Long-term performance of precision crystal oscillators in a near-Earth orbital environment

The Navy Navigation Satellite System (NNSS) uses precision quartz crystal oscillators to provide time and frequency in the orbiting spacecraft. The frequency changes for multiple oscillators, which were observed for 28 years of operational service in the orbital environment, are discussed. The primary frequency changes are believed to be caused by mass transfer to and from the resonator, stress relief in the resonator mounting structure and electrodes, and ionizing radiation of the quartz resonator. Observations to a resolution of 10- 13 have been made from 1963 to 1991 on 20 operational satellites in near-Earth orbit. No oscillator failures have occurred during the entire program life of nearly 30 years. One oscillator provided continuous operational service for over 21 years, and several have served more than 15 years. No oscillator changed frequency more than two parts in 107 while in operational service. One of the best performing oscillators had a predictable drift rate of 9x10(-13)+/-1x10(-13) per day after three years of service.     

Hypervelocity research and the growing problem of space debris

Man-made orbital debris has increased in number so that it poses a potential barrier to the exploration of space. The ever-increasing number of objects in space has created an increasing hazard to all spacecraft, including manned shuttles, unmanned satellites, and manned space stations. Although international efforts are underway to reduce the proliferation of space debris, the number of objects continues to climb.

The majority of debris tracked by earth observation is classed either as ‘operational debris’ (spent boosters and satellites, discarded hardware from manned flight, etc.) or as “fragmentation debris” (debris created by explosions aboard boosters or satellites or by impacts between objects in orbit). While there is considerable information available about operational debris, statistics on fragmentation debris are more suspect, since it is difficult to predict with any accuracy the fragments resulting from an explosion or impact on a space structure.

As realization of the importance of the problem grows, the hypervelocity launcher and impact communities are becoming increasingly involved. This paper defines the major problems to be solved and outlines the requirements for launchers, diagnostics, and modeling. A bew U.S. space program to model and the fragmentation of satellites impacted by space debris described. The results of tests against actual satellites are discribed in terms of their importance to the modeling effort.     

Passive microwave remote sensing of the atmosphere and ocean

The microwave remote sensing experiments conducted during the last decade using airborne and spaceborne sensors are now evolving as operational spacecraft observation systems. Microwave sounding unit (MUS) onboard TIROS-N regularly provides global atmospheric temperature profile required for numerical weather prediction. The monitoring of composition profile and geophysical parameters from space platform is now a reality and holds a great promise for future meteorological and oceanographic research. This review paper summarises the recent advances and future opportunities in passive microwave radiometry. Indian microwave remote sensing programme and achievements to date are also described.     

Fault-tolerant spacecraft attitude control system

Spacecraft perform a variety of useful tasks in our day-to-day life. These are such that spacecraft need to function properly without interruptions for 7 to 15 years in space without any maintenance. Though most spacecraft have redundant systems to serve as back-ups in case of failures, they greatly depend on human assistance through ground stations for failure analysis, remedial actions and redundancy management, resulting in itnerruption in services rendered. There is, therefore, need for a fault-tolerant system that functions despite failures and takes remedial action, without human assistance/intervention, autonomously on board the spacecraft. Commonly used techniques for fault-tolerance in computers cannot be directly used for fault-tolerance in sensors and actuators of a closed loop control system. Further, for space applications fault-tolerance needs to be achieved without much penalty in weight and computational requirements. This paper describes briefly the attitude control system (acs) of a spacecraft and highlights the essential features of a fault-tolerant control system. Schemes for fault tolerance in sensors and actuators are presented with an analysis on various failure modes and their effects. Newly developed fault-detection, identification and reconfiguration (fdir) algorithms for various elements ofacs are described in detail. Also an optimum symmetrically skewed configuration for the attitude reference system using dynamically tuned gyros is developed. Some of the schemes have already been used in Indian Spacecraft. As future Indian space missions will directly cater to various applications on an operational basis, the ultimate objective is to have a totally fault-tolerant ‘intelligent’ autonomous spacecraft.     

