Microscope Instrument Development,Lessons for GOCE

Two space missions are presently under development with payload based on ultra-sensitive electrostatic accelerometers. The GOCE mission takes advantage of a three axis gradiometer accommodated in a very stable thermal case on board a drag-free satellite orbiting at a very low altitude of 250 km. This ESA mission will perform the very highly accurate mapping of the Earth gravity field with a geographical resolution of 100 km. The MICROSCOPE mission is devoted to the test of the “Universality of free fall” in view of the verification of the Einstein Equivalence Principle (EP) and of the search of a new interaction. The MICROSCOPE instrument is composed of two pairs of differential electrostatic accelerometers and the accelerometer proof-masses are the bodies of the EP test. The satellite is also a drag-free satellite exhibiting a fine attitude control and in a certain way, each differential accelerometer is a one axis gradiometer with an arm of quite null length. The development of this instrument much interests the definition and the evaluation of the sensor cores of the gradiometer. The in flight calibration process of both instruments is also very similar. Lessons form these parallel developments are presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

Orbital reference frame estimation with power spectral density constraints for drag-free satellites

The drag-free satellites are widely used in the field of fundamental science as they enable the high-precision measurement in pure gravity fields. This paper investigates the estimation of local orbital reference frame (LORF) for drag-free satellites. An approach, taking account of the combi-nation of the minimum estimation error and power spectral density (PSD) constraint in frequency domain, is proposed. Firstly, the relationship between eigenvalues of estimator and transfer func-tion is built to analyze the suppression and amplification effect on input signals and obtain the eigenvalue range. Secondly, an optimization model for state estimator design with minimum estima-tion error in time domain and PSD constraint in frequency domain is established. It is solved by the sequential quadratic programming (SQP) algorithm. Finally, the orbital reference frame estimation of low-earth-orbit satellite is taken as an example, and the estimator of minimum variance with PSD constraint is designed and analyzed using the method proposed in this paper.     

A geopause satellite system concept

The forthcoming 10 cm range tracking accuracy capability holds much promise in connection with a number of Earth and ocean dynamics investigations. These include a set of earthquake-related studies of fault motions and the Earth's tidal, polar and rotational motions, as well as studies of the gravity field and the sea surface topography which should furnish basic information about mass and heat flow in the oceans. The state of the orbit analysis art is presently at about the 10 m level, or about two orders of magnitude away from the 10 cm range accuracy capability expected in the next couple of years or so. The realization of a 10 cm orbit analysis capability awaits the solution of four kinds of problems, namely, those involving orbit determination and the lack of sufficient knowledge of tracking system biases, the gravity field, and tracking station locations. The Geopause satellite system concept offers promising approaches in connection with all of these areas. A typical Geopause satellite orbit has a 14 hour period, a mean height of about 4.6 Earth radii, and is nearly circular, polar, and normal to the ecliptic. At this height only a relatively few gravity terms have uncertainties corresponding to orbital perturbations above the decimeter level. The orbit s, in this sense, at the geopotential boundary, i.e., the geopause. The few remaining environmental quantities which may be significant can be determined by means of orbit analyses and accelerometers. The Geopause satellite system also provides the tracking geometery and coverage needed for determining the orbit, the tracking system biases and the station locations. Studies indicate that the Geopause satellite, tracked with a 2 cm ranging system from nine NASA affiliated sites, can yield decimeter station location accuracies. Five or more fundamental stations well distributed in longitude can view Geopause over the North Pole. This means not only that redundant data are available for determining tracking system biases, but also that both components of the polar motion can be observed frequently. When tracking Geopause, the NASA sites become a two-hemisphere configuration which is ideal for a number of Earth physics applications such as the observation of the polar motion with a time resolution of a fraction of a day. Geopause also provides the basic capability for satellite-to-satellite tracking of drag-free satellites for mapping the gravity field and altimeter satellites for surveying the sea surface topography. Geopause tracking a coplanar, drag-free satellite for two months to 0.03 mm per second accuracy can yield the geoid over the entire Earth to decimeter accuracy with 2.5° spatial resolution. Two Geopause satellites tracking a coplanar altimeter satellite can then yield ocean surface heights above the geoid with 7° spatial resolution every two weeks. These data will furnish basic boundary condition information about mass and heat flows in the oceans which are important in shaping weather and climate.     

