In view of the problems such as the lower automation level and the insufficient precision of the traditional fiber optic gyroscope （FOG） static north-finder, this paper focuses on the in-depth analysis of the FOG dy...In view of the problems such as the lower automation level and the insufficient precision of the traditional fiber optic gyroscope （FOG） static north-finder, this paper focuses on the in-depth analysis of the FOG dynamic north-finder principle and algorithm. The simulation model of the FOG dynamic north found algorithm with the least square method by points is established using Simulink toolbox, and then the platform rotation speed and sampling frequency, which affect FOG dynamic north found precision obviously, are simulated and calculated, and the optimization analysis is carried out as a key consideration. The simulation results show that, when the platform rotation speed is between 4.5°/s and 8.5°/s and the sampling frequency is at about 50Hz in the case of using the parameters of this paper, the FOG dynamic north finding system can reach the higher precision. And the conclusions can provide the reference and validation for the engineering and practical of FOG dynamic north-finder.展开更多
Using the NASA Earth Exchange platform,the North American Forest Dynamics(NAFD)project mapped forest history wall-to-wall,annually for the contiguous US(1986–2010)using the Vegetation Change Tracker algorithm.As with...Using the NASA Earth Exchange platform,the North American Forest Dynamics(NAFD)project mapped forest history wall-to-wall,annually for the contiguous US(1986–2010)using the Vegetation Change Tracker algorithm.As with any effort to identify real changes in remotely sensed time-series,data gaps,shifts in seasonality,misregistration,inconsistent radiometry and cloud contamination can be sources of error.We discuss the NAFD image selection and processing stream(NISPS)that was designed to minimize these sources of error.The NISPS image quality assessments highlighted issues with the Landsat archive and metadata including inadequate georegistration,unreliability of the pre-2009 L5 cloud cover assessments algorithm,missing growing-season imagery and paucity of clear views.Assessment maps of Landsat 5–7 image quantities and qualities are presented that offer novel perspectives on the growing-season archive considered for this study.Over 150,000+Landsat images were considered for the NAFD project.Optimally,one high quality cloud-free image in each year or a total of 12,152 images would be used.However,to accommodate data gaps and cloud/shadow contamination 23,338 images were needed.In 220 specific path-row image years no acceptable images were found resulting in data gaps in the annual national map products.展开更多
Among 33 cold frontal systems passed over the Beijing-Tianjin-Tangshan area from May to August during 1963—1979,there existed mesoscale rainbands in the warm sectors ahead of 31 cold fronts.These rainbands, according...Among 33 cold frontal systems passed over the Beijing-Tianjin-Tangshan area from May to August during 1963—1979,there existed mesoscale rainbands in the warm sectors ahead of 31 cold fronts.These rainbands, according to their orientation,movement,formation and development,can be divided into three types:parallel, angulate and perpendicular type.The basic characteristics,sources and propagation of the three types of rainbands and the severe convective weather associated with the bands are presented.The possible mechanis- ms responsible for the formation and development of the bands are also briefly discussed.展开更多
文摘In view of the problems such as the lower automation level and the insufficient precision of the traditional fiber optic gyroscope （FOG） static north-finder, this paper focuses on the in-depth analysis of the FOG dynamic north-finder principle and algorithm. The simulation model of the FOG dynamic north found algorithm with the least square method by points is established using Simulink toolbox, and then the platform rotation speed and sampling frequency, which affect FOG dynamic north found precision obviously, are simulated and calculated, and the optimization analysis is carried out as a key consideration. The simulation results show that, when the platform rotation speed is between 4.5°/s and 8.5°/s and the sampling frequency is at about 50Hz in the case of using the parameters of this paper, the FOG dynamic north finding system can reach the higher precision. And the conclusions can provide the reference and validation for the engineering and practical of FOG dynamic north-finder.
基金contributes to the North American Carbon Program,with grant support from NASA’s Carbon Cycle Science and Applied Sciences Programs[NNX11AJ78G]Previous NASA NACP grants[NNG05GE55G][NNX08AI26G]were critical in developing the foundations of the current NISPS.
文摘Using the NASA Earth Exchange platform,the North American Forest Dynamics(NAFD)project mapped forest history wall-to-wall,annually for the contiguous US(1986–2010)using the Vegetation Change Tracker algorithm.As with any effort to identify real changes in remotely sensed time-series,data gaps,shifts in seasonality,misregistration,inconsistent radiometry and cloud contamination can be sources of error.We discuss the NAFD image selection and processing stream(NISPS)that was designed to minimize these sources of error.The NISPS image quality assessments highlighted issues with the Landsat archive and metadata including inadequate georegistration,unreliability of the pre-2009 L5 cloud cover assessments algorithm,missing growing-season imagery and paucity of clear views.Assessment maps of Landsat 5–7 image quantities and qualities are presented that offer novel perspectives on the growing-season archive considered for this study.Over 150,000+Landsat images were considered for the NAFD project.Optimally,one high quality cloud-free image in each year or a total of 12,152 images would be used.However,to accommodate data gaps and cloud/shadow contamination 23,338 images were needed.In 220 specific path-row image years no acceptable images were found resulting in data gaps in the annual national map products.
文摘Among 33 cold frontal systems passed over the Beijing-Tianjin-Tangshan area from May to August during 1963—1979,there existed mesoscale rainbands in the warm sectors ahead of 31 cold fronts.These rainbands, according to their orientation,movement,formation and development,can be divided into three types:parallel, angulate and perpendicular type.The basic characteristics,sources and propagation of the three types of rainbands and the severe convective weather associated with the bands are presented.The possible mechanis- ms responsible for the formation and development of the bands are also briefly discussed.