Accelerator test of an angle detecting inclined sensor (ADIS) prototype with beams of Ca and fragments

The measurement of cosmic rays and Solar energetic particles in space is basic to our understanding of the Galaxy, the Sun, phenomena in the Heliosphere and what has come to be known broadly as “space weather”. For these reasons, cosmic ray instruments are common on both scientific spacecraft and operational spacecraft such as weather satellites.

The resource constraints on spacecraft generally mean that instruments that measure cosmic rays and Solar energetic particles must have low mass (a few kg) and low power (a few W), be robust and reliable yet still highly capable. Such instruments must identify ionic species (at least by element, preferably by isotope) from protons through the iron group. The charge and mass resolution of heavy ion instruments in space depends upon determining ions’ angles of incidence. The Angle Detecting Inclined Sensor (ADIS) system is a highly innovative and uniquely simple detector configuration used to determine the angle of incidence of heavy ions in space instruments. ADIS replaces complex position sensing detectors (PSDs) with a system of simple, reliable and robust Si detectors inclined at an angle to the instrument axis.

In August 2004, we tested ADIS prototypes with a ⁴⁸Ca beam at the National Superconducting Cyclotron Laboratory's (NSCL) Coupled Cyclotron Facility (CCF). Among the analyses performed on the data taken at the NSCL, we demonstrate that our prototype design with an ADIS system has a charge resolution of less than 0.25e. We also present a more generalized analytic derivation of instrument response and report on the corresponding analysis of Monte-Carlo modeling data.     

空间环境污染对空间制冷器的影响

空间环境染污对航天器的影响是目前高可靠长寿命航天器设计时非常重要的问题之一.对于观测卫星、天文卫星、监测和跟踪卫星等航天器,低温探测器、低温光学系统,空间制冷器是重要的部件.因此空间环境和污染对制冷器的影响危及航天器的可靠性.非常薄的污染气体沉积膜能严重影响低发射率表面的发射率,由于发射率的增加使空间机械制冷器的低温热负荷增加10%～20%,导致辐射制冷器的温度升高,造成被冷却器件失效.介绍了一些控制污染的具体方法及控制污染敏感度物理学,讨论了一些与污染有关的物理现象.控制空间机械制冷器污染的方法是使用多层绝热或物理障板使空间制冷器与外部污染源进行隔离.控制辐射制冷器污染要从地面开始,在安装、装配、试验及与航天器集成过程中都要在清洁环境中进行.控制污染的方法是正确的结构设计,并必须尽量减少辐射制冷器对污染的敏感性.     

液态金属离子源在航天器电位主动控制的应用

航天器在轨运行期间太阳光照条件下,受光电子发射影响,航天器光照表面的电位达到正的数十伏。航天器表面带电现象会对其在轨安全稳定运行和空间等离子体探测数据的准确性造成严重影响。带正电航天器周围电场中离子和电子的速率和空间分布被扭曲,等离子体电子在鞘层加速,光照产生的光电子被吸引进入传感器,在低能量下引起高计数率,而且加速仪器微通道板的老化,使低密度等离子体的测量变得异常困难。因此,必须对航天器的电位进行主动控制。液态金属离子源电位主动控制器已在许多航天器上得到广泛的应用,通过主要介绍国外关于液态金属离子源电位主动控制器在航天器上的应用,为自主研制电位主动控制器满足空间科学探测的需求提供参考,也为军事和应用卫星在轨可靠运行提供防护方法。     

A statistical analysis of weight and cost parameters of spacecraft with special reference to ‘Aryabhata’

The available data on the weights of several NASA and ESRO spacecraft have been statistically analysed using the regression analysis technique and empirical relationships of the type W_T = AW{kx/β} have been established, where W_T is the total weight of the spacecraft, W_x is the weight of one of its chosen subsystems andA and β are constants for the chosen subsystem. It is found that the weight data ofAryabhata show a fairly good fit with these empirical relationships. Further, using an established statistically derived relationship for NASA satellites, involving the cost of a spacecraft and its weight parameters, it is found that the cost ofAryabhata is much lower than that of other NASA satellites of comparable weight. The implications of these results are briefly discussed.     

