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  • Spacecraft Design, Testing and Performance  (2)
  • damage  (1)
  • 1
    Electronic Resource
    Electronic Resource
    Evaluation of anti-sheep tree-stem-protection products in silvopastoral agroforestry (1996)
    Springer
    Agroforestry systems 34 (1996), S. 259-264 
    Details
    ISSN: 1572-9680
    Keywords: Acer pseudoplatanus ; damage ; Fraxinus excelsior ; herbivory ; plastic tree-shelters
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Abstract Direct herbivore damage to trees in temperate silvopastoral agroforestry systems is a major management consideration. Individual plastic tree-shelters may sometimes cause poor tree development and they have limited life span. Therefore, some alternative tree-shelter products were tested on four year old Acer pseudoplatanus and Fraxinus excelsior. Damage to the main stem was assessed by determining the area of bark removed by sheep browsing. Wobra®, a paint-on abrasive substance applied to the main stem, was effective in reducing sheep damage. However, some damage did occur due to breach of the protective barrier as a result of stem diameter increases and its removal through rubbing by animals. Anipel®, a systemic bittering agent supplied through the root system, was not effective in preventing significant damage compared to no-treatment controls, possibly because of poor uptake of the active ingredient into the tree. Within treatments, there were significant differences in damage incurred. Most notably Fraxinus suffered more damage than Acer, an effect which may be related to differences in tree size and/or relative palatability.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00046926
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    Paper (German National Licenses)
    Fulltext
  • 2
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    Spaceborne Autonomous and Ground Based Relative Orbit Control for the TerraSAR-X/TanDEM-X Formation (2007)
    In:  CASI
    Details
    Publication Date: 2018-06-06
    Description: TerraSAR-X (TSX) and TanDEM-X (TDX) are two advanced synthetic aperture radar (SAR) satellites flying in formation. SAR interferometry allows a high resolution imaging of the Earth by processing SAR images obtained from two slightly different orbits. TSX operates as a repeat-pass interferometer in the first phase of its lifetime and will be supplemented after two years by TDX in order to produce digital elevation models (DEM) with unprecedented accuracy. Such a flying formation makes indeed possible a simultaneous interferometric data acquisition characterized by highly flexible baselines with range of variations between a few hundreds meters and several kilometers [1]. TSX has been successfully launched on the 15th of June, 2007. TDX is expected to be launched on the 31st of May, 2009. A safe and robust maintenance of the formation is based on the concept of relative eccentricity/inclination (e/i) vector separation whose efficiency has already been demonstrated during the Gravity Recovery and Climate Experiment (GRACE) [2]. Here, the satellite relative motion is parameterized by mean of relative orbit elements and the key idea is to align the relative eccentricity and inclination vectors to minimize the hazard of a collision. Previous studies have already shown the pertinence of this concept and have described the way of controlling the formation using an impulsive deterministic control law [3]. Despite the completely different relative orbit control requirements, the same approach can be applied to the TSX/TDX formation. The task of TDX is to maintain the close formation configuration by actively controlling its relative motion with respect to TSX, the leader of the formation. TDX must replicate the absolute orbit keeping maneuvers executed by TSX and also compensate the natural deviation of the relative e/i vectors. In fact the relative orbital elements of the formation tend to drift because of the secular non-keplerian perturbations acting on both satellites. The goal of the ground segment is thus to regularly correct this configuration by performing small orbit correction maneuvers on TDX. The ground station contacts are limited due to the geographic position of the station and the costs for contact time. Only with a polar ground station a contact visibility is possible every orbit for LEO satellites. TSX and TDX use only the Weilheim ground station (in the southern part of Germany) during routine operations. This station allows two scheduled contact per day for the nominal orbit configuration, meaning that the satellite conditions can be checked with an interval of 12 hours. While this limitation is usually not critical for single satellite operations, the visibility constraints drive the achievable orbit control accuracy for a LEO formation if a ground based approach is chosen. Along-track position uncertainties and maneuver execution errors affect the relative motion and can be compensated only after a ground station contact.
    Keywords: Spacecraft Design, Testing and Performance
    Type: Proceedings of the 20th International Symposium on Space Flight Dynamics; NASA/CP-2007-214158
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 3
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    First Results from a Hardware-in-the-Loop Demonstration of Closed-Loop Autonomous Formation Flying (2003)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: A closed-loop system for the demonstration of formation flying technologies has been developed at NASA s Goddard Space Flight Center. Making use of a GPS signal simulator with a dual radio frequency outlet, the system includes two GPS space receivers as well as a powerful onboard navigation processor dedicated to the GPS-based guidance, navigation, and control of a satellite formation in real-time. The closed-loop system allows realistic simulations of autonomous formation flying scenarios, enabling research in the fields of tracking and orbit control strategies for a wide range of applications. A sample scenario has been set up where the autonomous transition of a satellite formation from an initial along-track separation of 800 m to a final distance of 100 m has been demonstrated. As a result, a typical control accuracy of about 5 m has been achieved which proves the applicability of autonomous formation flying techniques to formations of satellites as close as 50 m.
    Keywords: Spacecraft Design, Testing and Performance
    Type: 26th Annual AAS Guidance and Control Conference; Feb 05, 2003 - Feb 09, 2003; Breckenridge, CO; United States
    Format: text
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    NASA TECHNICAL REPORTS
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