave measurements by collocating with the CloudSat radar2011Konferansepaper (Annet vitenskapelig)
  • 180.
    Holl, Gerrit
    et al.
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Eliasson, Salomon
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Mendrok, Jana
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    SPARE-ICE: Synergistic ice water path from passive operational sensors2014Inngår i: Journal of Geophysical Research: Atmospheres, ISSN 2169-8996, Vol. 119, nr 3, s. 1504-1523Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This article presents SPARE-ICE, the Synergistic Passive Atmospheric Retrieval Experiment-ICE. SPARE-ICE is the first Ice Water Path (IWP) product combining infrared and microwave radiances. By using only passive operational sensors, the SPARE-ICE retrieval can be used to process data from at least the NOAA 15 to 19 and MetOp satellites, obtaining time series from 1998 onward. The retrieval is developed using collocations between passive operational sensors (solar, terrestrial infrared, microwave), the CloudSat radar, and the CALIPSO lidar. The collocations form a retrieval database matching measurements from passive sensors against the existing active combined radar-lidar product 2C-ICE. With this retrieval database, we train a pair of artificial neural networks to detect clouds and retrieve IWP. When considering solar, terrestrial infrared, and microwave-based measurements, we show that any combination of two techniques performs better than either single-technique retrieval. We choose not to include solar reflectances in SPARE-ICE, because the improvement is small, and so that SPARE-ICE can be retrieved both daytime and nighttime. The median fractional error between SPARE-ICE and 2C-ICE is around a factor 2, a figure similar to the random error between 2C-ICE ice water content (IWC) and in situ measurements. A comparison of SPARE-ICE with Moderate Resolution Imaging Spectroradiometer (MODIS), Pathfinder Atmospheric Extended (PATMOS-X), and Microwave Surface and Precipitation Products System (MSPPS) indicates that SPARE-ICE appears to perform well even in difficult conditions. SPARE-ICE is available for public use.

  • 181.
    Hong, Gang
    et al.
    Department of Atmospheric Sciences, Texas A&M University, College Station, Texas.
    Heygster, Georg
    Institute of Environmental Physics, University of Bremen.
    Notholt, Justus
    Institute of Environmental Physics, University of Bremen.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Interannual to diurnal variations in tropical and subtropical deep convective clouds and convective overshooting from seven years of AMSU-B measurements2008Inngår i: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 21, nr 17, s. 4168-4189Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This study surveys interannual to diurnal variations of tropical deep convective clouds and convective overshooting using the Advanced Microwave Sounding Unit B (AMSU-B) aboard the NOAA polar orbiting satellites from 1999 to 2005. The methodology used to detect tropical deep convective clouds is based on the advantage of microwave radiances to penetrate clouds. The major concentrations of tropical deep convective clouds are found over the intertropical convergence zone (ITCZ), the South Pacific convergence zone (SPCZ), tropical Africa, the Indian Ocean, the Indonesia maritime region, and tropical and South America. The geographical distributions are consistent with previous results from infrared-based measurements, but the cloud fractions present in this study are lower. Land-ocean and Northern-Southern Hemisphere (NH-SH) contrasts are found for tropical deep convective clouds. The mean tropical deep convective clouds have a slightly decreasing trend with -0.016% decade(-1) in 1999-2005 while the mean convective overshooting has a distinct decreasing trend with -0.142% decade(-1). The trends vary with the underlying surface (ocean or land) and with latitude. A secondary ITCZ occurring over the eastern Pacific between 2 degrees and 8 degrees S and only in boreal spring is predominantly found to be associated with cold sea surface temperatures in La Nina years. The seasonal cycles of deep convective cloud and convective overshooting are stronger over land than over ocean. The seasonal migration is pronounced and moves south with the sun from summer to winter and is particularly dramatic over land. The diurnal cycles of deep convective clouds and convective overshooting peak in the early evening and have their minima in the late morning over the tropical land. Over the tropical ocean the diurnal cycles peak in the morning and have their minima in the afternoon to early evening. The diurnal cycles over the NH and SH subtropical regions vary with the seasons. The local times of the maximum and minimum fractions also vary with the seasons. As the detected deep convective cloud fractions are sensitive to the algorithms and satellite sensors used and are influenced by the life cycles of deep convective clouds, the results presented in this study provide information complementary to present tropical deep convective cloud climatologies.

