Fabio Santoni

Spacecraft Angular Rate Estimation from Magnetometer Data Only Using an Analytic Predictor

A 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 assumption that the inertial geomagnetic field vector does not significantly change during the short sampling time. As the external disturbance torque is neglected, an analytic solution of Euler’s equations can be used in the filter’s propagation phase, allowing a significant savings of computation time compared to numerical integration of Euler’s equations. Contrary to most existing angular rate estimators, the spacecraft’s attitude is neither used nor estimated within the proposed algorithm. Moreover, the body-referenced geomagnetic field observations are not differentiated with respect to time as an external prefiltering procedure but are directly processed by the filter. This processing gives rise to a colored effective measurement noise, which is properly handled via approximate Markov modeling and application of Bryson and Henrikson’s reduced-order filtering theory. A simulation study employing a standard tenth-order International Geomagnetic Reference Field model is presented to demonstrate the performance of the algorithm.

Determination of disposed-upper-stage attitude motion by ground-based optical observations

The results of an observation campaign devoted to the determination of the rotational state of uncontrolled disposed upper stages by ground-based optical observations are presented. The photometric analysis is performed to extract light curves from photographs obtained by a camera based on charge-coupled devices. The rotation-axis direction and rotation period are determined comparing measurements with theoretical predictions obtained in the simplifying assumption of cylindrical diffusely reflecting cylinders. The angular speed is evaluated by the light-curve period. The direction of rotation axis is evaluated by measuring the difference between the maximum and minimum apparent magnitude of disposed upper stages. This evaluation can be performed from a single observing site by combining measurements collected during a suitable observation time span. The results achieved for two orbiting objects are reported showing a good agreement between determinations obtained in completely different observation geomet...

Risk management for micro-satellite design

Abstract It is shown how the methods of risk analysis have been used in the University of Rome micro-satellite program. One of the driving research topics related to this program is the reduction of cost in building spacecraft. The probability risk analysis techniques seem to be a powerful tool in the field of micro-satellites design, to outline possible faults. Innovation and limited budget forcing the designer to move in a very “risky” environment and can be faced with an as rigorous as possible decision making method. In our project, cost reduction is often attained relying on commercial, not space-rated components, which of course increases risk. This is why the design process should be led by the careful analysis of the risk associated with the selection of components and construction techniques. In standard applications risk can be evaluated from reliability data obtained in previous and well known similar applications. In our case, for many components, there is a lack of reliability data, due to the obvious missing experience when dealing with not yet space qualified, or even never flown before components. This lack is overcome using numerical simulations and practical engineering considerations, but does not allow a rigorous reliability assessment. A simple qualitative analysis is used to rank priorities among subsystems and allocate economic resources and development efforts. The main risk source is space radiation effect on CMOS electronic components. Therefore, development resources are directed to radiation effect mitigation. A procedure is proposed to lower risk without using space rated components.

Measurement of Electromagnetic Field Attenuation by Building Walls in the Mobile Phone and Satellite Navigation Frequency Bands

The electromagnetic (EM) wave attenuation by building walls is experimentally investigated, focusing on the frequency range 700 MHz-5 GHz. The results of this research can be used in the planning of the indoor radio coverage of wireless access networks like 2G, 3G, 4G, WiFi, and future 5G mobile phone systems, in the study of satellite navigation applications, and in the evaluation of the resident population exposure to electromagnetic fields emitted by the radio base stations (RBSs) of the mobile phone radio access network (RAN). Measurements are obtained using a portable vector network analyzer (VNA) connected to two light Vivaldi antennas. Time-domain methods are used to reduce errors caused by multiple paths. Measurements of EM building wall attenuation have been carried out in the city of Rome, Italy, in different buildings topologies: historical buildings from the Roman Empire up to middle 19th-century and modern reinforced concrete buildings. Measurements of EM shielding effectiveness (SE) show values even greater than 80 dB.

