Optimal control of a spinning double-pyramid Earth-pointing tethered formation

The dynamics and control of a tethered satellite formation for Earth-pointing observation missions is considered. For most practical applications in Earth orbit, a tether formation must be spinning in order to maintain tension in the tethers. It is possible to obtain periodic spinning solutions for a triangular formation whose initial conditions are close to the orbit normal. However, these solutions contain significant deviations of the satellites on a sphere relative to the desired Earth-pointing configuration. To maintain a plane of satellites spinning normal to the orbit plane, it is necessary to utilize “anchors”. Such a configuration resembles a double-pyramid. In this paper, control of a double-pyramid tethered formation is studied. The equations of motion are derived in a floating orbital coordinate system for the general case of an elliptic reference orbit. The motion of the satellites is derived assuming inelastic tethers that can vary in length in a controlled manner. Cartesian coordinates in a rotating reference frame attached to the desired spin frame provide a simple means of expressing the equations of motion, together with a set of constraint equations for the tether tensions. Periodic optimal control theory is applied to the system to determine sets of controlled periodic trajectories by varying the lengths of all interconnecting tethers (nine in total), as well as retrieval and simple reconfiguration trajectories. A modal analysis of the system is also performed using a lumped mass representation of the tethers.     

The overredundant spacecraft—A competitive approach to future telecommunication satellites

Present operational space telecommunication systems are based on simultaneous availability of more than one satellite on orbit, mainly a spare satellite in addition to the operational one.Considering the costs associated to the delivery of extra flight models and to extra launchers, the question is asked whether it would not be advantageous to launch a very limited number of “overredundant” spacecraft instead of several standard satellites.The paper gives main conditions of reliability, size and redundancy concept under which an “overredundant” spacecraft could be a competitive approach to future operational systems.     

25 YEARS OF SPACE AT SURREY—PIONEERING MODERN MICROSATELLITES

Space at Surrey has developed over 25 years from very modest beginnings in 1974 to an international space centre by 1998. It has pioneered small satellites and succeeded in launching 14 low cost but sophisticated microsatellites over the course of two decades. In the 1990s, small satellites have become fashionable—but this was not always so! This paper describes the 25 years history of “Space at Surrey”.     

Tracing the evolutionary path for space technologies

Proliferation and pace of advancing technologies warrant policy and strategic decision-making. Without thinking ahead, companies can loose marketshare and countries can yield comparative advantage. The rate at which burgeoning technologies progress, however, can make it difficult for corporations and governments alike to discern or better anticipate critical junctures in technology developments. This paper presents a conceptual, multidimensional framework, the “evolutionary path”, for understanding the stages of technological development in the civil space area. The analysis draws from three case studies — communications satellites, computers, and launch vehicles — and shows how the implications and developments of new, breakthrough technologies differ from the incremental technology upgrades or the later emergence of interconnected systems and infrastructures.     

Control of satellite imaging formations in multi-body regimes

Libration point orbits may be ideal locations for satellite imaging formations. Therefore, control of these arrays in multi-body regimes is critical. A continuous feedback control algorithm is developed that maintains a formation of satellites in motion that is bounded relative to a halo orbit. This algorithm is derived based on the dynamic characteristics of the phase space near periodic orbits in the circular restricted three-body problem (CR3BP). By adjusting parameters of the control algorithm appropriately, satellites in the formation follow trajectories that are particularly advantageous to imaging arrays. Image reconstruction and coverage of the (u, v) plane are simulated for interferometric satellite configurations, demonstrating potential applications of the algorithm and the resulting motion.     

Modifying the atlas of low lunar orbits using inert tethers

For long enough tethers, the coupling of the attitude and orbital dynamics may show non-negligible effects in the orbital motion of a tethered satellite about a central body. In the case of fast rotating tethers the attitude remains constant, on average, up to second order effects. Besides, for a tether rotating in a plane parallel to the equatorial plane of the central body, the attitude–orbit coupling effect is formally equal to the perturbation of the Keplerian motion produced by the oblateness of the central body and, therefore, may have a stabilizing effect in the orbital dynamics. In the case of a tethered satellite in a low lunar orbit, it is demonstrated that feasible tether lengths can help in modifying the actual map of lunar frozen orbits.     

The earth explorer candidates: Nine focused missions, all satellite classes

The size of a satellite should follow naturally from the mission requirements taking into account implementation constraints, notably the choice of launcher and programmatics. The key concepts are “focused mission” and “maximum return for the investment”. The nine candidates proposed for the ESA Earth Explorer programme of research missions form a good representative set to illustrate a discussion of satellite classes. These focused missions lead to satellites from less than 100 kg and 100 W, to more than 2000 kg and 2 kW of power generation and include flights of opportunity and precursor experiments.     