  • 182.
    Hong, Gang
    et al.
    Department of Atmospheric Sciences, Texas A&M University, College Station, Texas.
    Yang, Ping
    Department of Atmospheric Sciences, Texas A&M University, College Station, Texas.
    Baum, Bryan A.
    Space Science and Engineering Center, University of Wisconsin, Madison, Wisconsin.
    Heymsfield, Andrew J.
    National Center for Atmospheric Research, Boulder, Colorado.
    Weng, Fuzhong
    Satellite Meteorology and Climatology Division, Center for Satellite Applications and Research, NOAA NESDIS, Camp Springs, Maryland.
    Liu, Quanhua
    Joint Center for Satellite Data Assimilation, NOAA NESDIS, Camp Springs, Maryland.
    Heygster, Georg
    Institute of Environmental Physics, University of Bremen.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Scattering database in the millimeter and submillimeter wave range of 100-1000 GHz for nonspherical ice particles2009Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 114, s. D06201-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The inference of ice cloud properties from spaceborne sensors is sensitive to the retrieval algorithms and satellite sensors used. To approach a better understanding of ice cloud properties, it is necessary to combine satellite measurements from multiple platforms and sensors operating in visible, infrared, and millimeter and submillimeter-wave regions of the electromagnetic spectrum. The single-scattering properties of ice particles with consistent ice particle models are the basis for estimating the optical and microphysical properties of ice clouds from multiple satellite sensors. In this study, the single-scattering properties (extinction efficiency, absorption efficiency, single-scattering albedo, asymmetry factor, and scattering phase matrix) of nonspherical ice particles, assumed to be hexagonal solid and hollow columns, hexagonal plates, 3D bullet rosettes, aggregates, and droxtals, are computed from the discrete dipole approximation method for 21 millimeter and submillimeter-wave frequencies ranging from 100 to 1000 GHz. A database of the single-scattering properties of nonspherical ice particles are developed for 38 particle sizes ranging from 2 to 2000 μm in terms of particle maximum dimension. The bulk scattering properties of ice clouds consisting of various ice particles, which are the fundamental to the radiative transfer in ice clouds, are developed on the basis of a set of 1119 particle size distributions obtained from various field campaigns.

  • 183.
    Houman, Hakima
    et al.
    University of Toronto, Institute for Aerospace Studies.
    Emami, Reza
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Prioritizing orbital debris for active debris removal missions2017Inngår i: IEEE Aerospace Conference Proceedings, Piscataway, NJ: Institute of Electrical and Electronics Engineers (IEEE), 2017, artikkel-id 7943788Konferansepaper (Fagfellevurdert)
    Abstract [en]

    This paper introduces a method of prioritizing orbital debris for future active debris removal missions, based on the evaluation of the total collision probability of each cataloged debris. The United States Space Surveillance Network actively monitors, tracks, and catalogs space debris orbiting Earth. The catalog is updated routinely, and is available to the public in the form of two-line element data. The total collision probability of a debris is defined as the overall probability of the debris colliding with any other debris in a given time window. The proposed method uses the two-line element data pertaining to each debris in the catalog to project the future state of the debris, i.e., the classical orbital elements, at predefined time steps for a given time window. The effects of orbital perturbations are considered wherever applicable. The relative distances between all debris are estimated in each time step, and pairwise collision probabilities are calculated for any two debris objects in the catalog. To obtain the total collision probability for a debris, the pairwise collision probabilities pertaining to the debris are summed. Further, for every debris object the trend in the total collision probability as the time window progresses is quantified, and debris objects are ranked based on their chance of collision in the time window. The outcome of the study is compared with target debris proposed in other studies.

