The INTELSAT VII/VIIA generation of global communications satellites

Developed primarily to provide continuation of services from the INTELSAT V satellites in the Pacific Ocean Region, this latest generation of satellites also has utility for operation in other roles. Several new technologies are combined to give cost-effective international satellite communications on a global, regional and domestic basis. This new higher power satellite will form a fundamental part of the INTELSAT global network, having been designed to match the projected traffic demands and future service requirements.     

Stability analysis of an adaptive packet access scheme for mobile communication systems with high propagation delays

In this paper, we investigate a packet access scheme that is able to support mixed traffics in the presence of high propagation delays. Referring to a Time‐Code Division Multiple Access air interface, we propose a Medium Access Control (MAC) protocol based on a random access scheme. A successful attempt grants the use of a slot‐code resource. This protocol is named Adaptive Time Code‐Packet Reservation Multiple Access (ATC‐PRMA), since the access parameters are changed, depending on the traffic load conditions, so as to fulfil Quality of Service requirements. Numerical examples are carried out for the Low Earth Orbit (LEO)‐Mobile Satellite System (MSS) scenario, but all these considerations could be applied to High‐Altitude Platform Stations (HAPSs) as well. In both cases, high propagation delays prevent an immediate feedback to users. An analytical approach is proposed to study the stability of our MAC scheme. Accordingly, we define a criterion for optimizing system performance. The predicted ATC‐PRMA behaviour is supported by simulation results. Finally, we show the performance improvement of ATC‐PRMA with respect to a MAC protocol not employing adaptive parameters. Copyright © 2003 John Wiley & Sons, Ltd.     

地球静止轨道卫星在水利行业中应用综述

首先列举了涉及水利相关静止卫星的发展状况,阐述了静止卫星的发展历程,并从探测器精度、覆盖范围、成像时间、数据传输方式和姿态控制方式等方面对各国主流静止卫星进行了参数对比,指出静止卫星具有连续、同步、大尺度、多角度观测地球的能力,使得静止卫星不仅用于天气预报、气候预测等气象领域,还可以满足众多非气象领域包括水利行业的应用需求。其次,综述了静止卫星在水利行业中的应用领域和国内外的应用现状,分析了静止卫星水利相关应用参数,列举了静止卫星在水文监测、水旱灾害监测和水情传输中的应用实例,并进行了静止卫星在水利行业可用性分析,指出由于卫星传感器性能和在水利领域应用技术水平的限制是造成目前静止卫星在水利行业应用的业务水平不高的原因,需要加大稳定、高轨高精度和多种荷载传感器的研发投入。最后,对未来发展方向提出了设想,认为提高数据处理、同化技术水平,加强进入水利行业的应用能力,可以提高静止卫星数据在水利行业的业务化应用水平。     

Satellite-Based Energy Balance to Assess Within-Population Variance of Crop Coefficient Curves

Quantifying evapotranspiration (ET) from agricultural fields is important for field water management, water resources planning, and water regulation. Traditionally, ET from agricultural fields has been estimated by multiplying the weather-based reference ET by crop coefficients (Kc) determined according to the crop type and the crop growth stage. Recent development of satellite remote sensing ET models has enabled us to estimate ET and Kc for large populations of fields. This study evaluated the distribution of Kc over space and time for a large number of individual fields by crop type using ET maps created by a satellite based energy balance (EB) model. Variation of Kc curves was found to be substantially larger than that for the normalized difference vegetation index because of the impacts of random wetting events on Kc, especially during initial and development growth stages. Two traditional Kc curves that are widely used in Idaho for crop management and water rights regulation were compared against the satellite-derived Kc curves. Simple adjustment of the traditional Kc curves by shifting dates for emergence, effective full cover, and termination enabled the traditional curves to better fit Kc curves as determined by the EB model. Applicability of the presented techniques in humid regions having higher chances of cloudy dates was discussed.     

Real-Time Natural Disasters Detection and Monitoring from Smart Earth Observation Satellite