The Cassini Cosmic Dust Analyzer

The Cassini-Huygens Cosmic Dust Analyzer (CDA) is intended to provide direct observations of dust grains with masses between 10⁻¹⁹ and 10⁻⁹ kg in interplanetary space and in the jovian and saturnian systems, to investigate their physical, chemical and dynamical properties as functions of the distances to the Sun, to Jupiter and to Saturn and its satellites and rings, to study their interaction with the saturnian rings, satellites and magnetosphere. Chemical composition of interplanetary meteoroids will be compared with asteroidal and cometary dust, as well as with Saturn dust, ejecta from rings and satellites. Ring and satellites phenomena which might be effects of meteoroid impacts will be compared with the interplanetary dust environment. Electrical charges of particulate matter in the magnetosphere and its consequences will be studied, e.g. the effects of the ambient plasma and the magnetic field on the trajectories of dust particles as well as fragmentation of particles due to electrostatic disruption.The investigation will be performed with an instrument that measures the mass, composition, electric charge, speed, and flight direction of individual dust particles. It is a highly reliable and versatile instrument with a mass sensitivity 10⁶ times higher than that of the Pioneer 10 and 11 dust detectors which measured dust in the saturnian system. The Cosmic Dust Analyzer has significant inheritance from former space instrumentation developed for the VEGA, Giotto, Galileo, and Ulysses missions. It will reliably measure impacts from as low as 1 impact per month up to 10⁴ impacts per second. The instrument weighs 17 kg and consumes 12 W, the integrated time-of-flight mass spectrometer has a mass resolution of up to 50. The nominal data transmission rate is 524 bits/s and varies between 50 and 4192 bps.This revised version was published online in July 2005 with a corrected cover date.     

卫星授时与时间传递技术进展

近年来,卫星导航技术发展迅速.卫星导航系统以精密时间测量技术为基础,实现了伪距测量,进而实现定位.同时,卫星导航系统还提供了高精度授时功能.综述了卫星导航系统的授时和时间频率传递技术、基于通信卫星的授时技术以及双向卫星时间频率传递(TWSTFT)技术等.随着我国北斗卫星导航系统(BDS)的建成和提供服务,BDS授时应用研究正在快速发展.基于BDS/GNSS多系统的精密单点定位(PPP)时间传递技术已成为重点研究方向,未来将会应用于国际时间比对.同时,随着卫星通信技术尤其是低轨通信卫星技术的快速发展,低轨通信卫星授时会成为一个有潜力的研究方向.     

The Swedish Small Satellite Program for Space Plasma Investigations

The success of the Swedish small satellite program, in combination with an active participation by Swedish research groups in major international missions, has placed Sweden in the frontline of experimental space research. The program started with the development of the research satellite Viking which was launched in 1986, for detailed investigations of the aurora. To date, Sweden has developed and launched a total of six research satellites; five for space plasma investigations; and the most recent satellite Odin, for research in astronomy and aeronomy. These fall into three main categories according to their physical dimension, financial cost and level of ambition: nano-satellites, micro-satellites, and mid-size satellites with ambitious scientific goals. In this brief review we focus on five space plasma missions, for which operations have ended and a comprehensive scientific data analysis has been conducted, which allows for a judgement of their role and impact on the progress in auroral research. Viking and Freja, the two most well-known missions of this program, were pioneers in the exploration of the aurora. The more recent satellites, Munin, Astrid, and Astrid-2 (category 1 and 2), proved to be powerful tools, both for testing new technologies and for carrying out advanced science missions. The Swedish small satellite program has been internationally recognized as cost efficient and scientifically very successful.     

VI: FUTURE CONCEPTS: Attitude and Drag Control: An Application to the GOCE Satellite

The Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite, currently planned to he launched in the course of 2006, will require a precise drag compensation and a fine attitude control along the Local Orbiting Reference Frame (LORF) of a polar Sun-synchronous low orbit, allowing the Earth gravity field to be recovered with unprecedented accuracy by post-processing the scientific telemetry. To this aim, the spectral density of the spacecraft linear and angular accelerations must be limited below 0.025 respectively, in the frequency range from 5 mHz to 0.1 Hz, the gradiometer measurement bandwidth. In the same range, the orientation errors of the spacecraft in the LORF and of the LORF in the inertial frame must be kept below 10 . The Drag-Free Mode, encharged of drag-free and attitude control (DFAC) during measurement phases, determines the spacecraft state vector using a very precise gradiometer, one large Field-of-View Star Tracker and a Satellite-to-Satellite Tracking Instrument. Force and torque commands are actuated by two assemblies of thrusters: a single ion-thruster acting along the orbital direction, a set of eight micro-thrusters acting along the other five degrees of freedom. To cover every mission scenario, other control modes have been studied and designed: the Coarse Pointing Mode dedicated to rate damping and Sun acquisition, the Fine Pointing Mode handling the transition to Drag-Free Mode and the Ultimate Safe Mode, a survival operative mode to improve mission reliability. Results presented in this paper give a positive perspective on the solidity of the current DFAC design. This revised version was published online in August 2006 with corrections to the Cover Date.     

Quasistatic electric field measurements with spherical double probes on the GEOS and ISEE satellites

Spherical double probes for measurements of electric fields on the GEOS-1, GEOS-2, and ISEE-1 satellites are described. An essential feature of these satellites is their conductive surfaces which eliminate errors due to differential charging and enable meaningful diagnostic experiments to be carried out. The result of these experiments is a good understanding of interactions between the plasma, the satellite and the probes, including photo-electron emission on satellite and probes. Electric field measurements are compared with measurements of plasma drift perpendicular to the magnetic field in the solar wind and the magnetosphere and the error bar for the absolute values of the electric field is found to be in the range ±(0.5–1.0) mV m^-1 whereas relative variations can be determined with much better accuracy. A useful by-product from a spherical double probe system is the determination of satellite floating potential which is related to the plasma electron flux. This measurement allows high time resolution studies of boundary crossings. Examples of electric field measurements, which reflect the recent scientific results, are given for different regions of the magnetosphere from the bow shock, the inner magnetosphere and the tail. Several examples of simultaneous GEOS-ISEE observations are described.     

Identification Algorithms for Micro-Propulsion System Parameters Using Drag-Free Test Masses

Propulsion system characteristics determine to a large extent the dynamic behavior of a spacecraft. For many future science missions technologically novel micro-propulsion systems are required. In order to support its characterization, in-orbit experiments and subsequent data processing on ground can be an appropriate add-on to ground-based laboratory measurements. In this paper two identification methods for three major thruster parameters, thrust gain, thrust direction, and lever arm, are presented and compared. They are based on measurements of a precise inertial instrument that consists of two test masses, whose degrees of freedom are “mixed” with respect to its control principle, i.e. they are either drag-free controlled (free-flying) or suspension controlled (accelerometer mode). Using drag-free coordinates is a novel approach. It is related and compared to the more conventional approach using “accelerometer-like” measurements.     

基于变结构控制器的敏捷卫星姿态机动方法

随着遥感卫星观测能力的逐步提升,对卫星敏捷机动能力提出了更高的要求。针对敏捷卫星大角度姿态机动问题,以6个单框架控制力矩陀螺(SGCMG)组成五棱锥构型的姿态控制系统执行机构,在构建敏捷卫星姿态运动数学模型以及设计SGCMG系统操纵律的基础上,对卫星绕Euler轴进行姿态机动的角轨迹进行规划,并设计了一种基于误差四元数与误差角速度的变结构控制器。仿真及在轨验证结果表明,该控制器能够完成规划轨迹的良好跟踪且具有较强的鲁棒性,研究成果对敏捷卫星姿态控制系统的设计具有重要的参考意义。     

A new high performance multipurpose satellite tracking system

The European Space Agency's (ESA) multipurpose satellite tracking system is introduced. The system is able to perform accurate satellite ranging and Doppler measurements for a variety of mission types, i.e., from near-Earth satellites to deep space probes. The ranging signal is analyzed and described in an analytical manner from which the limits of the system performance are derived. A model of the overall system is presented and a few simulation results obtained thereby are compared with measurements performed with the ESA's Giotto and Hipparcos scientific missions     

Line-of-sight-pointing for satellites in high altitude inclined elliptic orbits

With a growing demand for space communications and resulting overcrowding of geostationary orbit (GEO), the importance of high altitude inclined elliptic orbits is gaining impetus. However, the satellites in these orbits suffer from a severe problem of apparent periodic angular drift around their line-of-sight. This paper addresses this problem and proposes a cost effective method based on tether to continually tilt the satellites in order to compensate for longitudinal and lateral drifts relative to the ground station. The proposed system comprises two satellites connected by a flexible tether at a point on each satellite with offsets. A control strategy is developed for tether offset variations that ensures judiciously controlled changes in the satellite orientations. The numerical simulation of the governing nonlinear equations of motion establishes the feasibility of the concept. A high degree of line-of-sight pointing of dual satellites as well as the simplicity of the proposed control mechanism makes the concept particularly attractive for future space applications.     