  • 184.
    Håkansson, Marcus
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Back-tracing of water ions at comet 67P/Churyumov–Gerasimenko2017Independent thesis Advanced level (degree of Master (Two Years)), 20 poäng / 30 hpOppgave
    Abstract [en]

    This paper examines the neutral coma of comet 67P/Churyumov–Gerasimenko by using measurements of charged particles (water ions) and tracing them back to their place of ionisation. The measurements were taken from Rosetta’s Ion Composition Analyser. The simulations made use of an existing program which traces particles forward, which was changed to trace particles backwards, with new conditions for terminating the simulation.

    Two types of simulations were made. The first type is referred to as ”one-day simulations”. In these, simulations are made using data from a single occasion, with nine occasions studied per selected day. The days were selected so that the spacecraft was in different positions in relation to the comet. The second is referred to as the ”full-hemisphere” simulation. In this simulation, data from all usable days are used to produce an image of the hemisphere facing the Sun.

    The full-hemisphere simulation suffers from lack of simultaneous measurements, and indeed it is impossible to obtain in-situ measurements at all positions at once. Both simulations could be improved using more precise models, which could not be done within the allotted time of this work.

  • 185.
    Höpfner, M.
    et al.
    Karlsruhe Institute of Technology.
    Milz, Mathias
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Orphal, J.
    Karlsruhe Institute of Technology.
    Stiller, G.
    Karlsruhe Institute of Technology.
    The natural greenhouse effect of atmospheric oxygen (O2) and nitrogen (N2)2012Inngår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 39, s. L10706-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The effect of collision-induced absorption by molecular oxygen (O 2) and nitrogen (N 2) on the outgoing longwave radiation (OLR) of the Earth's atmosphere has been quantified. We have found that on global average under clear-sky conditions the OLR is reduced due to O 2 by 0.11 Wm -2 and due to N 2 by 0.17 Wm -2. Together this amounts to 15% of the OLR-reduction caused by CH 4 at present atmospheric concentrations. Over Antarctica the combined effect of O 2 and N 2 increases on average to about 38% of CH 4 with single values reaching up to 80%. This is explained by less interference of H 2O spectral bands on the absorption features of O 2 and N 2 for dry atmospheric conditions

  • 186.
    Ishtiaq, Shehryar
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Optimization of a Laser Coarse Pointing Assembly for Low Earth Orbit Satellite Missions2016Independent thesis Advanced level (degree of Master (Two Years)), 20 poäng / 30 hpOppgave
    Abstract [en]

    A laser communications terminal was developed by the Deutsches Zentrum für Luft- und Raumfahrt and is already being flown over the BIROS satellite, however it is desired to improvepointing performance with the utilization of a coarse pointing assembly. This study presents theseries of optimizations towards space applicability performed on an existing coarse pointingassembly for such a laser communications terminal. This module was initially built for airborneapplications and tested upon the tornado aircraft. The idea was to use this existing flight performanceproven system and with the help of commercial off the shelf components providebetter structural stability and tolerance to the Low Earth Orbit space environment. Criticalpoints were highlighted and the scope of the study was defined to provide reliable modal andthermal analysis results for its performance under the specified mission design parameters. Acircular low earth orbit of 450 km altitude with an inclination similar to that of the BIROS satellitewas chosen to simulate the space environmental effects for this analysis. A preliminary designreview highlighted the need to increase the link budget, which brought down the beamdivergence to 50 μrad from a previous 100 μrad. This made the entire system sensitive andmore susceptible to environmental factors requiring a more rigid and deformation tolerantstructure. In this study the effect of thermal loadings in worst-case scenarios were analysed.Aside from the optimization changes, the results of the analysis pointed out the need to utilizethermal control measures for optimum performance. Therefore, some passive thermal controlmeasures are also presented in this study, which are necessary to meet the requirements.

