Logica

About: Logica is a based out in . It is known for research contribution in the topics: Formal specification & Enterprise architecture. The organization has 455 authors who have published 508 publications receiving 7126 citations. The organization is also known as: Logica plc.

Voyager 2 at neptune: imaging science results.

Abstract: Voyager 2 images of Neptune reveal a windy planet characterized by bright clouds of methane ice suspended in an exceptionally clear atmosphere above a lower deck of hydrogen sulfide or ammonia ices. Neptune's atmosphere is dominated by a large anticyclonic storm system that has been named the Great Dark Spot (GDS). About the same size as Earth in extent, the GDS bears both many similarities and some differences to the Great Red Spot of Jupiter. Neptune's zonal wind profile is remarkably similar to that of Uranus. Neptune has three major rings at radii of 42,000, 53,000, and 63,000 kilometers. The outer ring contains three higher density arc-like segments that were apparently responsible for most of the ground-based occultation events observed during the current decade. Like the rings of Uranus, the Neptune rings are composed of very dark material; unlike that of Uranus, the Neptune system is very dusty. Six new regular satellites were found, with dark surfaces and radii ranging from 200 to 25 kilometers. All lie inside the orbit of Triton and the inner four are located within the ring system. Triton is seen to be a differentiated body, with a radius of 1350 kilometers and a density of 2.1 grams per cubic centimeter; it exhibits clear evidence of early episodes of surface melting. A now rigid crust of what is probably water ice is overlain with a brilliant coating of nitrogen frost, slightly darkened and reddened with organic polymer material. Streaks of organic polymer suggest seasonal winds strong enough to move particles of micrometer size or larger, once they become airborne. At least two active plumes were seen, carrying dark material 8 kilometers above the surface before being transported downstream by high level winds. The plumes may be driven by solar heating and the subsequent violent vaporization of subsurface nitrogen.

The use of multi-temporal NDVI measurements from AVHRR data for crop yield estimation and prediction

Abstract: This paper investigates the potential of using vegetation index profiles from AVHRR data to monitor crop yield. A retrospective study and predictive study is presented, for 1986-1988 and 1989 respectively, for an area in Northern Greece. Results are encouraging for operational crop monitoring. Yield for wheat, cotton, rice and maize crops has been estimated to a high degree of accuracy using a simple linear relationship between NDVI and yield. However input from an agro-meteorological model is recommended to modify the model during the grain-filling period of the wheat crop. The estimates stabilize 50-100 days prior to harvest enabling an early assessment of crop yield to be made.

Linear mixture modelling applied to AVHRR data for crop area estimation

Abstract: A technique for estimating crop coverage using linear mixture modelling of multi-temporal Advanced Very High Resolution Radiometer (AVHRR) data is presented for a study area in northern Greece. This paper identifies some of the problems associated with using satellite sensor data with coarse spatial resolution for crop area estimation. Using satellite sensor imagery with a high spatial resolution to extrapolate ground measurements to AVHRR scales, the paper shows how the mixture model can be applied to AVHRR data in a mixed agricultural system. Crop areas are estimated to an average accuracy of 89 percent on regional scale using this technique. The results show that this linear mixture modelling has potential for operational crop area monitoring on a regional basis.

Solar sail parking in restricted three-body systems

Abstract: Stationary solutions to the restricted three-body problem for solar sail spacecraft in the Earth-sun and Earth-moon systems are investigated. It is found that the usual five Lagrange points are extended to a continuum of new artificial points that form level surfaces parameterized by the sail mass per unit area. Analytic expressions for the sail mass per unit area and the sail attitude required for these stationary solutions are obtained and the stability of the solutions examined. It is found that although in general the solutions are unstable, a simple closed-loop control scheme may be developed to ensure asymptotic stability.