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

Real-Time Hardware-in-the-Loop Laboratory Testing for Multisensor Sense and Avoid Systems

This paper focuses on a hardware-in-the-loop facility aimed at real-time testing of architectures and algorithms of multisensor sense and avoid systems. It was developed within a research project aimed at flight demonstration of autonomous non-cooperative collision avoidance for Unmanned Aircraft Systems. In this framework, an optionally piloted Very Light Aircraft was used as experimental platform. The flight system is based on multiple-sensor data integration and it includes a Ka-band radar, four electro-optical sensors, and two dedicated processing units. The laboratory test system was developed with the primary aim of prototype validation before multi-sensor tracking and collision avoidance flight tests. System concept, hardware/software components, and operating modes are described in the paper. The facility has been built with a modular approach including both flight hardware and simulated systems and can work on the basis of experimentally tested or synthetically generated scenarios. Indeed, hybrid operating modes are also foreseen which enable performance assessment also in the case of alternative sensing architectures and flight scenarios that are hardly reproducible during flight tests. Real-time multisensor tracking results based on flight data are reported, which demonstrate reliability of the laboratory simulation while also showing the effectiveness of radar/electro-optical fusion in a non-cooperative collision avoidance architecture

Real-Time Hardware-in-the-Loop Laboratory Testing for Multisensor Sense and Avoid Systems

This paper focuses on a hardware-in-the-loop facility aimed at real-time testing of architectures and algorithms of multisensor sense and avoid systems. It was developed within a research project aimed at flight demonstration of autonomous non-cooperative collision avoidance for Unmanned Aircraft Systems. In this framework, an optionally piloted Very Light Aircraft was used as experimental platform. The flight system is based on multiple-sensor data integration and it includes a Ka-band radar, four electro-optical sensors, and two dedicated processing units. The laboratory test system was developed with the primary aim of prototype validation before multi-sensor tracking and collision avoidance flight tests. System concept, hardware/software components, and operating modes are described in the paper. The facility has been built with a modular approach including both flight hardware and simulated systems and can work on the basis of experimentally tested or synthetically generated scenarios. Indeed, hybrid operating modes are also foreseen which enable performance assessment also in the case of alternative sensing architectures and flight scenarios that are hardly reproducible during flight tests. Real-time multisensor tracking results based on flight data are reported, which demonstrate reliability of the laboratory simulation while also showing the effectiveness of radar/electro-optical fusion in a non-cooperative collision avoidance architecture

Digital Sun Sensor Multi-Spot Operation

The operation and test of a multi-spot digital sun sensor for precise sun-line determination is described. The image forming system consists of an opaque mask with multiple pinhole apertures producing multiple, simultaneous, spot-like images of the sun on the focal plane. The sun-line precision can be improved by averaging multiple simultaneous measures. Nevertheless, the sensor operation on a wide field of view requires acquiring and processing images in which the number of sun spots and the related intensity level are largely variable. To this end, a reliable and robust image acquisition procedure based on a variable shutter time has been considered as well as a calibration function exploiting also the knowledge of the sun-spot array size. Main focus of the present paper is the experimental validation of the wide field of view operation of the sensor by using a sensor prototype and a laboratory test facility. Results demonstrate that it is possible to keep high measurement precision also for large off-boresight angles

Micro-sun-sensor performance validation in ground-reproduced orbital conditions

This papers deals with the large-FOV, micro-sun-sensor under development at the University of Naples. The sensor exploits a multi-hole mask to measure the sun line with high precision. Nevertheless, multi-spot operation exhibits failures near FOV borders, due to the uncertainty in the number of spots that can be reli-ably acquired. After a short description of the sensor concept, hardware model, and laboratory test equipment, this paper focuses on the latest upgrades of the sensor operation mode, aimed at getting reliable operation ability and improved precision over a wider FOV (at least 90x80 deg). Specifically, an original tech-nique is implemented in which the sensor shutter-time is automatically adapted based on detected image intensity to improve precision near FOV borders. Re-sults of the validation and performance assessment campaign of the upgraded operation mode executed with the ground facility are presented. In these tests sensor precision is characterized as a function of the separation of the illumina-tion direction from FOV centre, showing that the upgraded operation mode al-lows getting high precision (about 0.001°) also near FOV borders

Mission-oriented Micro-Sun-Sensor Laboratory Testing in Real-Time Operation Mode

This paper describes the digital sun sensor under development at the University of Naples. The sensor optical head exploits a CMOS photodetector and a mask with multiple holes. This original design allows forming multiple simultaneous images of the sun on the focal plane, which, once processed, produce multiple measurements of the sun line orientation. These are then averaged to improve sensor precision. To achieve high accuracy in sun line determination a calibration function based on neural networks is developed. The sensor can be operated either with a fixed number or with a variable number of sun spots, depending on the required field of view and sun-line measurement precision. This improves sensor flexibility and operation range but, at the same time, increases the computational load when multiple spots are used. Specific techniques are developed to minimize the computational load needed to process multiple acquisitions and to extract the sun line orientation with high accuracy and precision, and adequate updating frequency. These techniques have been tested by extensive test campaigns, carried out by using a laboratory test facility reproducing sun spectrum, apparent size and distance, and variable illumination directions. Specifically, tests reproducing the in-orbit planned experiment profile in terms of a sequence of sun line orientations are performed. Test results validate the sensor concept, confirming the precision improvement achievable with multiple apertures, and the sensor capability of operating with a variable number of sun spots