  • 187.
    Isoz, Oscar
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Estimation of the Impact on GNSS Receivers From Hall Thruster Engines2014Rapport (Fagfellevurdert)
  • 188.
    Isoz, Oscar
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Interference in Global Positioning System Signals and its Effect on Positioning and Remote Sensing2015Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    GPS and the other GNSS systems (GLONASS, Galileo and Beidou-2/COMPASS) is used to position billions of devices and is saving lives, the environment and money on a daily basis. GNSS enables anyone to determine their own unique global location. But the system can be fragile, it can easily be disabled or manipulated so that the calculated position from the receivers becomes incorrect. This can be done either intenionally or unintentionally. Further, many GNSS signals are located in shared frequency bands where other transmitters are allowed to broadcast as well. These transmitters can forexample be long range radars or distance montitoring equipment for aviation.In this thesis, it is demonstrated how one such radar can be detected and localized using data collected by the GNSS receiver for atmospheric sounding (GRAS). It is shown that the detected radar did not cause any measurable degradation of the temperature profiles generated from the collected data. Measurements from the GRAS sensor is also used as a reference to compare temperature soundings from the passive Advanced MicrowaveSounding Unit-A (AMSU-A) sensors that measures emission from Oxygen around 56 GHz.Further, work focusing on the detection of ground based interference is presented. It is shown how low cost independent units can be used for long term montitoring of the interference environment at key locations. Using collected data from the measurements at an area closed to the public, it is further shown how these units can be used to localize sources of broadband interference. Interference can also be generated from certain types of engines. One of the included contributions presents a theoretical analysis of the impact on GPS from an electrical engine intended for satellite propulsion. Even if the engine generates powerful broadband emission, since it is pulsed, the impact on the GPS receiver will most likely be minimal.

  • 189.
    Isoz, Oscar
    et al.
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Eriksson, Per
    Chalmers University of Technology, Department of Earth and Space Sciences.
    Intercalibration of microwave temperature sounders using radio occultation measurements2015Inngår i: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 120, nr 9, s. 3758-3773Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This is a study of the usefulness of radio occultation (RO) data for intercomparing different microwave temperature (MWT) sounding instruments. The RO data used are from the Global Navigational Satellite System Receiver for Atmospheric Sounding on the Metop-A and B satellites. The MWT sounders investigated are the Advanced Microwave Sounding Unit-A instruments on the satellites NOAA 15, 16, and 18 and Metop-A. We collocate RO and MWT data and then use these collocations to study various aspects of the MWT instruments. In addition, two different versions of the MWT data are compared: standard operational data (OPR) and the NOAA Integrated Microwave Intercalibration Approach data (IMICA). The time series of monthly mean differences shows that there are robust patterns for each satellite and data version, which mostly drift only slowly over time. The intersatellite spread, measured by the standard deviation of the yearly mean values by all satellites, is between 0.1 and 0.4 K, depending on channel, with no significant differences between OPR and IMICA data. The only notable exception is Channel 8 of NOAA 16, which appears to have a time-varying offset of 0.5–1 K relative to the other instruments. At this point it is not clear whether this deviation is real or a sampling artifact, so further study is needed. Due to the large number of collocations used, it is possible to also investigate the scene brightness and scan angle dependence of the MWT bias (relative to RO). First results of such an analysis are presented and discussed. Particularly, the investigation of the scan angle dependence is novel, since this bias pattern is difficult to assess without RO data. Further work is needed on these angular dependences, before conclusions are robust enough to include in data recalibration efforts, but our overall conclusion is that RO collocations are a powerful tool for intercomparing MWT sounders.

