Magnetosphere Imaging Instrument (MIMI) on the Cassini Mission to Saturn/Titan

The magnetospheric imaging instrument (MIMI) is a neutral and charged particle detection system on the Cassini orbiter spacecraft designed to perform both global imaging and in-situ measurements to study the overall configuration and dynamics of Saturn’s magnetosphere and its interactions with the solar wind, Saturn’s atmosphere, Titan, and the icy satellites. The processes responsible for Saturn’s aurora will be investigated; a search will be performed for substorms at Saturn; and the origins of magnetospheric hot plasmas will be determined. Further, the Jovian magnetosphere and Io torus will be imaged during Jupiter flyby. The investigative approach is twofold. (1) Perform remote sensing of the magnetospheric energetic (E > 7 keV) ion plasmas by detecting and imaging charge-exchange neutrals, created when magnetospheric ions capture electrons from ambient neutral gas. Such escaping neutrals were detected by the Voyager l spacecraft outside Saturn’s magnetosphere and can be used like photons to form images of the emitting regions, as has been demonstrated at Earth. (2) Determine through in-situ measurements the 3-D particle distribution functions including ion composition and charge states (E > 3 keV/e). The combination of in-situ measurements with global images, together with analysis and interpretation techniques that include direct “forward modeling’’ and deconvolution by tomography, is expected to yield a global assessment of magnetospheric structure and dynamics, including (a) magnetospheric ring currents and hot plasma populations, (b) magnetic field distortions, (c) electric field configuration, (d) particle injection boundaries associated with magnetic storms and substorms, and (e) the connection of the magnetosphere to ionospheric altitudes. Titan and its torus will stand out in energetic neutral images throughout the Cassini orbit, and thus serve as a continuous remote probe of ion flux variations near 20R _S (e.g., magnetopause crossings and substorm plasma injections). The Titan exosphere and its cometary interaction with magnetospheric plasmas will be imaged in detail on each flyby. The three principal sensors of MIMI consists of an ion and neutral camera (INCA), a charge–energy–mass-spectrometer (CHEMS) essentially identical to our instrument flown on the ISTP/Geotail spacecraft, and the low energy magnetospheric measurements system (LEMMS), an advanced design of one of our sensors flown on the Galileo spacecraft. The INCA head is a large geometry factor (G ∼ 2.4 cm² sr) foil time-of-flight (TOF) camera that separately registers the incident direction of either energetic neutral atoms (ENA) or ion species (≥5^∘ full width half maximum) over the range 7 keV/nuc < E < 3 MeV/nuc. CHEMS uses electrostatic deflection, TOF, and energy measurement to determine ion energy, charge state, mass, and 3-D anisotropy in the range 3 ≤ E ≤ 220 keV/e with good (∼0.05 cm² sr) sensitivity. LEMMS is a two-ended telescope that measures ions in the range 0.03 ≤ E ≤ 18 MeV and electrons 0.015 ≤ E≤ 0.884 MeV in the forward direction (G ∼ 0.02 cm² sr), while high energy electrons (0.1–5 MeV) and ions (1.6–160 MeV) are measured from the back direction (G ∼ 0.4 cm² sr). The latter are relevant to inner magnetosphere studies of diffusion processes and satellite microsignatures as well as cosmic ray albedo neutron decay (CRAND). Our analyses of Voyager energetic neutral particle and Lyman-α measurements show that INCA will provide statistically significant global magnetospheric images from a distance of ∼60 R _S every 2–3 h (every ∼10 min from ∼20 R _S). Moreover, during Titan flybys, INCA will provide images of the interaction of the Titan exosphere with the Saturn magnetosphere every 1.5 min. Time resolution for charged particle measurements can be < 0.1 s, which is more than adequate for microsignature studies. Data obtained during Venus-2 flyby and Earth swingby in June and August 1999, respectively, and Jupiter flyby in December 2000 to January 2001 show that the instrument is performing well, has made important and heretofore unobtainable measurements in interplanetary space at Jupiter, and will likely obtain high-quality data throughout each orbit of the Cassini mission at Saturn. Sample data from each of the three sensors during the August 18 Earth swingby are shown, including the first ENA image of part of the ring current obtained by an instrument specifically designed for this purpose. Similarily, measurements in cis-Jovian space include the first detailed charge state determination of Iogenic ions and several ENA images of that planet’s magnetosphere.This revised version was published online in July 2005 with a corrected cover date.     