Systems approach to the satellite operations problem

It is noted that the projected increase in satellites needed to satisfy future military, scientific, and commercial missions will soon be overwhelming current ground-operations capabilities. An intelligent processing architecture, IntelliSTAR, that addresses the potential of automating the entire satellite operations domain (planning, scheduling, execution, and analysis) is discussed. The architecture has been developed with flexibility in mind. In particular, IntelliSTAR has been structured to allow for development and validation on the ground, prior to deployment in space. The overall architecture is described, with particular emphasis placed on the scheduling of satellite missions     

Dynamics and Control Modeling for Gravity Probe B

A generic drag-free simulator has been developed to aid in the design, on-orbit and post-mission data analysis phases of increasingly complex future missions such as Gaia and STEP. Adaptation to the recent science mission Gravity Probe B (GP-B) has been carried out for a first simulator verification with actual flight data. Lessons learned from GP-B have shown that the controls simulator, developed concurrently with GP-B, has been invaluable to test flight control design and furthermore to resolve on-orbit anomalies in a time-saving manner. A complete mission software simulator including controls, full-body dynamics and comprehensive spacecraft environment disturbances has been established for Gravity Probe B. This simulator provides a reference and development platform for future mission design. The importance of this effort lies in the challenge to meet rising science requirements for future missions in the area of maximum disturbance rejection.     

下一代数据中继卫星系统发展思考

通过系统阐述中继卫星系统的发展过程,给出了主要国家和组织的中继卫星系统技术体制和现状.再结合卫星、载人航天器和深空探索的未来发展趋势,分析了下一代中继卫星系统的发展需求.在此基础上,从体系结构、卫星平台、链路调制体制、网络协议等方面,探讨并给出了下一代中继卫星系统的发展趋势和技术途径.为满足未来近地、深空航天任务,以及临近、低空快速移动用户的不同要求,节约系统成本,下一代中继卫星系统将向专业化和与其他系统融合的方向发展:星间链路将增加激光链路,数据速率可达到10 Gbit/s以上;多址业务成为主用,同时支持用户数能力将极大提高;对于链路调制体制,在采用CR(Cognitive Radio,认知无线电)和SDR(Software Defined Radio,软件定义无线电)技术的基础上,可实现实时自适应调整和根据需求加载配置;数据传输将采用网络化方式,天地间构成一体化DTN(Delay Tolerant Network,容延迟网络).     

Geology of the Icy Satellites

In the last 25 years, the explorations of the Voyager and Galileo missions have resulted in an entirely new view of the icy worlds orbiting the giant outer planets. These objects show a huge diversity in their characteristics, resulting from their formation histories, internal processes and interactions with their space environments. This paper will review the current state of knowledge about the icy satellites and discuss the exciting prospects for the upcoming Cassini/Huygens mission as it begins a new era of exploration of the Saturn satellite system.     

A data transmission scheduling algorithm for rapid-response earth-observing operations

With the development of rapid-response Earth-observing techniques, the demand for reducing a requirements-tasking-effects cycle from 1 day to hours grows rapidly. For instance, a satellite user always wants to receive requested data in near real-time to support their urgent mis- sions, such as dealing with wildfires, volcanoes, flooding events, etc. In this paper, we try to reduce data transmission time for achieving this goal. The new feature of a responsive satellite is that users can receive signals from it directly. Therefore, the traditional satellite control and operational tech- niques need to be improved to accommodate these changes in user needs and technical upgrading. With that in mind, a data transmission topological model is constructed. Based on this model, we can deal with the satellite data transmission problem as a multi-constraint and multi-objective path- scheduling problem. However, there are many optional data transmission paths for each target based on this model, and the shortest path is preferred. In addition, satellites represent scarce resources that must be carefully scheduled in order to satisfy as many consumer requests as possible. To efficiently balance response time and resource utilization, a K-shortest path genetic algorithm is proposed for solving the data transmission problem. Simulations and analysis show the feasibility and the adaptability of the proposed approach.     