  • 190.
    Isoz, Oscar
    et al.
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Kinch, Kjartan
    Niels Bohr Institute, Copenhagen University.
    Bonnedal, Magnus
    RUAG Space, Gothenburg.
    Akos, Dennis
    Luleå tekniska universitet, Institutionen för system- och rymdteknik.
    Interference from terrestrial sources and its impact on the GRAS GPS radio occultation receiver2014Inngår i: Radio Science, ISSN 0048-6604, E-ISSN 1944-799X, Vol. 49, nr 1, s. 1-6Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    [1] It is well known that terrestrial GPS/Global Navigation Satellite Systems (GNSS) receivers are vulnerable and have suffered from intentional and unintentional interference sources. Unfortunately, space-based GPS/GNSS receivers are not exempt from radio frequency interference originating from the Earth. This paper explores data recorded by the GNSS Receiver for Atmospheric Sounding (GRAS) instrument onboard MetOp-A in September 2007, which is assumed to be representative of the typical environment for GPS/GNSS instrumentation in LEO orbit. Within these data it is possible to detect both pulsed interference and variations in the background noise. One plausible source of the pulsed interference is identified. We also show that neither the pulsed interference nor the variations in the background noise degrades the performance of the higher level products from GRAS

  • 191. Jarret, Mark
    et al.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Child, R. K.
    Jimenez, C.
    Courcoux, N.
    Emde, C.
    Heygster, G.
    Sreerekha, T. R.
    Eriksson, P.
    Rydberg, B.
    Foster, P. R.
    Rose, T.
    Davis, C.
    Evans, K. F.
    Heymsfield, A.
    Lohmann, U.
    Stubenrauch, C.
    Establishment of mission and instrument requirements to observe cirrus clouds at sub-millimetre wavelengths: Final report, ESTEC contract no. 19053/05/NL/AR2007Rapport (Annet vitenskapelig)
  • 192.
    Jegatheesan, Anjana
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Development of Rear Entry HUT/PLSS Design for Aouda.X Spacesuit Simulator2018Independent thesis Advanced level (degree of Master (Two Years)), 20 poäng / 30 hpOppgave
    Abstract [en]

    The Aouda.X Space suit simulator, developed by The Austrian Space Forum( OeWF) currently consists of a system that weighs 48 kg of which nearly 57% is comprised of the Suit's HUT (Hard Upper Torso), PLSS (Portable Life Support System) and OBDH (On- board data handling). In addition to this, the current conguration requires 3 hours of assisted donning/doffing. To improve the ergonomics of the design, a relatively lighter HUT/PLSS design prototype with efficient donning capabilities, preferably self -donning, must be developed. This issue can be addressed by proposing a Rear - Entry Design that when implemented on the Aouda.X, can potentially ease these impediments.This study aims at identifying a suitable Rear entry closure design for the current conguration of Aouda.X based on planetary suit performance indicators and operational requirements. The Aouda.X rear entry design is also targeted to be compatible with the NDX -Suitport developed by the University of North Dakota's Human Space Flight Laboratory. The thesis work comprises of the development of a suitable methodology to distinguish a rear entry design for the HUT and PLSS of the spacesuit simulator with the identication of a self-sealing/locking mechanism based on these requirements. A full scale CAD model of the HUT and PLSS with optimal dimensions of compatibility for the Spacesuit with the suitport is designed as a result of this study. Static load bearing analysis is performed to validate the feasibility of the structure and make suitabe recommendations for choice of materials. Methods for further improvement for rear entry suit development are outlined.