Analysis of the UNISAT-3 solar array in-orbit performance

DOI: 10.2514/1.32392 A technological experiment to assess the in-orbit performance of non-space-rated solar arrays is onboard the microsatellite UNISAT-3. These include terrestrial-technology monocrystalline silicon and low-efficiency triplejunction solar arrays, assembled using commercial off-the-shelf materials and following conventional non-spacerated procedures. A space-rated monocrystalline silicon solar array is also onboard, used for comparison of the achieved results. The manufacturing process is described andthe solararray performance is evaluated based on the data collected in the first two-and-a-half years of operation in orbit. For both terrestrial-technology and lowefficiencytriple-junctionsolararrays,theobservedsolararraydegradationisveryfastinthe firsthalf-year,inwhich approximately one-fourth of the initial efficiency is lost. Then degradation stabilizes, showing a long-term decaying rate with a time constant of ten years, which makes the suggested technology suitable for small scientific and educational low-Earth-orbit spacecraft.

Passive magnetic attitude stabilization system of the EduSAT microsatellite

Abstract Passive magnetic attitude stabilization systems are simple, easy to realize, cheap, and do not require software development and on-board energy consumption. Owing to these features, passive magnetic attitude stabilization systems were selected for the EduSAT (Educational Satellite) microsatellite, a student-built satellite funded by the Italian Space Agency, scheduled to be launched in the last quarter year of 2010. The passive magnetic attitude stabilization system is based on a permanent magnet, which provides a restoring torque to align an oriented axis of the satellite with the Earths magnetic field direction, and an energy dissipation system, which can consist of a set of permeable rods magnetized by the oscillation of the geomagnetic field along their axis. UNISAT-3 attitude determination results after 1 year from its launch demonstrated the necessity of an accurate design and manufacturing process of soft magnetic strips. Predicting system performance in orbit and evaluating the obtainable accuracy are not trivial: the main problem is knowing the effective magnetization of the permeable rods. The paper deals with sizing, choice of material, manufacturing process, and arrangement of a set of permeable rods on board the EduSAT microsatellite on the basis of previous flight experience.

Nanosatellite Cluster Launch Collision Analysis

AbstractNanospacecraft are usually launched in clusters from a single launch, in which they are hosted as secondary payloads. The nanospacecraft, e.g., CubeSats, are usually released simultaneously in small groups from one single container, to simplify the launcher interface manufacturing and the launch operations. The release sequence must be designed to minimize the risk of collisions among the nanospacecraft themselves. Considering the increasing number of micro- and nanosatellites in recent space activity, a collision risk analysis and a release systems configuration optimization are of interest. This paper describes a collision probability analysis for a nanosatellite cluster deployment, performed by a series of Monte Carlo simulations, comparing results obtained with different release mechanisms, procedures, and orbital dynamics modeling assumptions. The collision risk in the first orbits after launch depends mostly on the in-orbit injection initial conditions, such as relative initial position and ...

