Giancarlo Rufino

Enhancement of the centroiding algorithm for star tracker measure refinement

Abstract The application of the hyperacuity technique to image processing of star trackers is analysed. An analytical study of the error introduced by the centroiding algorithm is presented and it is shown that a systematic contribution and a random one exist. They result from image processing assumptions and photometric measure uncertainty, respectively. Their behaviour is characterised by means of numerical simulations that are based on optics theoretical point spread functions. The latter ones take into account both defocus and diffraction effects. First, measured star position uncertainty is evaluated as a function of defocus. As a result, a criterion for optimal defocus is presented. Subsequently, an original procedure for systematic centroiding error correction by means of a backpropagation neural network is described. It is also suitable for real hardware calibration. When applied to one of the considered numerical models, the position computation accuracy is improved from 0.01 to 0.005 pixels.

Spaceborne along-track SAR interferometry: performance analysis and mission scenarios

A system study of a spaceborne along-track synthetic aperture radar (SAR) interferometer is presented. This sensor has been successfully experienced for detecting moving targets by using only airborne installations. Several key issues must be addressed when spaceborne configurations are envisaged. To this end, a quantitative evaluation of system performance and measurement accuracy has been conducted. First, the identification of possible space configurations has been accomplished. In particular, the two antennas can operate on a single satellite or they can be carried along appropriate trajectories by two spacecrafts. Then, an error budget of radial velocity measurement accuracy has been performed. Finally, two possible mission scenarios are dealt in details, and numerical results are reported.

Brightness-independent start-up routine for star trackers

Initial attitude acquisition by a modern star tracker is investigated here. Criteria for efficient organization of the on-board database are discussed with reference to a brightness-independent initial acquisition algorithm. Star catalog generation preprocessing is described, with emphasis on the identification of minimum star brightness for detection by a sensor based on a charge coupled device (CCD) photodetector. This is a crucial step for proper evaluation of the attainable sky coverage when selecting the stars to be included in the on-board catalog. Test results are also reported, both for reliability and accuracy, even if the former is considered to be the primary target. Probability of erroneous solution is 0.2% in the case of single runs of the procedure, while attitude determination accuracy is in the order of 0.02/spl deg/ in the average for the computation of the inertial pointing of the boresight axis.

Performance of spaceborne bistatic synthetic aperture radar

This paper reports on a model developed for evaluating major system performance of a spaceborne bistatic synthetic aperture radar (SAR) for remote sensing applications. The procedure accounts for formation flying aspects. It is particularly aimed at comparison of monostatic and bistatic cases, and, as a test case, it is applied to study a novel configuration, based on a small satellite equipped with a receiving-only antenna orbiting in tandem with a large, noncooperative transmitting spacecraft, the Italian COSMO-SkyMed mission. Numerical results and plots show the effectiveness of the procedure as a mission design tool and put in evidence key issues and characteristics of the proposed spaceborne bistatic formation.

BISSAT: a bistatic SAR for Earth observation

This paper summarizes scientific rationale and technical approach for a bistatic synthetic aperture radar (SAR) mission (BISSAT). The study has been funded by the Italian Space Agency for a competitive Phase-A study along with other five missions. Its concept consists in flying a passive SAR on board a small satellite, which observes the area illuminated by an active SAR, operating on an already existing large platform.

Integrated VIS-NIR Hyperspectral / Thermal-IR Electro-Optical Payload System for a Mini-UAV

This paper presents the development of a modern electro-optical payload system for remote sensing from a mini-UAV. It is aimed at applications of natural disasters monitoring, in particular forest fires. Both the sensor and the mini-UAV platform are being developed at the Dept. of Space Science and Engineering (DISIS) of the University of Naples “Federico II.” The core of the system is an integrated, multi-band sensor that includes a thermal imager and a hyperspectral sensor in VNIR band. Instrument characterization, laboratory tests, and payload architecture are discussed.