  • 193.
    Jerker, Bergström
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Path Planning with Weighted Wall Regions using OctoMap2018Independent thesis Advanced level (degree of Master (Two Years)), 20 poäng / 30 hpOppgave
    Abstract [en]

    In the work of the Control Engineering research group of the Department of Computer Science, Electrical and Space Engineering, Signals and systems at Luleå University of Technology a need had arisen for a path planning algorithm. The ongoing research with Unmanned Aerial Vehicles(UAVs) had so far been done with any complicated paths being created manually with waypoints set by the uses. To remove this labourious part of the experimental process a path should be generated automatically by simply providing a program with the position of the UAV, the goal to which the user wants it to move, as well as information about the UAV's surroundings in the form of a 3D map.In addition to simply finding an available path through a  3D environment the path should also be adapted to the risks that the physical environment poses to a flying robot. This was achieved by adapting a previously developed algorithm, which did the simple path planning task well, by adding a penalty weight to areas near obstacles, pushing the generated path away from them.The planner was developed working with the OctoMap map system which represents the physical world by segmenting it into cubes of either open or occupied space. The open segments of these maps could then be used as vertices of a graph that the planning algorithm could traverse.The algorithm itself was written in C++ as a node of the Robot Operating System(ROS) software framework to allow it to smoothly interact with previously developed software used by the Control Engineering Robotics Group.The program was tested by simulations where the path planner ROS node was sent maps as well as UAV position and intended goal. These simulations provided valid paths, with the performance of the algorithm as well as the quality of the paths being evaluated for varying configurations of the planners parameters.The planner works well in simulation and is deemed ready for use in practical experiments.

  • 194.
    Jimenez, Carlos
    et al.
    Observatoire de Paris, Laboratoire d'Etudes de Rayonnement et de la Matire en Astro-physique, Centre National de la Recherche Scientifique, Paris.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Rydberg, Bengt
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    Eriksson, Patrick
    Chalmers University of Technology, Department of Radio and Space Science, Gothenburg.
    Evans, K. F.
    Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder.
    Performance simulations for a submillimetre-wave satellite instrument to measure cloud ice2007Inngår i: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 133, nr Suppl.2, s. 129-149Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The performance of a conically scanning satellite instrument for the measurement of cloud ice was studied. The instrument measures radiances in 12 channels placed around the 183, 325 and 448 GHz water vapour lines and the 243, 664 and 874 GHz window channels, and is designed to provide estimations of ice water path (IWP), the equivalent sphere diameter (DME), and the median ice mass height (ZME). Overall median relative errors of around 20% for IWP, 33 µm for DME, and 240 m for ZME for a midlatitude winter scenario, and 17% for IWP, 30 µm for DME, and 310 m for ZME for a tropical scenario were found. Detection limits (relative retrieval error reaching 100%) of around 2 gm-2 were estimated for both scenarios. The performance of a five-receiver instrument, where either the 664 or 874 GHz channel is dropped, was close, but with increased errors for very thin and high clouds. A trade-off between having the 874 GHz receiver or two infrared channels at 10.7 and 12 µm emerged, as very similar performance was found between the six-receiver instrument and the five-receiver instrument with the infrared channels. Another trade-off between receiver selection and noise was also apparent, with some of the four-receiver selections operating at half noise levels being able to compete with the standard six-receiver instrument. Dual-polarized measurements were also tested, but they did not significantly improve the retrievals of IWP or DME.

  • 195. Jiménez, Carlos J.
    et al.
    Gulkis, Samuel L.
    Beaudin, Gérard
    Encrenaz, Th. H.
    Eriksson, Patrick
    Chalmers University of Technology.
    Kamp, Lucas W.
    Lee, S.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Sub-millimeter observations of the terrestrial atmosphere during an Earth flyby of the MIRO sounder on the Rosetta spacecraft2013Inngår i: Planetary and Space Science, ISSN 0032-0633, E-ISSN 1873-5088, Vol. 82-83, s. 99-112Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Sub-millimeter spectra recorded by the MIRO sounder aboard the Rosetta spacecraft have been used at the time of an Earth flyby (November 2007) to check the consistency and validity of the instrumental data. High-resolution spectroscopic data were recorded in 8 channels in the vicinity of the strong water line at 557 GHz, and in a broad band continuum channel at 570 GHz. An atmospheric radiative transfer code (ARTS) and standard terrestrial atmospheres have been used to simulate the expected observational results. Differences with the MIRO spectra suggest an anomaly in the behavior of four spectroscopic channels. Further technical investigations have shown that a large part of the anomalies are associated with an instability of one of the amplifiers. The quality of the MIRO data has been further tested by inverting the spectra with an atmospheric inversion tool (Qpack) in order to derive a mesospheric temperature profile. The retrieved profile is in good agreement with the one inferred from the Earth Observing System Microwave Limb Sounder (EOS-MLS). This work illustrates the interest of validating instruments aboard planetary or cometary spacecraft by using data acquired during Earth flybys.

