1,721,241 research outputs found
1H-NMR and Photo-CIDNP Spectroscopies show a possible Role for Trp23 and Phe31 in Nucleic Acid Binding by P2 Ribonuclease from the archaeon Sulfolobus solfataricus
No full textInvestigations were performed on recombinant ribonuclease P2 from Sulfolobus solfataricus, previously cloned and expressed in Escherichia coli [Fusi, P., Grisa, M., Mombelli, E., Consonni, R., Tortora, P. and Vanoni, M. (1995) Gene 154, 99-103]. NMR and photo-CIDNP spectroscopies showed that the enzyme possesses an aromatic cluster consisting of Phe5, Tyr7, Phe31 and Tyr33 while Trp23 is fully exposed to solvent. Phe31, Tyr33 and Trp23 are located within a triple stranded antiparallel beta-sheet, each one being part of an amino acid stretch matching consensus sequences for RNA binding. Phe31 and Trp23 are exposed to and specifically interact with a flavin dye used as a model ligand, with a topology reminiscent of that found in several eubacterial and eukariotic RNA-binding proteins
Small satellites beyond boundaries
Full text linkIt is a great pleasure for me to present this special issue of the CEAS Space Journal devoted to Small Satellites Beyond Boundaries. This brings together some of the most interesting presentations delivered throughout several conferences held in the last couple of years in the area of Small Satellites and CubeSats.
Small satellites (and, more recently, CubeSats) were initially developed for educational purposes, however their capabilities have opened a new design space for exploration that includes stand-alone spacecraft, constellations, and mother–daughter systems that collaborate. In light of recent advances in science instruments and spacecraft miniaturization technologies that have emerged in just the past few years, small spacecraft can now be considered for use in planetary exploration, either as adjuncts to larger missions on which they could “catch a ride” to the most remote and challenging destinations in the solar system, or in some cases, as stand-alone missions of their own. The first ever experience of an interplanetary mission carried out using CubeSats is the Mars Cube One (MarCO) mission to Mars, flown as a companion to the InSight mission to Mars. Next, a spectacular set of 13 CubeSats will be delivered in 2021 to a high lunar orbit within the frame of NASA’s Artemis-1 mission—the first launch of their new space launch system (SLS) rocket
A GPS based attitude determination algorithm for the spin-stabilized microsatellite UNISAT
No full textGPS receivers for attitude determination have been used in several satellite missions. The typical configuration exploits the carrier phase differences among four GPS antennas and estimates the attitude achieving accuracy of 1 deg or better. For spinning satellites it has been shown that GPS based attitude determination is possible using only two antennas. This simplifies the attitude estimation algorithm, avoiding to solve for the so-called integer ambiguity, and to take into account the line-bias. Several methods have been proposed in the literature to estimate the attitude of a spinning spacecraft: most of them make use of a two-stage procedure where, first the spin and nutation frequencies are determined, then they are used in the second step, to estimate satellite's attitude. In this paper a different approach is presented. A Recursive Least Square algorithm is implemented to fit the so-called displacement vector, computed making use of the double-differenced phase measurements, to an analytically predicted function. It allows to estimate at once six independent parameters which define the spinning motion properties. This paper deals with an application of this GPS based attitude determination algorithm to the design of the attitude determination system of the University of Rome microsatellite UNISAT. The angular momentum direction is determined to within 10-2 deg, spin and nutation rates to within 10-5 rad/s, and phase to within 10-1 deg
The microsatellite research program at Universita' di Bologna
No full textIn the Second Faculty of Engineering of the University of Bologna, the Aerospace group in Forlì has started a new microsatellite research program. The first step consists of the design and setup of an amateur radio ground station, recently installed and implemented in the University laboratories. At the same time, researchers, PhD and graduate students are directly involved in the satellite design. The microsatellite weighs about 20kg and consists of a cubical prism, 300mm side, with a modular structure made of six shop-machined Al trays, kept together by eight stainless steel bars. Four Al/Al honeycomb sandwich lateral panels, which are the support structure of glued solar panels, complete the satellite structure. This architecture has been selected in order to have a multi-purpose bus, to be used with minor changes for several missions, accommodating payloads with different volume and power requirements. This paper reports on the current status of the ground station implementation and of the microsatellite bus design and manufacturing
Spacecraft angular rate estimation from magnetometer data only
