Flat-sat facility for processor-in-the-loop verification and testing of nanosatellite ADCS
Authors:   Politecnico Di Milano, Milano, Italy, Colagrossi Andrea , Silvestrini Stefano , Lavagna Michèle 
Published: January 2024

Small spacecraft missions are experiencing an increasing interest from the space community, because of their capability to reduce the cost of space access and of their potential to accomplish operations, complementary or even similar to larger monolithic spacecraft. However, these miniaturized systems typically suffer from poor reliability and their infant mortality rate remains quite high [1]. This is primarily due to the limited cost budget not allowing extensive verification and testing activities, which are still the key elements to ensure a high-quality standard guaranteeing space mission success and survivability. On the other hand, the verification and testing phase cannot reach the complexity and the level of classical spacecraft missions, since in that case the mission cost would dramatically increase. For these reasons, advanced and tailored AIV/AIT processes are needed.
The ADCS and, more generally, the GNC subsystems are commonly absorbing a relevant fraction of the AIV/AIT budget because of their need of dedicated and very specific facilities. For example, full hardware-in-the-loop (HIL) testing of a basic ADCS subsystem requires, at the minimum, a 3DOF frictionless bearing, a Sun simulator, and a Helmholtz cage [2]. More complex sensor architectures may require additional environmental simulation devices, and each specific mission may impose a specific facility customization. Moreover, the calibration and the set-up of such hardware-in-the-loop test benches is difficult and time consuming. Thus, a flexible and efficient verification and testing facility is extremely beneficial for the verification and testing of nanosatellite GNC subsystems.
The main idea to realize this verification and testing equipment, described in this work, is based on the virtualization of all the components associated with a dynamical measurement. In this way, it is possible to recreate a digital twin of the moving platform with all the associated sensors and actuators, leaving the on-board processor as the flight component under test without the need for any movable element. However, not to reduce the coverage and the significance of the verification and testing activities, all the real electrical and data interfaces shall be maintained and the on-board data handling (OBDH) subsystem shall not experience significant timing and data communication differences with respect to the real scenario.
The facility designed and built by the ASTRA team at Politecnico di Milano, Department of Aerospace Science and Technology, is able to achieve these goals and implements an enhanced processor-in-the-loop facility for nanosatellites. The current development status allows to assess the functionalities of the facility, and to apply it for the real-time verification and testing of a specific nanosatellite ADCS subsystem. It is still in a breadboarding status for a fast and efficient implementation and integration, but it is ready for consolidation and upgrade to a definitive version. Specifically, the final status will exploit standard industrial connectors and micro-controllers in place of the jumper wires and the Arduino boards. Moreover, generic ADCS and GNC subsystem will be fully compatible with the facility.
The processor-in-the-loop facility is based on a software simulation section and a hardware interface section. The software part is based on a validated Functional Engineering Simulator (FES) running on MATLAB/Simulink and exploiting the Simulink Real-Time capabilities on a Windows PC. It contains a Dynamics-Kinematics-Environment (DKE) section developed according to the ECSS standards [3-4] and the simulators of sensors and actuators. These are high-fidelity functional models of the specific sensors and actuators on-board the spacecraft, whose output is numerically identical to the one of the real components. In particular, the output data are defined in terms of data-type, output format and frequency. These sensor and actuator models are calibrated with dedicated hardware-in-the-loop testing campaigns on the specific spacecraft components.
The sensor data exits from the simulator environment through a real-time serial peripheral component. Then, they are received from the Arduino micro-controllers and formatted according to the data protocol and communication standards of the real component. The current development status implements I2C, Serial RS-232, RS-422, SPI data communication standards, and the data interface protocol is written according to the component specifications. The facility also includes the possibility to transduce PWM signals in order to be interfaced with analogic components requiring this modulation technique. The programming of the micro-controllers is made in C language allowing a great flexibility in reproducing different communication and data protocols for different sensor and actuator typologies with respect to the one currently in use.
After the data processing, all the data are made available to the on-board computers through the same hardware interfaces and connections that will be used on the spacecraft. However, to guarantee the safety of the components under test, the facility is electrically isolated from the spacecraft section. For this purpose, dedicated digital isolator boards have been designed. At the end of the process, the output commands of the ADCS for the actuators are sent back to the software section through the same digital isolator to micro controller to serial peripheral to MATLAB/Simulink path. In this way, a closed loop real-time simulation is performed on the developed ADCS algorithms running on the real hardware exploiting an equivalent digital twin of the on-board set-up.
The paper presents the main features and characteristics of this flat-sat facility for PIL verification and testing of nanosatellite ADCS. It outlines the main design, implementation and verification steps. Furthermore, it discusses the future extension of the facility and its consolidation in a final development status for generic applications to any ADCS or GNC subsystem
REFERENCES
[1] Villela, Thyrso, et al. "Towards the thousandth CubeSat: A statistical overview." International Journal of Aerospace Engineering, 2019.
[2] Modenini, Dario, et al. "A dynamic testbed for nanosatellites attitude verification." Aerospace 7.3 (2020): 31.
[3] ECSS. Space engineering: Space environment. Technical Report ECSS-E-ST-10-04C, European Cooperation for Space Standardization, 2008.
[4] ECSS. Space engineering: System modelling and simulation. Technical Report ECSS-E-TM-10-21A, European Cooperation for Space Standardization, 2010