  • 196.
    Johansson, Mattias
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Evaluation of Sensor Solutions & Motor Speed Control Methods for BLDCM/PMSM in Aerospace Applications2017Independent thesis Advanced level (degree of Master (Two Years)), 20 poäng / 30 hpOppgave
    Abstract [en]

    The goal of this thesis was to evaluate sensors and motor speed control methods for BLDC/PMSM motors in Aerospace applications. The sensors and methods were evaluated by considering accuracy, robustness, cost, development gain and parameter sensitivity. The sensors and methods chosen to simulate were digital Hall sensors and sensorless control of BLDC motors. Using Matlab Simulink/Simscape some motor speed control methods and motor speed estimation methods were simulated using the Hall sensors and sensorless control as a basis. It was found that the sensorless control methods for BLDC motors couldn't estimate the speed accurately during dynamic loads and that the most robust and accurate solution based on the simulations was using the digital Hall sensors for both speed estimation and commutation and this was tested on a hardware setup.

  • 197.
    John, V. O.
    et al.
    University of Miami.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Kuvatov, M.
    Universität Bremen.
    Soden, B. J.
    University of Miami.
    Sreerekha, T. R.
    Met Office Hadley Centre, Exeter.
    Toward a long-term homogenized UTH data set derived from satellite microwave measurements2006Inngår i: Microwave remote sensing of the atmosphere and environment. V. / [ed] Azita Valinia; Seiho Uratsuka; Tapan Misra, Bellingham, Wash: SPIE - International Society for Optical Engineering, 2006, s. 64100K-Konferansepaper (Fagfellevurdert)
    Abstract [en]

    This article presents some ideas and issues related to the creation of a long-term upper tropospheric humidity (UTH) data set using satellite based microwave measurements. Polar orbiting satellites have been measuring UTH for more than a decade now. There are three microwave instruments which can measure UTH from Space: Special Sensor Microwave/Temperature-2 (SSM/T2), Advanced Microwave Sounding Unit-B (AMSU-B), and Microwave Humidity Sounder (MHS). These instruments have channels at 183.31±1.00GHz which are sensitive to UTH. Retrieval of UTH and cloud issues are discussed in detail. Advantage of microwave measurements of UTH over infrared measurements are demonstrated. Preliminary results on the inter-calibration of these instruments are also shown.

  • 198.
    John, Viju O.
    et al.
    Met Office Hadley Centre, Exeter.
    Allan, Richard P.
    Department of Meteorology, University of Reading.
    Bell, William
    Met Office Hadley Centre, Exeter.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Kottayil, Ajil
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Assessment of intercalibration methods for satellite microwave humidity sounders2013Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 118, nr 10, s. 4906-4918Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Three methods for ntercalibrating humidity sounding channels are compared to assess their merits and demerits. The methods use the following: (1) natural targets (Antarctica and tropical oceans), (2) zonal average brightness temperatures, and (3) simultaneous nadir overpasses (SNOs). Advanced Microwave Sounding Unit-B instruments onboard the polar-orbiting NOAA 15 and NOAA 16 satellites are used as examples. Antarctica is shown to be useful for identifying some of the instrument problems but less promising for intercalibrating humidity sounders due to the large diurnal variations there. Owing to smaller diurnal cycles over tropical oceans, these are found to be a good target for estimating intersatellite biases. Estimated biases are more resistant to diurnal differences when data from ascending and descending passes are combined. Biases estimated from zonal-averaged brightness temperatures show large seasonal and latitude dependence which could have resulted from diurnal cycle aliasing and scene-radiance dependence of the biases. This method may not be the best for channels with significant surface contributions. We have also tested the impact of clouds on the estimated biases and found that it is not significant, at least for tropical ocean estimates. Biases estimated from SNOs are the least influenced by diurnal cycle aliasing and cloud impacts. However, SNOs cover only relatively small part of the dynamic range of observed brightness temperatures

