Publications and Conferences

Design of the Active Attitude Determination and Control System for the e-st@r CubeSat

One of the most limiting factors which affects pico/nano satellites capabilities is the poor accuracy in attitude control. To improve mission performances of this class of satellites, the capability of controlling satellite’s attitude shall be enhanced. The paper presents the design, development and verification of the Active Attitude Determination and Control System (A-ADCS) of the E-ST@R Cubesat developed at Politecnico di Torino. The heart of the system is an ARM9 microcontroller that manages the interfaces with sensors, actuators and the on-board computer and performs the control tasks. The attitude manoeuvres are guaranteed by three magnetic torquers that contribute to control the satellite in all mission phases. The satellite attitude is determined elaborating the data provided by a COTS Inertial Measurement Unit, a Magnetometer and the telemetries of the solar panels, used as coarse Sun sensor. Different algorithms have been studied and then implemented on the microprocessor in order to determine the satellite attitude. Robust and optimal techniques have been used for the controller design, while stability and performances of the system are evaluated to choose the best control solution in every mission phase. A mathematical model of the A-ADCS and the external torques acting on the satellite, its dynamics and kinematics, is developed in order to support the design. After the design is evaluated and frozen, a more detailed simulation model is developed. It contains non-ideal sensors and actuators models and more accurate system disturbances models. New numerical simulations permit to evaluate the behaviour of the controller under more realistic mission conditions. This model is the basic element of the Hardware In The Loop (HITL) simulator that is developed to test the A-ADCS hardware (and also the whole satellite). Testing an A-ADCS on Earth poses some issues, due to the difficulties of reproducing real orbit conditions (i.e. apparent sun position, magnetic field, etc). This is especially true in the case of low cost projects, for which complex testing facilities are usually not available. Thanks to a good HITL simulator it is possible to test the system and its “real in orbit” behaviour to a certain grade of accuracy saving money and time for verification. The paper shows the results of the verification of the ADCS by means of the HITL strategy, which are consistent with the expected values.

The active attitude determination and control system for e-st@r-2

The last decade has been characterized by a rapid growth in the development of University satellites, in particular those belonging to the CubeSat standard. In order to make these small satellites suitable for more complex mis- sions (e.g. remote sensing, communication or navigation), they should be able to actively control their attitude about three axes. For example, to allow the use of electro-optic sensors or scientific instrumentation, it is fundamental an accurate pointing of the satellite or, more in general, the ability of the satellite to re-orient itself. One of the most challenging aspects of the re- search activity of the AeroSpace Systems Engineering Team (ASSET) at Politecnico di Torino is the development of an Active Attitude Determination and Control System (A-ADCS) for nano-satellites application. This pa- per illustrates the design, development and verification phases of the A- ADCS as a payload for the e-st@r-2 (E-ST@R stands for Educational saTel- lite at politecnico di toRino) CubeSat. The purpose is to show the changes (and the reasons which led to that) compared to the A-ADCS of the previous CubeSat e-st@r-1. The A-ADCS architecture is then described in details. The core of the system is still an ARM9 microcontroller. It manages the interfaces with sensors, actuators and the On-Board Computer (OBC), per- forming the control tasks and acting as a backup for vital OBC functions. The attitude manoeuvres are guaranteed by three magnetic torquers (MTs) properly sized that contribute to control the satellite in all the mission phases. The satellite attitude is determined processing the data provided by a Commercial-Off-The-Shelf (COTS) Inertial Measurement Unit, solar panels and a magnetometer. Different algorithms such as QMETHOD and QUEST (QUaternion ESTimator) have been studied and then implemented on the microprocessor in order to estimate the attitude. As for the determination, several ad-hoc control strategies, based on best known and most relevant techniques for control system, have been compared. The adopted methodology is based on the Model-and-Simulation Design: during the feasibility study and the preliminary design, simple mathematical models for the orbit environment and satellite dynamics and kinematics have been used in order to support the system development. In next phases, more complex models are implemented for the verification campaign. Many simulation sessions are carried out and the results show how the performances (in terms of stability, power consumption, time to reach desired attitude) vary according to algorithms used for determination and control. The entire design process is described, focusing on control and determination strategies adopted and a discussion of the results is given.