Legal consequences of a SETI detection

If a detection of ETI takes place, this will in all probability be the result of either: (a) detecting and recognising a signal or other emission of ETI; or (b) the finding of an alien artifact (for instance on the Moon or other Celestial Body of our Solar System); or (c) the highly improbable event of an actual encounter. First and foremost, legal consequences regarding any of these contingencies will result from immediate consultations between nations on Earth. Understandings, memoranda and even agreements might be proposed and/or concluded. Such results within the field of terrestrial law will surely be a new branch of International Law, and particularly of International Space Law. At the same time, terrestrial nations will have to realize that any ETI will be self-determined intelligent individualities or organizations who might have their own understanding of “rules of behaviour” and thus, be legal subjects. Whether one calls such rules “law” or not: if two intelligent races—both of which have specific rules of behaviour—come into contact with each other, the basic understanding of such mutual rules will lead to a kind of “code of conduct”. This might be the starting point for a kind of Law—Metalaw—between different races in the Universe.     

Remote sensing of natural formations from measurements of radiance coefficients

This paper is devoted to the consideration of some problems of remote sensing of natural formations using radiance coefficients in the narrow spectral invervals. The techniques for the determination of the most informative spectral intervals, as developed by the authors of the paper, are given and the results obtained are presented. The technique for obtaining the training sample of spectral radiance coefficients, with the help of a four-objective camera applied by the authors, is described. Some problems regarding the construction of a “universal” alphabet for classes of natural formations are discussed.     

Satellite communications and the Internet: implications for the outer space treaty

The speedily expanding Internet is in the process of transforming the technological, economic, and policy bases for nation-state regulation of telecommunications, including space-based satellite networks. Deployment of the packet-switched Internet has accelerated the liberalization of telecommunications markets and has led to far-reaching regulatory restructuring and policy shifts regarding state ownership and control of networks and information flows. As space-based GMPCS networks become integral parts of the globalizing Internet infrastructure, the state-centric legal paradigm requiring state “authorization and continuing supervision” of space activities by “non-governmental entities” stipulated under Article VI of the OST and associated treaties forming the outer space legal regime will be called increasingly into question. This paper examines the technological, economic/trade, and security issues that question whether the existing state-centric paradigm for regulating Internel-based GMPCS satellite systems will remain in legal phase with emerging liberalized regulatory regimes for terrestrial Internet-based infractructures.     

Stability of a ring of connected satellites

The motion of a large number of artificial satellites connected in a ring one after another by tethers of variable length is considered. Every satellite is supposed to have a control system programmed according to some tether tension law as a function of the distance between tethered satellites. The effect of the tension control law on the stability of stationary rotation of this ring is investigated. The final stability condition includes two requirements: 1) the nominal tether tension should be less than a definite limit equal, up to numerical coefficient, to one satellite weight divided by the number of satellites; 2) tether tension should decrease (or remain constant) with the increase of the distance between tethered satellites. In dynamics the artificial rings of this kind are much like their natural prototype—meteor rings. On the other hand, the investigation of the artificial rings contributes to developing an unexpected view upon meteor rings, suggesting a model of an imaginary equivalent string.     

Technology readiness and risk assessments: A new approach

Systems that depend upon the application of new technologies inevitably face three major challenges during development: performance, schedule and budget. Technology research and development (R&D) programs are typically advocated based on argument that these investments will substantially reduce the uncertainty in all three of these dimensions of project management. However, if early R&D is implemented poorly, then the new system developments that plan to employ the resulting advanced technologies will suffer from cost overruns, schedule delays and the steady erosion of initial performance objectives. It is often critical for senior management to be able to determine which of these two paths is more likely—and to respond accordingly. The challenge for system and technology managers is to be able to make clear, well-documented assessments of technology readiness and risks, and to do so at key points in the life cycle of the program.Several approaches have been used to evaluate technology maturity and risk in order to better anticipate later system development risks. The “technology readiness levels” (TRLs), developed by NASA, are one discipline-independent, programmatic figure of merit (FOM) that allows more effective assessment of, and communication regarding the maturity of new technologies. Another broadly used management tool is of the “risk matrix”, which depends upon a graphical representation of uncertainty and consequences. However, for the most part these various methodologies have had no explicit interrelationship.This paper will examine past uses of current methods to improve R&D outcomes and will highlight some of the limitations that can arise. In this context, a new concept for the integration of the TRL methodology, and the concept of the “risk matrix” will be described. The paper will conclude with observations concerning prospective future directions for the important new concept of integrated “technology readiness and risk assessments”.     