中巴地球资源卫星

介绍了中巴地球资源卫星概况及与国外同类卫星的比较。描述了五谱段CCD相机和四谱段红外扫描仪等有效载荷。扼要介绍了卫星的公用服务平台各分系统和在轨测试性能。在首发星的基础上充分汲取用户意见,把后继星的工作做好。     

Formal Specification and Quantitative Analysis of a Constellation of Navigation Satellites

Navigation satellites are a core component of navigation satellite‐based systems such as Global Positioning System, Global Navigation Satellite System and Galileo, which provide location and timing information for a variety of uses. Such satellites are designed for operating on orbit to perform tasks and have lifetimes of 10 years or more. Reliability, availability and maintainability analysis of systems has been indispensable in the design phase of satellites in order to achieve minimum failures or to increase mean time between failures and thus to plan maintenance strategies, optimise reliability and maximise availability. In this paper, we present formal models of both a single satellite and a navigation satellite constellation and logical specification of their reliability, availability and maintainability properties, respectively. The probabilistic model checker PRISM has been used to perform automated analysis of these quantitative properties. Copyright © 2014 John Wiley & Sons, Ltd.     

新型整星隔振系统动力学分析

在卫星的服役周期中,发射阶段的动力学环境最为恶劣,期间受到多种复杂载荷的作用。传统的星箭连接适配器由于刚度较大,给航天器的参数设计及生产带来了极大的困难。为了达到多轴隔振的目的,基于粘弹性材料受到剪切力作用时产生较大阻尼损耗的原理设计了一种新型整星隔振器。针对该隔振系统进行了简化,以质量刚度阻尼耦合的形式来模拟卫星和隔振器之间的关系,对其进行了隔振效果的分析。其中,重点对仿真中出现的隔振系统的一阶模态结构阻尼系数的饱和现象提供了详尽的理论和数值分析。最后,针对新型整星隔振器的工程具体应用提出了两点参考准则。     

Timing Traceability and the Link Between ISRO-NPLI

We present an algorithm for timescale prediction of IRNSS Network Time (IRNWT) using a GNSS link for daily exchange of data between Indian Space Research Organization and CSIR-NPL. A link between CSIR-NPL and IRNWT through Two Way Satellite Time and Frequency Transfer is also being established.     

SeaWiFS long-term solar diffuser reflectance and sensor noise analyses

The NASA Ocean Biology Processing Group's Calibration and Validation (Cal/Val) team has undertaken an analysis of the mission-long Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) solar calibration time series to assess the long-term degradation of the solar diffuser reflectance over 9 years on orbit. The SeaWiFS diffuser is an aluminum plate coated with YB71 paint. The bidirectional reflectance distribution function of the diffuser was not fully characterized before launch, so the Cal/Val team has implemented a regression of the solar incidence angles and the drift in the node of the satellite's orbit against the diffuser time series to correct for solar incidence angle effects. An exponential function with a time constant of 200 days yields the best fit to the diffuser time series. The decrease in diffuser reflectance over the mission is wavelength dependent, ranging from 9% in the blue (412 nm) to 5% in the red and near infrared (670-865 nm). The Cal/Val team has developed a methodology for computing the signal-to-noise ratio (SNR) for SeaWiFS on orbit from the diffuser time series corrected for both the varying solar incidence angles and the diffuser reflectance degradation. A sensor noise model is used to compare on-orbit SNRs computed for radiances reflected from the diffuser with prelaunch SNRs measured at typical radiances specified for the instrument. To within the uncertainties in the measurements, the SNRs for SeaWiFS have not changed over the mission. The on-orbit performance of the SeaWiFS solar diffuser should offer insight into the long-term on-orbit performance of solar diffusers on other instruments, such as the Moderate-Resolution Imaging Spectrometer [currently flying on the Earth Observing System (EOS) Terra and Aqua satellites], the Visible and Infrared Radiometer Suite [scheduled to fly on the NASA National Polar-orbiting Operational Environmental Satellite System (NPOESS) and NPOESS Preparatory Project (NPP) satellites] and the Advanced Baseline Imager [scheduled to fly on the National Oceanic and Atmospheric Administration Geostationary Environmental Operational Satellite Series R (GOES-R) satellites].     