E-st@r-I lessons learned and their application

CubeSats are characterised to be small and cheap platforms, born within universities with educational objectives. However, these systems are becoming more and more attractive for other missions, such as for example technology demonstration, science application, and Earth observation. This requires an increase of CubeSat performance and reliability, because educationally-driven missions have often failed. Nowadays, ESA Education Office is conducting its first edition of Fly Your Satellite! Program devoted to provide support to selected University CubeSat developers of ESA specialists for verification phase of their CubeSats. The goal of the initiative is to increase CubeSat mission reliability through several actions: to improve design implementation, to define best practice for conducting the verification process, and to make the CubeSat community aware of the importance of verification. Within this initiative, CubeSat team at Politecnico di Torino developed the e-st@r-II CubeSat as follow-on of the e-st@r-I satellite, launched in 2012 on the VEGA Maiden Flight. Both 1U satellites are developed to give hands-on experience to university students and to test an active attitude determination and control system. The present work describes the lessons learned gathered during e-st@r-I development and operations, and their application to improve the new CubeSat, from design to operations. In particular, design improvements have been applied to reduce assembly procedure complexity and to deal with possible on-board computer failures. ECSS rules have been considered to design and assess new procedures for the verification campaign, tailoring them when possible with the support of ESA specialists. Different operative modes have been implemented to deal with some anomalies observed during the operations of the first satellite; mainly leading to a new version of the on-board software. In particular, a new activation sequence has been considered to have a stepwise switch-on of the satellite. In conclusion, the know-how gained during e-st@r-I development and operations have been crucial for the development and verification of the e-st@r-II CubeSat.

A tool for nano-satellite functional verification: comparison between different in-the-loop simulation configurations

This paper describes the simulator technology and the verification campaign for the e-st@r CubeSats family, developed at Politecnico di Torino. The satellites’ behavior has been investigated using a Model and Simulation Based Approach. One of the critical issue in the verification and validation of any space vehicle is the impossibility to fully test some features due to the particular and often un-reproducible environment in which it will operate. Simulations result as one of the best means for testing space system capabilities as it may help to overcome the abovementioned problem. In order to perform different simulation configurations for e-st@r CubeSats, an in-house simulator (named StarSim) has been developed. It is a unique infrastructure, modular and versatile, capable of supporting any desired configuration of the system under test, ranging from full algorithm in the loop simulations (AIL), and gradually inserting satellite hardware, until a complete hardware in the loop (HIL) simulation is performed. When a verification campaign is led on a real object, pure AIL computer based simulations (in which all the equipment and mission conditions are reproduced by virtual models) are not sufficient to test the actual software and hardware to a high degree of confidence since real systems can exhibit random and unpredictable dynamics difficult to be perfectly modeled (i.e. communication delays, uncertainties, and so on). For these reasons, Software In The Loop (SIL), Controller In The Loop (CIL) and HIL simulations were planned. SIL simulations foresee that algorithms are written in the final programming language and executed on ground hardware. In CIL simulations, the software runs on the flight processor while other system’s element are still kept virtual. In HIL simulation, the real hardware (i.e. sensors, actuators, and power sources) are included in the loop. In this paper, after the details of the simulator architecture and its characteristics are described, an exhaustive comparison between AIL and HIL simulations is presented, highlighting main differences and singularities: similar trends of the sensible system’s variables are reached but not identical performances (i.e. absolute and average pointing error and stability, attitude determination accuracy, battery charging and discharging duration) arose analyzing the values. Moreover, it is demonstrated how the technology here presented can effectively support and improve the verification and validation activities for a nano-satellite, by increasing the confidence level on the mission objectives achievement.

