A multiple-rendezvous,sample-return mission to two near-Earth asteroids

We propose a dual-rendezvous mission, targeting near-Earth asteroids, including sample-return. The mission, Asteroid Sampling Mission (ASM), consists of two parts: (i) flyby and remote sensing of a Q-type asteroid, and (ii) sampling of a V-type asteroid. The targeted undifferentiated Q-type are found mainly in the near-Earth space, and to this date have not been the target of a space mission. We have chosen, for our sampling target, an asteroid from the basaltic class (V-type), as asteroids in this class exhibit spectral signatures that resemble those of the well-studied Howardite–Eucrite–Diogenite (HED) meteorite suite. With this mission, we expect to answer specific questions about the links between differentiated meteorites and asteroids, as well as gain further insight into the broader issues of early Solar System (SS) evolution and the formation of terrestrial planets. To achieve the mission, we designed a spacecraft with a dry mass of less than 3 tonnes that uses electric propulsion with a solar-electric power supply of 15 kW at 1 Astronomical Unit (AU). The mission includes a series of remote sensing instruments, envisages landing of the whole spacecraft on the sampling target, and employs an innovative sampling mechanism. Launch is foreseen to occur in 2018, as the designed timetable, and the mission would last about 10 years, bringing back a 150 g subsurface sample within a small re-entry capsule. This paper is a work presented at the 2008 Summer School Alpbach,“Sample return from the Moon, asteroids and comets” organized by the Aeronautics and Space Agency of the Austrian Research Promotion Agency. It is co-sponsored by ESA and the national space authorities of its Member and Co-operating States, with the support of the International Space Science Institute and Austrospace.     

Robust attitude stabilization of spacecraft subject to actuator failures

A robust nonlinear control scheme is developed to stabilize the 3-axis attitude of the spacecraft for cases where there is no control available on either roll or yaw axis. The stability conditions for robustness against unmatched uncertainties and disturbances are derived to establish the regions of asymptotic 3-axis attitude stabilization. The properties of the proposed sliding surface are investigated to obtain the domains of sliding mode for the closed-loop system. Several numerical simulations are presented to demonstrate the efficacy of the proposed controller and validate the theoretical results. The control algorithm is shown to compensate for time-varying external disturbances including solar radiation pressure, aerodynamic forces, and magnetic disturbances; and uncertainties in the spacecraft inertia parameters. The numerical results also establish the robustness of the proposed control scheme to negate disturbances caused by orbit eccentricity.     

Variable structure control for satellite attitude stabilization in elliptic orbits using solar radiation pressure

The paper proposes the use of solar radiation pressure for satellite attitude control in elliptic orbits based on variable structure control. The system comprises of a satellite with two-oppositely placed solar flaps. Sliding mode control and terminal sliding mode control techniques have been adopted to develop nonlinear control laws for suitably rotating the control solar flaps to neutralize the adverse effect of eccentricity normally responsible for a considerable deterioration in the attitude control performance. The detailed numerical simulation of the governing nonlinear equation of the motion including the effects of various system parameters on the controller performance, establishes the feasibility of the proposed control strategy. The proposed controller is found to be robust against parameter uncertainties and external disturbances and thereby, the control strategy presented in the paper may be applicable to future satellite missions.     

Operations of a spaceflight experiment to investigate plant tropisms

Plants will be an important component in bioregenerative systems for long-term missions to the Moon and Mars. Since gravity is reduced both on the Moon and Mars, studies that identify the basic mechanisms of plant growth and development in altered gravity are required to ensure successful plant production on these space colonization missions. To address these issues, we have developed a project on the International Space Station (ISS) to study the interaction between gravitropism and phototropism in Arabidopsis thaliana. These experiments were termed TROPI (for tropisms) and were performed on the European Modular Cultivation System (EMCS) in 2006. In this paper, we provide an operational summary of TROPI and preliminary results on studies of tropistic curvature of seedlings grown in space. Seed germination in TROPI was lower compared to previous space experiments, and this was likely due to extended storage in hardware for up to 8 months. Video downlinks provided an important quality check on the automated experimental time line that also was monitored with telemetry. Good quality images of seedlings were obtained, but the use of analog video tapes resulted in delays in image processing and analysis procedures. Seedlings that germinated exhibited robust phototropic curvature. Frozen plant samples were returned on three space shuttle missions, and improvements in cold stowage and handing procedures in the second and third missions resulted in quality RNA extracted from the seedlings that was used in subsequent microarray analyses. While the TROPI experiment had technical and logistical difficulties, most of the procedures worked well due to refinement during the project.     