The navigational experiments on microgravitational space platform “FOTON-M2”

This paper presents the review of results of the navigating experiments which have been carried out during flight of microgravitational space platform (MSP) Foton-M2 in May–June 2005. The brief characteristic of the created MIRAGE–M equipment consisting from magnitometric system and satellite radionavigation receiver is given. The measurements have allowed to restore unguided MSP movement and to estimate a level of microaccelerations (microgravitations) onboard during flight, and have provided precision time-position binding of the research experiments. The data from the equipments transmitted on the telemetering channel have allowed testing the information technologies of virtual support of experiments in space. Flight testing of the equipment has allowed make a conclusion on usefulness of accommodation onboard the small-sized auxiliary navigating system focused for work with users of research experiments. The experiments on MSP Foton-M2 are the development of experiments with MIRAGE equipment carried out in 1999 during flight time of MSP Foton-12 [N.D. Semkin, V.V. Ivanov, V.I. Abrushkin, V.L. Balakin, I.V. Belokonov, K.E. Voronov, The experiments with magnetic fields formed by technical equipment inside Foton-12 spacecraft: the results of the MIRAGE experiments, in: Proceedings of International Conference “Scientific and Technological Experiments on Russian Foton/Bion Recoverable Satellites: Results, Problems and Outlooks”, 25–30 June 2000, pp. 116–122; V.L. Balakin, I.V. Belokonov, V.V. Ivanov, “Determination of motion of spacecraft Foton-12 as a result of magnetic fields measurement in MIRAGE experiment”, pp. 231–238 (published in the same place)].Paper is executed within the framework of the grant of the Russian Fund of Fundamental Researches 06-08-00244.     

International coordination of space solar power related activities

Because the need for energy is global, and many energy networks are already interdependent, because no one country has sufficient technological capability or sufficient funds to provide a space solar powered solution on its own, and because any such solution will require international regulation, international coordination will be vital to any attempt to produce energy for Earth from space. This will be made easier by the fact that work on the subject has already been widely publicized and distributed and cooperative efforts have already been made. Various coordinating approaches are described and the need to forge partnerships between government, industry and academia — with greater involvement of all non-space groups concerned with energy — is emphasized. A “terracing approach” to the actual implementation of SPS is suggested and outlined.     

Fuel consumption and collision avoidance strategy in multi-static orbit formations

This paper analyses the fuel consumption of interferometric radar missions employing small satellite formations like, e.g., Cross-track Pendulum, Cartwheel, CarPe, or Trinodal Pendulum. Individual analytic expressions are provided for each of the following contributions: separation from a simultaneously injected master satellite, formation set-up, orbit maintenance, formation maintenance, and distance maintenance. For this, a general system of equations is derived describing the relative motion of the small satellites in a co-rotating reference frame. The transformation into Keplerian elements is carried out. To evaluate fuel consumption, three master satellites are assumed in different orbital heights, which are typical for Earth observation missions. The size of the exemplarily analysed formations is defined by remote sensing aspects and their respective fuel requirements are estimated. Furthermore, a collision avoidance concept is introduced, which includes a formation separation and formation set-up after a desired time period.     

Index: piggy-back satellite for aurora observation and technology demonstration

This paper describes outline of the piggy-back satellite “INDEX” for demonstration of advanced satellite technologies as well as for observation of fine structure of aurora. Aurora observation will be carried out by three cameras(MAC) with a monochromatic UV filter. Electron and ion spectrum analyzer (ESA/ISA) will measure the particle phenomena together with the aurora imaging. INDEX satellite will be launched in 2002 by Japanese H2-A. The satellite is mainly controlled by the high-speed, fault-tolerant on-board RICS processor (three-voting system of SH-3). The attitude control is a compact system of three-axis stabilization. Although the size of INDEX is small (50Kg class), several newly-developed technologies are applied to the satellite system, including silicon-on-insulator devices, variable emittance radiator, solar-concentrated paddles, lithium-ion battery, and GPS receiver with all-sky antenna-coverage.     

Bifurcation Regime of a Synchronous Resonance in the Translational–Rotational Motion of Nonspherical Natural Satellites of Planets

Period-doubling bifurcations of the synchronous spin-orbit resonance in the motion of a nonspherical natural planetary satellite along the elliptic orbit are studied. The satellite spin axis is assumed to coincide with the axis of its largest principal moment of inertia and is perpendicular to the orbital plane. The period-doubling bifurcations take place when the value of satellite's dynamical asymmetry parameter falls in the parametric resonance domain. Theoretical dependences of the amplitude of the bifurcation oscillations of a satellite at the pericenter of its orbit upon the eccentricity and dynamical asymmetry parameter are investigated. Three different methods of calculating the amplitude of bifurcation oscillations are presented and compared. These theoretical estimates can be used to predict the opportunity to observe the bifurcation regime. The possibility of the occurrence of the bifurcation regime in the motion of natural planetary satellites is studied. It is concluded that the bifurcation regime is possible in the motion of Deimos, Epimetheus, Helen, Pandora, and Phobos. Phobos is the most probable candidate for finding the bifurcation regime of a synchronous rotation. The identification of such a regime would allow one to impose stringent constraints on the values of the inertial parameters of the satellite observed.     