e-st@r-I experience: valuable knowledge for improving the e-st@r-II design

Many universities in the world have now permanent hands-on education programs based on CubeSats. These small and cheap platforms are becoming more and more attractive also for other-than-educational missions, such as for example technology demonstration, science application, and Earth observation. This will require the development of adequate technology to increase CubeSat performance. Furthermore, it is necessary to improve mission reliability, because educationally-driven missions have often failed. In 2013 the ESA Education Office launched the Fly Your Satellite! Initiative devoted to provide six university teams with the support of ESA specialists for the verification phase of their CubeSat. The project aims at increasing CubeSat mission reliability through several actions: to improve design implementation, to define best practice for conducting the verification process, and to make the CubeSat community aware of the importance of verification. Within this framework, the CubeSat team at Politecnico di Torino developed the e-st@r-II CubeSat as follow-on of the e-st@r-I satellite, launched in 2012 on the VEGA Maiden Flight. E-st@r-I and e-st@r-II are both 1U satellites with educational and technology demonstration objectives: to give hands-on experience to university students and to test an active attitude determination and control system based on inertial and magnetic measurements with magnetic actuation. The paper describes the know-how gained thanks to the e-st@r-I mission and how they have been used to improve the new CubeSat in several areas, from design to operations. The CubeSat design has been improved to reduce the complexity of the assembly procedure and to deal with possible failures on the on-board computer, for example implementing a new communication software in the communications subsystem. New procedures have been designed and assessed for the verification campaign accordingly to ECSS rules and with the support of ESA specialists. Different operative modes have been implemented to deal with some anomalies observed during the operations of the first satellite. The main difference is a new version of the on-board software. In particular, the activation sequence of the satellite has been modified to have a stepwise switch-on of the satellite. In conclusion, the e-st@r-I experience has provided valuable lessons during its development, requirements verification and on-orbit operations. This know-how has become crucial for the development of the e-st@r-II CubeSat.

Lessons learned of a systematic approach for the e-st@r-II CubeSat environmental test campaign

CubeSat-standard satellites have become more and more popular during last years. Education objectives, mainly pursued in the first CubeSat projects, have given way to the design of missions with other-than-education objectives, like Earth observation and technology demonstration. These new objectives require the development of appropriate technology. Moreover, is necessary to ensure a certain level of reliability, because education-driven mission often failed. In 2013 the ESA Education Office launched the Fly Your Satellite! Initiative devoted to provide six university teams with the support of ESA specialists for the verification phase of their CubeSats. Within this framework, the CubeSat Team at Politecnico di Torino developed the e-st@r-II CubeSat. E-st@r-II is a 1U satellite with educational and technology demonstration objectives: to give hands-on experience to university students; to demonstrate the capability of autonomous attitude determination and control, through the design, development and test in orbit of an A-ADCS; and to test in orbit COTS technology and in-house developed hardware and software (as UHF communication subsystem and software for on-board and data handling subsystem). The paper describes the application of a systematic approach to the definition, planning and execution of environmental test campaign of e- st@r-II CubeSat and the gathered lessons learned. The approach is based on procedures designed and assessed for the vibrations and thermal-vacuum cycling tests of a CubeSat accordingly to ECSS rules and with the support of ESA specialists. Concretely, ECSS application, tailored to fit a CubeSat project, allowed to define a test plan oriented to reduce verification duration and cost, which lead to a lean verification execution. Moreover, the interaction with ESA thermal and mechanical experts represented a valuable aid to increase the Team know-how and to improve and optimise the verification plan and its execution. The planning encompasses the analysis of the requirements to be verified that have been gathered in such a way that the tests duration has been reduced. The required tests, like thermal- vacuum cycling and bake-out tests, have been combined in order to speed-up the verification campaign. The tests outputs shown that the satellite is able to withstand launch and space environment. Furthermore, satellite expected functionalities have been tested and verified when the CubeSat is subjected to space environment, in terms of temperature and vacuum conditions. In conclusion, it has been successfully demonstrated that the proposed approach allows executing a lean CubeSat verification campaign against environmental requirements following a systematic approach based on ECSS.

Autonomous neuro-fuzzy solution for fault detection and attitude control of a 3U CubeSat

In recent years, thanks to the increase of the know-how on machine-learning techniques and the advance of the computational capabilities of on-board processing, algorithms involving artificial intelligence (i.e. neural networks and fuzzy logics) have begun to spread even in the space applications. Nowadays, thanks to these reasons, the implementation of such techniques is becoming realizable even on smaller platforms, such as CubeSats. The paper presents an algorithm for the fault detection and for the fault-tolerant attitude control of a 3U CubeSat, developed in MathWorks Matlab & Simulink environment. This algorithm involves fuzzy logic and multi-layer feed-forward offline-trained neural network. It is utilized in a simulation of a CubeSat satellite placed in LEO, considering as available attitude control actuators three magnetic torquers and one reaction wheel. In particular, fuzzy logics are used for the fault detection and isolation, while the neural network is employed for adapting the control to the perturbation introduced by the fault. The simulation is performed considering the attitude of the satellite known without measurement error. In addition, the paper presents the system, simulator and algorithm architecture, with a particular focus on the design of fuzzy logics (connection and implication operators, rules and input/output qualificators) and the neural network architecture (number of layers, neurons per layer), threshold and activation functions, offline training algorithm and its data management. With respect to the offline training, a model predictive controller has been adopted as supervisor. In conclusion the paper presents the control torques, state variables and fuzzy output evolution, in the different faulty configurations. Results show that the implementation of the fuzzy logics joined with neural networks provide good robustness, stability and adaptability of the system, allowing to satisfy specified performance requirements even in the event of some malfunctioning of a system actuator.