  • 199.
    John, Viju O
    et al.
    Met Office Hadley Centre, Exeter.
    Holl, Gerrit
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Allen, Richard P.
    Department of Meteorology, University of Reading.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Parker, David E.
    Met Office Hadley Centre, Exeter.
    Soden, Brian J.
    Rosenstiel School of Marine and Atmospheric Science, University of Miami.
    Clear-sky biases in satellite infrared estimates of upper tropospheric humidity and its trends2011Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 116Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We use microwave retrievals of upper tropospheric humidity (UTH) to estimate the impact of clear-sky-only sampling by infrared instruments on the distribution, variability, and trends in UTH. Our method isolates the impact of the clear-sky-only sampling, without convolving errors from other sources. On daily time scales, IR-sampled UTH contains large data gaps in convectively active areas, with only about 20-30 % of the tropics (30 degrees S-30 degrees N) being sampled. This results in a dry bias of about -9 % RH in the area-weighted tropical daily UTH time series. On monthly scales, maximum clear-sky bias (CSB) is up to -30 % RH over convectively active areas. The magnitude of CSB shows significant correlations with UTH itself (-0.5) and also with the variability in UTH (-0.6). We also show that IR-sampled UTH time series have higher interannual variability and smaller trends compared to microwave sampling. We argue that a significant part of the smaller trend results from the contrasting influence of diurnal drift in the satellite measurements on the wet and dry regions of the tropics.

  • 200.
    John, Viju O.
    et al.
    Met Office Hadley Centre, Exeter.
    Holl, Gerrit
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Atkinson, Nigel
    Met Office Hadley Centre, Exeter.
    Buehler, Stefan
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.
    Monitoring scan asymmetry of microwave humidity sounding channels using simultaneous all angle collocations (SAACs)2013Inngår i: Journal of Geophysical Research: Atmospheres, ISSN 2169-8996, Vol. 118, nr 3, s. 1536-1545Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Simultaneous all angle collocations (SAACs) of microwave humidity sounders (AMSU-B and MHS) on-board polar orbiting satellites are used to estimate scan-dependent biases. This method has distinct advantages over previous methods, such as that the estimated scan-dependent biases are not influenced by diurnal differences between the edges of the scan and the biases can be estimated for both sides of the scan. We find the results are robust in the sense that biases estimated for one satellite pair can be reproduced by double differencing biases of these satellites with a third satellite. Channel 1 of these instruments shows the least bias for all satellites. Channel 2 has biases greater than 5 K, thus needs to be corrected. Channel 3 has biases of about 2 K and more and they are time varying for some of the satellites. Channel 4 has the largest bias which is about 15 K when the data are averaged for 5 years, but biases of individual months can be as large as 30 K. Channel 5 also has large and time varying biases for two of the AMSU-Bs. NOAA-15 (N15) channels are found to be affected the most, mainly due to radio frequency interference (RFI) from onboard data transmitters. Channel 4 of N15 shows the largest and time varying biases, so data of this channel should only be used with caution for climate applications. The two MHS instruments show the best agreement for all channels. Our estimates may be used to correct for scan-dependent biases of these instruments, or at least used as a guideline for excluding channels with large scan asymmetries from scientific analyses.

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