Current remote sensing satellites with multispectral sensors capture high-resolution images and produce vast quantities of data. The size and volume of this information has dramatically increased in the last decade as sensor resolution and capabilities have significantly improved, without a similar improvement on the satellite system capacity to accommodate these changes. Remote sensing satellites currently operate on a “store and forward” paradigm, where data is stored on the satellite until the satellite is in view of the ground station. Low Earth orbit satellites may only see a ground station for a 10–15 min window per pass, in which time all the collected information must be telemetered to the ground. This process requires large and expensive onboard storage resources and places tremendous stress on communication channels. Hence, a complete image may not be successfully telemetered in one pass causing a significant delay between capture and analysis and limiting the benefits of these images. Smart satellites are more technologically advanced, require less ground station support and data storage, and are capable of transmitting required information quickly and easily to ground stations. With onboard reconfigurable data processing, these satellites have faster data product turnaround, less communication requirements, and provide more useful information. The high performance computing (HPC-I) payload on board the Australian satellite FedSat, launched in December 2002, is a demonstration device of the feasibility of reconfigurable computing technology in space. This device is small in size, requires low power, and has the processing capacity to handle large data volumes. Using this device in conjunction with a high-resolution imaging sensor, such as the bispectral infrared detection (BIRD) sensor, smart dedicated satellites become a feasible and cost effective solution to remote sensing needs. This paper elaborates on the system level design of a real-time fire observation system in the context of a smart satellite mission for detecting and monitoring natural disasters. The proposed system is built upon flight tested field programmable gate arrays based HPC-I technology, and would be capable of producing useful information about natural disasters directly broadcasted to interested parties within rapid timeframes. The algorithms for onboard real-time detection of direction, intensity, and location of fires are discussed, and reliable algorithms for detecting and verifying these fires using smoke plume detection are presented. Further work is described including fire-front analysis and the tracking of fire movement.     

μ Control for Satellites Formation Flying

In this paper, a μ controller is designed for a satellite formation flying system around the Earth based on an uncertainty model derived from a nonlinear relative position equation. In this model, nonzero eccentricity and varying semimajor axis are included as parametric uncertainties. J2 perturbation, atmospheric drag, and actuation and sensor noise are bounded by functional uncertainties. The μ controller design based on the nominal mission (an 800?km altitude circular reference orbit) is capable of achieving desired performance, is robust to uncertainties, and satisfies fuel consumption requirements even in a challenge nonnominal mission (a 0.1 eccentricity and 7,978?km semimajor axis elliptic reference orbit) with the same control gain. In this nonnominal mission, the designed μ controller is able to keep formation with almost the same level of the ΔV budget (43.86?m/s/year) as used in the nominal mission (39.65?m/s/year). For comparison, linear quadratic regulator (LQR) and sliding mode controllers (SMC) are developed and extensively tuned to get the same ΔV consumption as that of the designed μ controller for the nominal mission. However, as shown in the simulation, the designed linear robust controller (LQR) and nonlinear robust controller (SMC) have a serious ΔV consumption penalty (1.72?km/s/year for SMC) or are unstable (for LQR) in the nonnominal mission.     

卫星角反射器的设计

以角反射器远场衍射理论为依据,提出了利用角反射器参数补偿速差的技术方案。即通过改变角反射器参数的大小,使得接收光斑的极值中心刚好回到测站位置,以弥补速差效应的影响。采用角反射器衍射光学理论与采用传统理论所得的卫星角反射器补偿角之间存在差异,这种差异随着轨道高度的不同而发生变化。以德国Champ卫星上角反射器为例,运用角反射器衍射光学理论进行了设计,设计所得的单角误差-3.70″与Champ卫星提供的-3.8″非常相近,而且按两者结果模拟得到的接收强度仅相差0.6%,从而验证了设计方法的正确性和可靠性。     

天文学在卫星技术中的应用

天文学在卫星技术中的应用越来越广泛．针对卫星目标识别技术,本文详细探讨了该项技术的最一般问题：坐标系的转换和星对的角距,并概述其实现方法以及试验结果分析,通过实验证明了该方法在卫星的空中定态技术中是很有效的。     

变粒度搜索算法与整周模糊度解算

LAMBDA方法应用于GNSS单频单点载波相位定位时,其整周模糊度的搜索域过大,解算速度过慢,制约了单频单点精确定位技术的应用。针对这一问题,结合粗糙整数映射理论,提出变粒度搜索算法,通过改善整周模糊度搜索区域的拓扑结构,并采用有粒度的粗糙整数作为搜索单元,可有效提高搜索效率。仿真结果表明,基本实现了单频单点载波相位准实时定位。     

国际新一代对地观测系统在气溶胶遥感方面的应用

大气气溶胶在很多生物地球化学循环中具有重要作用, 但是由于它的来源广泛并且具有很大的时空变化, 难以在全球范围内精确、实时确定气溶胶的性质、组成及时空分布, 因而对大气气溶胶的研究依赖于监测手段的发展。对大气气溶胶实验观测的观测方法和测量仪器有很多, 而对地观测卫星利用其特有的优势, 可以实现对气溶胶的大范围、实时探测, 促进了大气气溶胶这一边缘学科的研究发展。简要回顾了对地观测系统( Ear th Observ ing System) 的发展历程, 然后介绍了对地观测卫星遥感气溶胶的发展及现状, 对国际新一代对地观测系统在气溶胶遥感方面的应用现状做了详细的描述和总结。