CubeSat Team of Politecnico di Torino: past, present and future projects

The CubeSat Team is a student team of Politecnico di Torino involved in the design and development of small platforms for scientific space missions and for testing new technologies in orbit. The team was created in 2008 on the initiative of students and professors of the Aerospace Engineering course. More than 150 students got involved in the program since then. The CubeSat Team is coordinated by the Systems and Technologies for Aerospace Research team of the Mechanical and Aerospace Engineering Department, under the supervision of Professor Sabrina Corpino. The objectives of the program are summarised in the following mission statement: “To educate aerospace- engineering students on systems development, management, and team work. To achieve insight in the development of scenarios and enabling technologies for future space missions” In these years, the CubeSat Team reached great achievements, above all the launch of the e-st@r-I spacecraft, one of the two first Italian CubeSats in orbit. Students got involved in the design, development, and verification effort and participated actively in the integration and launch campaign. At the moment, two other CubeSats are being developed at the Systems and Technologies for Aerospace Research laboratory (STARLab): e-st@r-II, and 3-STAR, which are respectively part of the ESA “Fly Your Satellite!” and GEOID programs. The primary scientific payload of e- st@r-II is an active Attitude Determination and Control System for which innovative determination algorithms have been developed. The CubeSat successfully completed the environmental test campaign on June 2015 at ESA-ESTEC. 3-STAR is a 3U CubeSat that will take part in the GEOID constellation for the validation of the GENSO network through the HumSat communication payload. In addition, 3-STAR carries a remote sensing GNSS-based payload. This experiment will open the door to several applications, from Earth monitoring to civil protection warning services, and eventually military missions. The team is also working on the definition of new mission concepts to define innovative solutions targeted to establish low-cost/fast-delivery space assets for science and exploration of the Solar system. The aim of these missions is to increase the scientific and technological knowledge with unprecedented measurements and by exploiting the potentialities of interplanetary CubeSats as distributed systems. At the present moment, two main destinations are considered: Mars and Near Earth Asteroids. For these studies, two important international collaborations have been established, with MIT and NASA’s JPL. All these activities will be described into the details in the paper.

E-st@r-I experience: valuable knowledge for improving the e-st@r-II design

Many universities all over the world have now established hands-on education programs based on CubeSats. These small and cheap platforms are becoming more and more attractive also for other-than- educational missions, such as technology demonstration, science applications, and Earth observation. This new paradigm requires the development of adequate technology to increase CubeSat performance and mission reliability, because educationally-driven missions have often failed. In 2013 the ESA Education Office launched the Fly Your Satellite! Programme which aims at increasing CubeSat mission reliability through several actions: to improve design implementation, to define best practices for conducting the verification process, and to make the CubeSat community aware of the importance of verification. Within this framework, the CubeSat team at Politecnico di Torino developed the e- st@r-II CubeSat as follow-on of the e-st@r-I satellite, launched in 2012 on the VEGA Maiden Flight. E-st@r-I and e-st@r-II are both 1U satellites with educational and technology demonstration objectives: to give hands- on experience to university students and to test an active attitude determination and control system based on inertial and magnetic measurements with magnetic actuation. This paper describes the know-how gained thanks to the e-st@r-I mission, and how this heritage has been translated into the improvement of the new CubeSat in several areas and lifecycle phases. The CubeSat design has been reviewed to reduce the complexity of the assembly procedure and to deal with possible failures of the on-board computer, for example re-coding the software in the communications subsystem. New procedures have been designed and assessed for the verification campaign accordingly to ECSS rules and with the support of ESA specialists. Different operative modes have been implemented to handle some anomalies observed during the operations of the first satellite. A new version of the on-board software is one of the main modifications. In particular, the activation sequence of the satellite has been modified to have a stepwise switch-on of the satellite. In conclusion, the e-st@r-I experience has provided valuable lessons during its development, verification and on-orbit operations. This experience has been crucial for the development of the e-st@r-II CubeSat as illustrated in this article.