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81.
The authors present a new scientific space mission consisting of a satellite carrying a receiving- only SAR which receives the signal transmitted by the ENVISAT-1 SAR. The integration of ENVISAT-1 SAR and bistatic radar data offers an improved potentiality of surface classification, three-dimensional observation, and the opportunity of advanced scientific experiments in the field of bistatic scattering. The small satellite nominal orbit and the attitude manoeuvres are designed in order to maintain an adequate overlap between the two radar swaths along the whole orbit, taking into account the ENVISAT-1 attitude and pointing. A preliminary satellite design (2-year lifetime) is then performed to evaluate the orbit decay and to determine the appropriate orbit manoeuvres (every 4 days) to control the satellites relative phase. The numerical simulation shows that a spacecraft of about 584kg is able to meet the mission requirements. 相似文献
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In extension to common applications such as groundtrack displays and antenna steering, the SGP4 orbit model is proposed for operational orbit determination in small satellite missions. SGP4 is an analytical orbit model for Low-Earth orbiting satellites that is widely used for the propagation of NORAD twoline elements. Twoline elements may hence be generated completely independent of NORAD. Their use as exclusive source of orbital information simplifies the operations concept and reduces mission costs through the extensive use of existing low-cost mission support software. Due to small computer resource requirements of 8–10kByte, the SGP4 model may also be applied for onboard orbit computations making use of e.g. a 80186 processor, thus ensuring full compatibility of ground-based and onboard operations. The proposed approach is particularly suited in combination with a space-borne GPS receiver, were the C/A-code navigation solutions are treated as measurements that are adjusted in a least-squares sense using the SGP4 model. As consequence, inherent drawbacks of the pure navigation solutions such as data gaps and scatter as well as limited velocity accuracy are avoided, while the operational navigation activities are kept at a minimum. The feasibility of the concept is illustrated based on real GPS navigation data from the TOPEX/Poseidon and the MIR space station with an inherent data quality of 50–100 m. It is shown that 3 hours of data within a 4 day period are sufficient to keep the position error within 4 km, that is considered sufficient for most applications. 相似文献
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陕西省复合型人才奇缺,尤其是特色经济和支柱产业人才奇缺,造成人才缺口原因虽然是多方面的,但是市场经济决定了人才走势。根据今年市场对人才的需求,向在校毕业生们提出了几点就业的建议。 相似文献
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基于X射线探伤机空气比释动能率测量的数学模型,对其测量不确定度进行了评定。 相似文献
88.
经过对误差分离技术理论进行了分析之后,又对误差补偿技术进行了研究,并建立了多点法测量形状误差的补偿控制系统。通过补偿主轴径向误差运动,提高了工件的圆度误差和圆柱度误差。 相似文献
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高职高专英语是指在生产、管理、技术和服务等各行业第一线工作的业务人员所需的英语。在理论上被界定为专门用途英语分支下,介于学术英语与职业英语之间的业务用途英语。本文作者主要从专门用途英语的两个核心特点(目标需求分析与学习需求分析)的角度对高职高专英语教学做了一项实证研究。 相似文献