A multiobjective optimization approach to determine the parameters of stepped frequency pulse train

Frequency stepping techniques are commonly used in modern radar system to get high range resolution with the disadvantage that its autocorrelation function (ACF) yield undesirable “grating lobes”. Wider mainlobe deteriorates the range resolution capability of the waveform and higher peak sidelobe either hides the small targets or causes the false target detection. Several techniques have been used to choose the parameters of linear frequency modulated (LFM) pulse train to suppress the grating lobes without paying much attention to the mainlobe width and peak sidelobe level. In this paper a multiobjective optimization (Nondominated Sorting Genetic Algorithm-II (NSGA-II)) approach is proposed to optimize the parameters of the LFM pulse train to achieve reduced grating lobes, low peak sidelobe level and narrow mainlobe width. The optimization problem has been studied in two different ways: first one is associated with the reduction of grating lobes and the minimization of peak sidelobe level of the ACF with constraints and second one is related to the minimization of the peak sidelobe level and mainlobe width of the ACF with constraints. Simulation studies have been carried out to justify the potentiality of the proposed approach.     

A PC based test system for an on-board computer

This paper presents a robust method of testing a processor based on-board computer used in the Indian remote sensing satellite (IRS-1C). The novelty of this scheme lies in a judicious mix of software and hardware approaches adopted to realise this PC-based system. All the electrical interfaces (like telemetry, Telecommand, timer and star sensor) are simulated. They are fabricated as add-on cards to PC-AT (ISAbus). The on-board computer controls two star sensors apart from handling three telemetry streams and accepting more than one hundred Telecommands     

Orbital and Attitude Prediction Accuracy Requirements for Satellites Engaged in Remote Sensing and Scientific Missions

The criteria for selecting the orbital and attitude prediction accuracy requirements for communications satellites have been investigated in a previous paper by the author. The present paper extends the analysis to the satellites engaged in remote sensing and scientific missions. A unified approach has been developed to examine the problem of the prediction accuracy requirements for the two missions.     

Feasibility of using statellites in non-equatorial 24-hour circular orbits for communications

The early sixties witnessed the debate among competing candidate orbits that led to the emergence of perfect geostationary systems as virtually the sole “instruments” for satellite communication. The subsequent problem of overcrowding of geostationary ring on one hand and explosive growth in demand on communication capacity on the other led comsat experts to focus on the alternate routes through various near-earth and medium attitude satellite constellations later proposed for uninterrupted communication. However, the opportunities thrown up by quasi-stationary orbits for augmentation of the space communication capacity have gone abegging. This paper attempts to draw attention of communication satellite designers/planners to the immense potential for utilization of the non-equatorial, 24-hour circular orbits for communication. For the proposed quasi-stationary orbits, the change and/or control of the inclination of the plane is not envisaged in the launch and/or operational phase. The resulting significant payload weight advantage is associated with the problem of periodic as well as secular apparent angular satellite drift relative to the ground terminal. However, the problem may be largely overcome through controlled satellite tilting using solar radiation pressure or through the use of tethered auxiliary mass attachment. Alternatively, it may be possible to overcome the attitude control problem by the use of systems such as on-bard electronically steerable phased array antenna capable of following the line-of-sight to the co-operative ground station.     

Correlation of Binarized Images

Correlator technique has been used in pattern recognition applications. We introduce a correlation performance measure, peak to background ratio (PBR), defined as the ratio of the peak intensity to the average background variation in a single correlation output. Using this performance measure, we analyze the binary correlator and show that the binarization of the images can result in better performance. We consider a real-time implementation of the binary correlator, using VLSI technology. The analytical results are supported by computer simulations.