Close approach analysis in the geosynchronous region using optical measurements

E ARTH orbiting spacecraft are exposed to a hazardous micrometeoroid and orbital debris environment. These objects can collide at very high velocity, causing significant damage to operational satellites in any orbital regime, from low Earth orbit (LEO) to geosynchronous orbit (GEO). In particular, the rapid growth of the number of spacecraft and apogee kick motors in the geosynchronous region increases the concern regarding the risk of collision between space objects in GEO [1]. Even a small number of potential explosions could seriously affect long-term use and exploitation of this orbital regime [2]. According to the results of observations given in [3], the number of objects present in the GEO protected region includes 391 operational and 706 nonoperational spacecraft. These include nonfunctional satellites, spent upper stages and apogee kick motors, fragments, and objects of undetermined type. Unintentional close approaches involving operational spacecraft and failed ones are not infrequent and the possibility of having dangerous close approaches is likely to increase in the future as the GEO belt becomes more crowded. A well-known example is the failure of Telstar 401, leading to several close encounters with operational GEO satellites, at distances as low as 2 km, with 2 km uncertainty [4]. Hence, a series of specific mitigation and collision avoidance measures have to be applied to objects orbiting in this region. To perform a close approach (CA) analysis, the two objects’ states, namely position and velocity vectors as well as the state covariance, are necessary. Many of these objects are catalogued and regularly monitored by NORAD, which distributes the two line element (TLE) sets. These orbital parameters could in principle be used to perform a risk assessment analysis based on the impact probability between orbiting objects. Comparison of TLEpredictions in GEO with astrometric optical observations reported in [5] show that expectation value and standard deviation for the distances between the catalog ephemerides propagated with SDP4 and the observed astrometric position of observations is in the order of 4.0 23.25 (1σ) km in track and 4.96 7.54 (1σ) km cross track. However, there are specific instances in which the accuracy of a TLE prediction can be much worse than average, in particular, when the most recent TLE (reference epoch) is several days old. In these cases, the accuracy of the provided state vectors might not be sufficient for accurate collision analyses, and further orbit determination should be performed in order to determine accurately the state of the objects and the related covariance [6]. As a matter of fact, as discussed in [7,8], collision risk management in geosynchronous orbit can be very costly using data of insufficiently high accuracy because data errors induce high action frequency for even modest amounts of collision risk reduction. Moreover, an accurate covariance matrix is required for optimizing the collision avoidance maneuvers wherever required [9]. This level of accuracy can be enhanced by integrating the information provided by the TLE with optical measurements. The main aim of the analysis presented is to evaluate the effectiveness of using optical astrometric measurements in support of CA management compared to predictions based only on TLEs. The emphasis is placed on experimental results obtained in the observations of a CA in the GEO region between GORIZONT 11 and COSMOS 1738, which occurred on 22 December 2011.

Soft Magnets for Passive Attitude Stabilization of Small Satellites

The spinning and oscillatory motions of small orbiting satellites can be damped exploiting the magnetic energy dissipation occurring in onboard soft magnetic strips, cyclically excited by the oscillation of the earth field component along their axis. In this paper we investigate the role played by the intrinsic magnetic properties of the material, the aspect ratio of the strips, and their mutual arrangement in achieving maximum energy dissipation under typical spacecraft working conditions. Grain-oriented Fe-Si, mumetal, and Fe-based amorphous alloys, all endowed with near-rectangular hysteresis loops, are considered. Their energy loss behaviour is calculated when, either as single strip samples or arranged into an array of strips, they are subjected to a slowly oscillating magnetic field of defined peak value, emulating the action of the earth magnetic field on the travelling satellite. The strip size and array layout leading to maximum energy loss are predicted. Amorphous alloys, combining high saturation magnetization with flexible hysteresis loop properties, are shown to lead to the best damping behaviour under both oscillating and spinning satellite motions. In the latter case the Fe-Si strips appear to provide comparably high damping effects, while inferior behaviour is always predicted with mumetal samples.

Dynamics of Spring-Deployed Solar Panels for Agile Nanospacecraft

AbstractAdvanced performances in nanospacecraft are typically associated with high onboard power availability, requiring deployed solar arrays. Nanospacecraft geometrical and weight constraints do not allow the use of reversible dynamics and lock systems used in conventional spacecraft. Most nanospacecraft deployed solar panel systems are therefore based on compact and reliable miniaturized mechanisms, such as simple passively deployed preloaded springs. The use of these kinds of deployed solar arrays may interfere with the performance of the attitude control system. Agile nanospacecraft, designed to perform rapid attitude maneuvers, may potentially be affected by the solar array dynamical interaction with the main satellite body motion. In addition, the presence of inherently nonlinear elements, such as bounds on allowed rotations and preloaded springs, deserve a dedicated analysis. The analysis developed in this paper is based on the performance of state-of-the art agile nanosatellite attitude control s...