The data compression problem for the “GAIA” astrometric satellite of ESA

Space-based astrometry has a great tradition at ESA. The first space-based astrometric satellite in history, “Hipparcos”, was launched by ESA in 1989 and, in spite of orbital problems, was able to accomplish almost all of its tasks until it was finally shut down in 1993. The results of the Hipparcos mission were published by ESA in 1997 in the form of six CD-ROMs: the Hipparcos Catalogue contains 118,218 entries with median astrometric precision of around 1 milliarcsec, and specific results for double and multiple systems. In practice, Hipparcos drew for the first time the three-dimensional “map” of the spherical region of the Galaxy surrounding the Sun and having a radius of roughly 1,000 light years.

Then, in 1995, ESA launched the study of a new astrometric satellite, named “GAIA” and about a hundred times more powerful than Hipparcos, i.e. with median astrometric precision of around 10 microarcsec. This new satellite is intended to measure the parallaxes of over 50 million stars in the Galaxy, at least for the brightest stars, and this would mean to “draw” the three-dimensional map of the whole Galaxy, reaching out even to the Magellanic Clouds, 180,000 light years away.

The team of European scientists and engineers now designing GAIA, however, is facing hard technological difficulties. One of these is the design and coding of radically new and ultra-powerful mathematical algorithms for the on-board compression of the 50-million-stars data that GAIA will send to Earth from its intended geostationary orbit. Preliminary estimates of the raw data rates from the GAIA focal plane, in fact, are of the order of a few Gigabits per second. To reduce the data stream to the envisaged telemetry link of 1 Megabit per second, on-board data compression with a 1 to 1,000 ratio is the target. Clearly, this is far beyond the capabilities of any lossless compression technique (enabling compression ratios of 1 to some tens), and so some “wise” lossy compression mathematical procedure must be adopted.

In this paper a GAIA-adapted lossy data compression technique is presented, based on the Karhunen-Loève Transform (KLT). The essence of this method was already used by NASA for the Galileo mission when the large antenna got stuck and the mission was rescued by re-programming the on-board computer in terms of the KLT. That transform was officially named ICT — “Integer Cosine Transform” — by the NASA-JPL team led by Dr. Kahr-Ming Cheung. But the KLT here described for GAIA will of course differ from the JPL one in many regards, owing to the advances in computer technology.

Finally, estimates are also given about the possibility of using the KLT for onboard data compression in case GAIA is going to be put into orbit around the Lagrangian point L2 of the Earth-Sun system, and, above all, in case the number of stars to be observed is actually raised from 50 millions to one billion, as ESA currently appears to be likely to pursue.     

含固定转速驱动电机的挠性卫星自旋稳定性分析

分析一类含驱动电机挠性卫星的自旋稳定性,应用不变集定理证明了系统轨迹收敛于系统平衡点集,并通过构造正定的单调递减的类能量函数来判断各平衡点的稳定性质。获得的结论包括：由于挠性振动阻尼特性,在无外扰力矩作用时,含固定转速驱动电机的挠性卫星存在稳定的自旋;不含驱动电机的挠性卫星,稳定的自旋轴为其最大主惯量轴;驱动电机可改变挠性卫星的自旋轴,给出了稳定的自旋轴与电机转速方向夹角公式,该夹角为锐角;当电机转动角动量幅值远小于系统角动量时,稳定自旋轴接近最大主惯量轴;电机转动角动量幅值越接近系统角动量,稳定自旋轴越接近电机转动方向。文中也给出了稳定自旋轴平行电机转轴的条件。     

High performance satellite networks

The high performance satellite communications networks of the future will have to be interoperable with terrestrial fiber cables. These satellite networks will evolve from narrowband analogue formats to broadband digital transmission schemes, with protocols, algorithms and transmission architectures that will segment the data into uniform cells and frames, and then transmit these data via larger and more efficient synchronous optional (SONET) and asynchronous transfer mode (ATM) networks that are being developed for the information “superhighway”. These high performance satellite communications and information networks are required for modern applications, such as electronic commerce, digital libraries, medical imaging, distance learning, and the distribution of science data.

In order for satellites to participate in these information superhighway networks, it is essential that they demonstrate their ability to: (1) operate seamlessly with heterogeneous architectures and applications, (2) carry data at SONET rates with the same quality of service as optical fibers, (3) qualify transmission delay as a parameter not a problem, and (4) show that satellites have several performance and economic advantages over fiber cable networks.