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  • Other Sources  (4)
  • 1980-1984  (4)
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  • 1
    Publication Date: 2019-06-28
    Description: This simulation study evaluates the response of a 21-cm radiometer, measuring the radiation emitted by a bare soil, to varying accumulations of rain. It is shown that correlations between the decrease in emissivity after a rain storm and the total accumulation depend strongly on the physical characteristics of the soil which affect its capacity to hold water. These are primarily soil texture and pre-rain soil moisture. A method is also discussed which would use the numerical models with remotely sensed microwave brightness and surface physical temperatures, along with conventional weather data, to estimate the total accumulation.
    Keywords: EARTH RESOURCES AND REMOTE SENSING
    Type: IEEE Transactions on Geoscience and Remote Sensing (ISSN 0196-2892); GE-22; 415-423
    Format: text
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  • 2
    Publication Date: 2019-06-28
    Description: Analytical formulas are derived for orbit-averaged behavior of magnetic control laws for unloading the excess angular momentum of a spacecraft reaction wheel control system in the presence of secular environmental torques. The specific example of an axially symmetric spacecraft with an inertially fixed attitude for which the dominant environmental torque is the gravity-gradient torque is treated in detail, but extensions of the general approach to other inertially fixed and earth-pointing spacecraft are discussed. The analytical formulas are compared to detailed simulations performed for the Solar Maximum Mission spacecraft, and agreement to within 10% is found. The analytical formulas can be used in place of detailed simulations for preliminary studies, and can be used to find selected cases giving the most stringent tests of momentum unloading capability for which detailed simulations may be performed.
    Keywords: SPACECRAFT DESIGN, TESTING AND PERFORMANCE
    Type: Journal of Guidance and Control; 3; Nov
    Format: text
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  • 3
    Publication Date: 2019-06-28
    Description: An energy and moisture balance model of the soil surface was used to estimate daily evaporation from wheat and barley fields in West Germany. The model was calibrated using remotely sensed surface temperature estimates. Complete atmospheric boundary layer models are difficult to use because of the number of parameters involved and a simplified model was used here. The resultant evaporation estimates were compared to eddy correlation evaporation estimates and good agreement was found.
    Keywords: EARTH RESOURCES AND REMOTE SENSING
    Type: Advances in Space Research (ISSN 0273-1177); 4; 11 1
    Format: text
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  • 4
    Publication Date: 2019-06-28
    Description: A computer program that simulates the flow of heat and moisture in soils is described. The space-time dependence of temperature and moisture content is described by a set of diffusion-type partial differential equations. The simulator uses a predictor/corrector to numerically integrate them, giving wetness and temperature profiles as a function of time. The simulator was used to generate solutions to diffusion-type partial differential equations for which analytical solutions are known. These equations include both constant and variable diffusivities, and both flux and constant concentration boundary conditions. In all cases, the simulated and analytic solutions agreed to within the error bounds which were imposed on the integrator. Simulations of heat and moisture flow under actual field conditions were also performed. Ground truth data were used for the boundary conditions and soil transport properties. The qualitative agreement between simulated and measured profiles is an indication that the model equations are reasonably accurate representations of the physical processes involved.
    Keywords: EARTH RESOURCES AND REMOTE SENSING
    Type: E83-10342 , NASA-TM-82121 , SM-G1-04086 , NAS 1.15:82121
    Format: application/pdf
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