Multi-Aperture CMOS Sun Sensor for Microsatellite Attitude Determination

This paper describes the high precision digital sun sensor under development at the University of Naples. The sensor determines the sun line orientation in the sensor frame from the measurement of the sun position on the focal plane. It exploits CMOS technology and an original optical head design with multiple apertures. This allows simultaneous multiple acquisitions of the sun as spots on the focal plane. The sensor can be operated either with a fixed or a variable number of sun spots, depending on the required field of view and sun-line measurement precision. Multiple acquisitions are averaged by using techniques which minimize the computational load to extract the sun line orientation with high precision. Accuracy and computational efficiency are also improved thanks to an original design of the calibration function relying on neural networks. Extensive test campaigns are carried out using a laboratory test facility reproducing sun spectrum, apparent size and distance, and variable illumination directions. Test results validate the sensor concept, confirming the precision improvement achievable with multiple apertures, and sensor operation with a variable number of sun spots. Specifically, the sensor provides accuracy and precision in the order of 1 arcmin and 1 arcsec, respectively.

A Model-Based 3D Template Matching Technique for Pose Acquisition of an Uncooperative Space Object

This paper presents a customized three-dimensional template matching technique for autonomous pose determination of uncooperative targets. This topic is relevant to advanced space applications, like active debris removal and on-orbit servicing. The proposed technique is model-based and produces estimates of the target pose without any prior pose information, by processing three-dimensional point clouds provided by a LIDAR. These estimates are then used to initialize a pose tracking algorithm. Peculiar features of the proposed approach are the use of a reduced number of templates and the idea of building the database of templates on-line, thus significantly reducing the amount of on-board stored data with respect to traditional techniques. An algorithm variant is also introduced aimed at further accelerating the pose acquisition time and reducing the computational cost. Technique performance is investigated within a realistic numerical simulation environment comprising a target model, LIDAR operation and various target-chaser relative dynamics scenarios, relevant to close-proximity flight operations. Specifically, the capability of the proposed techniques to provide a pose solution suitable to initialize the tracking algorithm is demonstrated, as well as their robustness against highly variable pose conditions determined by the relative dynamics. Finally, a criterion for autonomous failure detection of the presented techniques is presented.

Satellite Angular Velocity Estimation Based on Star Images and Optical Flow Techniques

An optical flow-based technique is proposed to estimate spacecraft angular velocity based on sequences of star-field images. It does not require star identification and can be thus used to also deliver angular rate information when attitude determination is not possible, as during platform de tumbling or slewing. Region-based optical flow calculation is carried out on successive star images preprocessed to remove background. Sensor calibration parameters, Poisson equation, and a least-squares method are then used to estimate the angular velocity vector components in the sensor rotating frame. A theoretical error budget is developed to estimate the expected angular rate accuracy as a function of camera parameters and star distribution in the field of view. The effectiveness of the proposed technique is tested by using star field scenes generated by a hardware-in-the-loop testing facility and acquired by a commercial-off-the shelf camera sensor. Simulated cases comprise rotations at different rates. Experimental results are presented which are consistent with theoretical estimates. In particular, very accurate angular velocity estimates are generated at lower slew rates, while in all cases the achievable accuracy in the estimation of the angular velocity component along boresight is about one order of magnitude worse than the other two components.

An Integrated Electro-Optical Payload System for Forest Fires Monitoring from Airborne Platform

This paper presents the preliminary results obtained within a research project aimed at the development of a remote sensing system for forest fires monitoring in missions of compact airborne platforms. The core of the system is an integrated, multi-/hyper-spectral suite of electro-optical sensors. They were selected to get enhanced ability in forest fire detection and monitoring. The system is completed by a dedicated on-board computer for sensor control. It is capable of autonomous operation and it is also in charge of data exchange with the on-board navigation and flight control system. Two different configurations are described, relevant to different aircrafts: a mini UAV and a certified, two-seat light aircraft. Results of the first flight experiments are presented, that highlight performance achievable by the system.