No full textA method is presented for fast estimation of the angular rate of a tumbling spacecraft in a low-Earth orbit, from sequential readings of Earth's magnetic field. Useful as a backup algorithm in cases of rate gyro malfunctions, or during the initial acquisition phase, the estimator consists of an extended Kalman filter, based on the underlying assumption that the geomagnetic field vector does not significantly change (relative to an inertial frame of reference) during the short sampling time. Contrary to previously introduced angular rate estimators, the spacecraft's attitude is not assumed to be known (nor is it estimated as part of the proposed procedure). Moreover, the body-referenced geomagnetic field observations are not differentiated with respect to time as a prefiltering procedure, but are directly processed by the filter. A simulation study employing the standard 8th order IGRF geomagnetic field model is presented to demonstrate the performance of the algorithm. © 2000 by P. Tortora and Y. Oshman. Published by the American Institute of Aeronautics and Astronautics, Inc
Stochastic gravitational wave background: Upper limits in the 10(-6) to 10(-3) Hz band
No full textWe have used precision Doppler tracking of the Cassini spacecraft during its 2001-2002 solar opposition to derive improved observational limits to an isotropic background of low-frequency gravitational waves. Using the Cassini multilink radio system and an advanced tropospheric calibration system, the effects of heretofore leading noises - plasma and tropospheric scintillation - were, respectively, removed and calibrated to levels lower than other noises. The resulting data were used to construct upper limits to the strength of an isotropic background in the 10-6 to 10-3 Hz band. Our results are summarized as limits on the strain spectrum Sh(f), the characteristic strain (hc = the square root of the product of the frequency and the one-sided spectrum of strain at that frequency), and the energy density (Ω = energy density in bandwidth equal to center frequency assuming a locally white energy density spectrum, divided by the critical density). Our best limits are Sh(f) < 6 × 10-27 Hz-1 at several frequencies in the millihertz band, hc < 2 × 10-15 at about 0.3 mHz, and Ω < 0.025 × h75-2, where h75 is the Hubble constant in units of 75 km s-1 Mpc-1, at 1.2 × 10-6 Hz. These are the best observational limits in the low-frequency band, the bound on Ω, for example, being about 3 orders of magnitude better than previous constraints from Doppler tracking
Nanosatellite-class dynamic attitude simulator for hands-on aerospace control education
Full text linkDue to their low size, mass, development cost and time, nanosatellites have become an increasingly popular tool at universities for providing students with hands-on experience in aerospace education. Among spacecraft subsystems, the attitude determination and control one surely represents a fruitful resource for practicing aerospace control applications. To enable on-ground verification of spacecraft attitude control hardware and software, however, the biggest challenge to overcome is that of providing a representative testing environment. Towards this end, at the μ3S laboratory at the University of Bologna a dynamic hardware in the loop facility has been developed, which allows for testing attitude control subsystems of nanosatellites in the range of 1U to 3U, according to the CubeSat form factor. This paper describes the educational impact that the facility has been having, during both its development and commissioning phases, as well as its early use as a testbed for CubeSats attitude control, which is currently focused on magnetic-based actuation
Large constellations of small satellites: A survey of near future challenges and missions
Full text linkConstellations of satellites are being proposed in large numbers; most of them are expected to be in orbit within the next decade. They will provide communication to unserved and underserved communities, enable global monitoring of Earth and enhance space observation. Mostly enabled by technology miniaturization, satellite constellations require a coordinated effort to face the technological limits in spacecraft operations and space traffic. At the moment in fact, no cost-effective infrastructure is available to withstand coordinated flight of large fleets of satellites. In order for large constellations to be sustainable, there is the need to efficiently integrate and use them in the current space framework. This review paper provides an overview of the available experience in constellation operations and statistical trends about upcoming constellations at the moment of writing. It highlights also the tools most often proposed in the analyzed works to overcome constellation management issues, such as applications of machine learning/artificial intelligence and resource/infrastructure sharing. As such, it is intended to be a useful resource for both identifying emerging trends in satellite constellations, and enabling technologies still requiring substantial development efforts
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