Chemical Composition of Icy Satellite Surfaces

Much of our knowledge of planetary surface composition is derived from remote sensing over the ultraviolet through infrared wavelength ranges. Telescopic observations and, in the past few decades, spacecraft mission observations have led to the discovery of many surface materials, from rock-forming minerals to water ice to exotic volatiles and organic compounds. Identifying surface materials and mapping their distributions allows us to constrain interior processes such as cryovolcanism and aqueous geochemistry. The recent progress in understanding of icy satellite surface composition has been aided by the evolving capabilities of spacecraft missions, advances in detector technology, and laboratory studies of candidate surface compounds. Pioneers 10 and 11, Voyagers I and II, Galileo, Cassini and the New Horizons mission have all made significant contributions. Dalton (Space Sci. Rev., 2010, this issue) summarizes the major constituents found or inferred to exist on the surfaces of the icy satellites (cf. Table 1 from Dalton, Space Sci. Rev., 2010, this issue), and the spectral coverage and resolution of many of the spacecraft instruments that have revolutionized our understanding (cf. Table 2 from Dalton, Space Sci. Rev., 2010, this issue). While much has been gained from these missions, telescopic observations also continue to provide important constraints on surface compositions, especially for those bodies that have not yet been visited by spacecraft, such as Kuiper Belt Objects (KBOs), trans-Neptunian Objects (TNOs), Centaurs, the classical planet Pluto and its moon, Charon. In this chapter, we will discuss the major satellites of the outer solar system, the materials believed to make up their surfaces, and the history of some of these discoveries. Formation scenarios and subsequent evolution will be described, with particular attention to the processes that drive surface chemistry and exchange with interiors. Major similarities and differences between the satellites are discussed, with an eye toward elucidating processes operating throughout the outer solar system. Finally we discuss the outermost satellites and other bodies, and summarize knowledge of their composition. Much of this review is likely to change in the near future with ongoing and planned outer planet missions, adding to the sense of excitement and discovery associated with our exploration of our planetary neighborhood.     

Large angle pitch attitude maneuver of a satellite using solarradiation pressure

The force exerted by the solar radiation, though very small in magnitude, produces significant effects, especially in the case of high altitude satellites. The solar radiation pressure represents one freely available environmental force that may be put to use for various purposes. This may lead to enhancement of the life of the satellite since it consumes a very nominal amount of on-board energy. The advantages offered by the solar radiation pressure have drawn the attention of several researchers. Various controllers were proposed for many space missions, particularly for attitude control and stabilization of satellites. A controller for achieving large angle pitch attitude maneuver is described. The proposed control law is very simple in its form and requires a minimum number of on-board computations. Varieties of cases are tried and the effect of various parameters is studied     

A Diagnostic Tree Approach for Fault Cause Identification in the Attitude Control Subsystem of Satellites

Space and Earth observation programs demand stringent guarantees ensuring smooth and reliable operations of space vehicles and satellites. Due to unforeseen circumstances and naturally occurring faults, it is desired that a fault-diagnosis system be capable of detecting, isolating, identifying, or classifying faults in the system. Unfortunately, none of the existing fault-diagnosis methodologies alone can meet all the requirements of an ideal fault- diagnosis system due to the variety of fault types, their severity, and handling mechanisms. However, it is possible to overcome these shortcomings through the integration of different existing fault-diagnosis methodologies. In this paper, a novel learning-based, diagnostic-tree approach is proposed which complements and strengthens existing efficient fault detection mechanisms with an additional ability to classify different types of faults to effectively determine potential fault causes in a subsystem of a satellite. This extra capability serves as a semiautomatic diagnostic decision support aid to expert human operators at ground stations and enables them to determine fault causes and to take quick and efficient recovery/reconfiguration actions. The developed diagnosis/analysis procedure exploits a qualitative technique denoted as diagnostic tree (DX-tree) analysis as a diagnostic tool for fault cause analysis in the attitude control subsystem (ACS) of a satellite. DX-trees constructed by our proposed machine-learning-based automatic tree synthesis algorithm are demonstrated to be able to determine both known and unforeseen combinations of events leading to different fault scenarios generated through synthetic attitude control subsystem data of a satellite. Though the immediate application of our proposed approach would be at ground stations, the proposed technique has potential for being integrated with causal model-based diagnosis and recovery techniques for future autonomous space vehicle missions.