Beam width and transmitter power adaptive to tracking system performance for free-space optical communication

The basic free-space optical communication system includes at least two satellites. To communicate between them, the transmitter satellite must track the beacon of the receiver satellite and point the information optical beam in its direction. Optical tracking and pointing systems for free space suffer during tracking from high-amplitude vibration because of background radiation from interstellar objects such as the Sun, Moon, Earth, and stars in the tracking field of view or the mechanical impact from satellite internal and external sources. The vibrations of beam pointing increase the bit error rate and jam communication between the two satellites. One way to overcome this problem is to increase the satellite receiver beacon power. However, this solution requires increased power consumption and weight, both of which are disadvantageous in satellite development. Considering these facts, we derive a mathematical model of a communication system that adapts optimally the transmitter beam width and the transmitted power to the tracking system performance. Based on this model, we investigate the performance of a communication system with discrete element optical phased array transmitter telescope gain. An example for a practical communication system between a Low Earth Orbit Satellite and a Geostationary Earth Orbit Satellite is presented. From the results of this research it can be seen that a four-element adaptive transmitter telescope is sufficient to compensate for vibration amplitude doubling. The benefits of the proposed model are less required transmitter power and improved communication system performance.     

Performance test of micro ion thruster using microwave discharge

Several types of electric propulsions have been developed and used for artificial satellites on geostationary orbit keeping or planetary exploration. A series of small-scale satellites is recently expected to be used instead of traditional large-scale satellites for reducing launch cost and risk. The trend towards needs of small spacecraft requires micro thrusters with high specific impulse. In this study, a micro ion engine utilizing a microwave discharge plasma was fabricated and was tested. This ion engine comprises some components, such as electrostatic grids, antenna, and microwave power supply, which decide its performance. The performance dependence on the components was examined under several experimental conditions. The typical performance parameters of the engine were ion production cost of 770 V, propellant utilization efficiency of 72%, and thrust of 0.66 mN in conditions of input 2.45 GHz microwave power of 8 W and propellant (xenon gas) flow rate of 0.2 sccm with a star type antenna.     

The NAL-ISRO acoustic test facility

The Indian Space Programme has the goals of launching a number of communication and remote sensing satellites for various uses. The acoustic exposure testing of such space bound components and systems on ground is mandatory to check their ability to withstand the extreme noise fields encountered during the trans-atmospheric flight to their final destinations in space. This paper outlines the nature and origin of this acoustic field, need for acoustic testing, establishment of a versatile groundbased high intensity acoustic-testing facility and its performance evaluation. The facility was set up as a joint project between the National Aeronautical Laboratory (nal) and the Indian Space Research Organisation (isro).     

空间环境和污染对光学器件的影响

用于观测卫星、气象卫星、空间望远镜上的各种光学器件及其光学涂层必须经受住空间环境和污染的考验才能保证航天器的可靠性。采取有效措施减少空间环境的影响是解决问题的关键。对于低温光学仪器是个相对独立的系统,在冷却期间必须保证仪器中最冷的传感器芯片保持最高温度,这样在冷却之前污染物就不会被吸附在表面上。最好的方法是在光学器件的适当位置安装加热器及污染控制罩。     

月球表面环境综合模拟系统的设想

相比于环绕地球飞行的卫星或载人飞船,探月航天器面临的空间环境更为复杂和严酷.月球表面的环境对登月飞船、月球车的环境适应性及可靠性提出了极为严格的要求.文章就月球表面综合环境模拟系统进行了初步的系统设计.该系统可实现月表尘埃、地形地貌、承载能力及摩擦效应、真空、温度交替变化等环境因素的综合模拟,可为登月飞船及月球车的设计、优化以及最终的系统验证提供试验平台.     

Effect of low earth orbit atomic oxygen on spacecraft materials

This review attempts to bring together the published data and analysis related to the effect of low earth orbit (LEO) atomic oxygen (AO) interaction with spacecraft materials. The basic interaction mechanism of AO with spacecraft materials and quantification of its effect on materials performance are briefly discussed. After providing a list of materials susceptible to the LEO environment, the paper focuses on the degradation mechanism of various spacecraft materials. Particular emphasis is given to the protective mechanisms for AO-susceptible materials and development of AO-resistant materials for long-term LEO spacecraft applications. Ground-simulation testing requirements and their present status are reviewed briefly. The need for further research is emphasized.