ASTROSAT

The ASTROSAT satellite is an Indian National Space Observatory under development in India. Due for launch in 2010, ASTROSAT will carry a complement of five scientific instruments enabling simultaneous observations from the optical through to the hard X-ray energy band. This capability will enable broad-band spectroscopy and high time-resolution monitoring of both galactic and extra-galactic targets, such as X-ray binaries and AGN. One of the instruments is being built in collaboration with the Canadian Space Agency and another in collaboration with the University of Leicester. ASTROSAT also carries a scanning sky monitor to observe the variable X-ray sky. After an initial period of science verification and guaranteed time, a certain fraction of ASTROSAT observing time will also be made available to the community via a call for proposals. Here I summarise the instrument complement and principle scientific objectives of the mission.     

Studies of Comets in the Ultraviolet: The Past and the Future

The remote sensing of comets in the ultraviolet bandpass has been a valuable tool for studying the structure, composition, variability, and physical processes at work in cometary comae. By extension, these studies of comae have revealed key insights into the composition of cometary nuclei. Here we briefly review the ultraviolet studies of comets, and then take a look toward the future of such work as anticipated by the advent of several key new instruments. This revised version was published online in June 2006 with corrections to the Cover Date.     

夜气辉对天基光学碎片探测的影响分析

设计并完成了夜气辉背景下的天基碎片观测试验，获取了综合谱段的观测数据。基于观测数据进行夜气辉的辐射特性及其对天基碎片探测的影响分析。结果表明，夜气辉辐射随纬度和高度呈现一定的分布特点，这些特点均符合夜气辉的理论分布规律。在约90 km处辐射最强，亮度可达到单位积分时间每平方角秒近15星等，这对恒星及碎片提取概率影响较大，其中，13等星的提取概率仅为40.5%。     

基于天通卫星系统的天基测控技术

针对卫星测控资源紧缺的现状,为满足未来大规模卫星用户对测控资源的使用便利性这一需求,研究利用天通卫星进行近地低轨卫星测控服务的能力。天通作为高轨移动通信卫星系统,具有传输实时方便、服务安全可靠和综合效费比高等多方面的特点。首先,分析了天通卫星对不同倾角、轨道高度卫星的测控覆盖时长,以及对载人航天任务空间站和火箭进行测控的服务能力。然后,基于天通民用系统现有的系统架构,详细介绍了天通系统支持低轨测控的系统改造方案以及工作流程。最后,结合目前的技术发展趋势和天通卫星能力现状,描述了天通卫星在天基测控和低速中继两种任务场景中的应用前景。     

基于天基光学探测图像初析南大西洋异常区影响

文章针对目前在轨运行的高灵敏度探测类可见光遥感器经过南大西洋异常区（SouthAtlanticAnomaly,SAA）时出现的探测能力降低、目标无法识别等现象,通过卫星在轨获取的SAA天基探测图像研究图像中出现的异常现象机理;提出了基于CCD探测器固有暗像元区的图像特征变化开展SAA影响评估的方法,根据在轨图像暗像元区的均值和标准差、图像区的热像素及活像元等特征参数的统计分析,判断经过SAA过程中连续获取的图像中出现显著影响时的拐点,通过曲线拟合估算出该轨道高度的SAA空间分布特征和边界,依此进一步提出利用在轨天基探测图像给出测量影响该轨道高度卫星SAA区域边界的简单方法;最后,基于SAA对天基探测器件的影响分析结果和认识,给出了高灵敏度探测类卫星避免或消除SAA影响的改进措施和建议。文章采用的数据分析和评估方法为提升SAA对天基探测影响的认识,以及下一步有效开展空间碎片探测工作提供参考。     

具有外部扰动及不确定系统参数的漂浮基空间机器人关节运动的增广鲁棒控制

讨论了载体位置与姿态均不受控制的漂浮基空间机器人系统的鲁棒控制问题. 利用拉格朗日方法及系统动量守恒关系导出了漂浮基空间机器人欠驱动形式的系统动力学方程, 以确保得到的系统动力学方程关于一组适当选择的组合惯性参数呈线性函数关系. 在此基础上, 借助增广变量法针对末端爪手所持载荷参数存在不确定的情况, 提出了对外部扰动具有鲁棒性的漂浮基空间机器人关节空间轨迹跟踪的拟增广鲁棒控制方案. 所提控制方案由于充分利用空间机器人系统动量守恒关系, 消除了动力学方程中载体位置相关量, 因此具有不需要测量、反馈载体的位置、移动速度和移动加速度的显著优点; 且由于对系统不确定参数及外部扰动始终采用保持鲁棒性的方式而非在线估计的方式, 有效减少了计算量, 因此更适用于机载计算机运算能力有限的空间机器人控制系统实时在线应用. 一个平面两杆空间机器人系统的数值模拟仿真, 证实了方法的有效性.      

Feasibility of performing space surveillance tasks with a proposed space-based optical architecture

Under ESA contract an industrial consortium including Aboa Space Research Oy (ASRO), the Astronomical Institute of the University of Bern (AIUB), and the Dutch National Aerospace Laboratory (NLR), proposed the observation concept, developed a suitable sensor architecture, and assessed the performance of a space-based optical (SBO) telescope in 2005. The goal of the SBO study was to analyse how the existing knowledge gap in the space debris population in the millimetre and centimetre regime may be closed by means of a passive optical instrument. The SBO instrument was requested to provide statistical information on the space debris population in terms of number of objects and size distribution. The SBO instrument was considered to be a cost-efficient with 20 cm aperture and 6° field-of-view and having flexible integration requirements. It should be possible to integrate the SBO instrument easily as a secondary payload on satellites launched into low-Earth orbits (LEO), or into geostationary orbit (GEO). Thus the selected mission concept only allowed for fix-mounted telescopes, and the pointing direction could be requested freely. Since 2007 ESA focuses space surveillance and tracking activities in the Space Situational Awareness (SSA) preparatory program. Ground-based radars and optical telescopes are studied for the build-up and maintenance of a catalogue of objects. In this paper we analyse how the proposed SBO architecture could contribute to the space surveillance tasks survey and tracking. We assume that the SBO instrumentation is placed into a circular sun-synchronous orbit at 800 km altitude. We discuss the observation conditions of objects at higher altitude, and select an orbit close to the terminator plane. A pointing of the sensor orthogonal to the orbital plane with optimal elevation slightly in positive direction (0° and +5°) is found optimal for accessing the entire GEO regime within one day, implying a very good coverage of controlled objects in GEO, too. Simulations using ESA’s Program for Radar and Optical Observation Forecasting (PROOF) in the version 2005 and a GEO reference population extracted from DISCOS revealed that the proposed pointing scenario provides low phase angles together with low angular velocities of the objects crossing the field-of-view. Radiometric simulations show that the optimal exposure time is 1–2 s, and that spherical objects in GEO with a diameter of below 1 m can be detected. The GEO population can be covered under proper illumination nearly completely, but seasonal drops of the coverage are possible. Subsequent observations of objects are on average at least every 1.5 days, not exceeding 3 days at maximum. A single observation arc spans 3° to 5° on average. Using a simulation environment that connects PROOF to AIUB’s program system CelMech we verify the consistency of the initial orbit determination for five selected test objects on subsequent days as a function of realistic astrometric noise levels. The initial orbit determination is possible. We define requirements for a correlator process essential for catalogue build-up and maintenance. Each single observation should provide an astrometric accuracy of at least 1”–1.5” so that the initially determined orbits are consistent within a few hundred kilometres for the semi-major axis, 0.01 for the eccentricity, and 0.1° for the inclination.     

机内通话器自动测试仪设计

介绍了一种机内通话器自动测试仪的硬件设计和软件设计方法;分析了测试仪的硬件及软件组成和设计中的关键技术。该测试仪可方便实现对机内通话器的检测,具有一定的军事和经济效益。     

基于LabVIEW的皮卫星接收地面站解调器设计

针对皮卫星接收地面站测试系统解调器的应用场合,研究和开发了一种基于LabVIEW虚拟仪器开发平台的软件解调器,提出了一种简化的遥测信号解调算法。利用LabVIEW控制数据的采集和存储,通过VisualC 实现了解调算法,并在LabVIEW中调用该程序来实现地面站遥测信号的解调。给出了解调算法的原理、Costas环跟踪方差的MATLAB仿真和软件设计,通过误码率实验,测试结果证明了该设计的有效性。本解调系统具有应用灵活和便于改进的优点,已应用于皮卫星的测试实验。